Regulation of Aerodynamic Loads on Aircraft and Missiles Using Azimuthally-Controllable, Segmented Aerodynamic Forebody Bleed Actuation
20260078990 ยท 2026-03-19
Inventors
Cpc classification
F42B10/28
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
International classification
Abstract
The present disclosure generally relates to systems and methods for controlling aerodynamic loads on an aerostructure. The present disclosure can include a system including an outer shell, the outer shell including at least one aperture, and an inner shell having an aperture therethrough. The at least one aperture of the outer shell can be part of an array of apertures azimuthally distributed on the outer shell. The system can further include an actuator configured to control a rotational alignment of one of the inner shell and the outer shell to alter aerodynamic bleed through a portion of the at least one aperture of the outer shell. The actuator can be configured alter the rotational alignment of the inner shell and the outer shell based at least in part on a desired steering direction of the aerostructure.
Claims
1. A system for controlling aerodynamic loads on an aerostructure, comprising: an outer shell comprising at least one aperture; an inner shell having an aperture therethrough, the aperture extending azimuthally around a portion of the inner shell; and an actuator configured to control a rotational alignment of the inner shell and the outer shell to alter aerodynamic bleed through at least a portion of at least one aperture of the outer shell.
2. The system of claim 1, wherein the actuator comprises a motor configured to axially rotate at least one of the inner shell and the outer shell.
3. The system of claim 2, wherein the outer shell is rotationally fixed to the aerostructure, and wherein the motor is configured to axially rotate the inner shell with respect to the outer shell.
4. The system of claim 1, wherein the outer shell is disposed on an end of the aerostructure.
5. The system of claim 1, wherein at least one aperture of the outer shell is part of an array of apertures azimuthally distributed on the outer shell.
6. The system of claim 5, wherein at least one aperture of the outer shell comprises at least one straight edge, and the aperture of the inner shell comprises at least one straight edge, such that the at least one straight edge of the outer shell is configured to align with the at least one straight edge of the inner shell.
7. The system of claim 6, wherein the array of apertures comprises a first row of apertures and a second row of apertures, wherein at least one aperture of the first row is aligned with at least one aperture of the second row substantially parallel to a longitudinal axis of the aerostructure.
8. The system of claim 1, wherein the actuator is configured to cause the inner shell to transition between a first rotational alignment with the outer shell in which the inner shell blocks aerodynamic bleed through at least a portion of at least one aperture of the outer shell and a second rotational alignment with the outer shell in which the inner shell allows aerodynamic bleed through the at least a portion of the at least one aperture of the outer shell.
9. The system of claim 1, wherein the actuator is configured to control the rotational alignment of the inner shell and the outer shell based at least in part on a side force component from a fluid flow on the aerostructure.
10. The system of claim 1, wherein at least one aperture of the outer shell is part of a plurality of apertures spanning a top azimuthal portion of the outer shell.
11. The system of claim 1, wherein the inner shell is part of a plurality of inner shells comprising a first inner shell and a second inner shell, wherein the first inner shell and the second inner shell overlap thereby forming a relative azimuthal aperture.
12. The system of claim 1, wherein the actuator is selected from a group consisting of: a rotary motor, an inflatable actuator, piezoelectric plates, and an electrostatic actuator.
13. The system of claim 1, wherein the actuator is configured to automatically control the rotational alignment of the inner shell and the outer shell based at least in part on a desired steering direction of the aerostructure.
14. An aerostructure comprising: a substantially cylindrical body; and a forebody section comprising: an outer shell comprising an array of apertures; and an inner shell having one or more apertures therethrough, the one or more apertures extending azimuthally around a portion of the inner shell; and an actuator configured to control a rotational alignment of the inner shell and outer shell, such that the rotational alignment is configured to alter aerodynamic bleed through at least a portion of at least one aperture of the outer shell.
15. The aerostructure of claim 14, wherein the forebody section further comprises a gap disposed between the outer shell and the inner shell.
16. The aerostructure of claim 14, wherein the outer shell and the inner shell are hollow, such that the forebody section further comprises a compartment disposed at least partially within the inner shell.
17. The aerostructure of claim 14, wherein the inner shell is part of a plurality of inner shells comprising a first inner shell and a second inner shell, wherein the first inner shell and the second inner shell overlap thereby forming a relative azimuthal aperture.
18. A method of controlling aerodynamic loads on an aerostructure, the aerostructure comprising an outer shell, the outer shell comprising at least one aperture, and an inner shell having an aperture therethrough, the aperture extending azimuthally around a portion of the inner shell, the method comprising: placing the aerostructure in motion, such that the aerostructure experiences aerodynamic loads; and altering a rotational alignment of the inner shell with respect to the outer shell to alter aerodynamic bleed through at least one aperture of the outer shell.
19. The method of claim 18, wherein altering the rotational alignment of the inner shell with respect to the outer shell comprises: substantially nullifying effects of side forces and yawing moments via a first rotational alignment adjustment; determining a desired directional change of the aerostructure; and making a second rotational alignment adjustment based at least in part on the desired directional change of the aerostructure.
20. The method of claim 18, wherein altering the rotational alignment of the inner shell with respect to the outer shell to alter aerodynamic bleed through at least one aperture of the outer shell comprises altering an azimuthal position of the aperture of the inner shell.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The following detailed description of specific embodiments of the disclosure will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the disclosure, specific embodiments are shown in the drawings. It should be understood, however, that the disclosure is not limited to the precise arrangements and instrumentalities of the embodiments shown in the drawings.
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[0036]
DETAILED DESCRIPTION
[0037] To facilitate an understanding of the principles and features of the present disclosure, various illustrative embodiments are explained below. The components, steps, and materials described hereinafter as making up various elements of the embodiments disclosed herein are intended to be illustrative and not restrictive. Many suitable components, steps, and materials that would perform the same or similar functions as the components, steps, and materials described herein are intended to be embraced within the scope of the disclosure. Such other components, steps, and materials not described herein can include, but are not limited to, similar components or steps that are developed after development of the embodiments disclosed herein.
[0038] It should also be noted that, as used in the specification and the appended claims, the singular forms a, an, and the include plural references unless the context clearly dictates otherwise. References to a composition containing a constituent is intended to include other constituents in addition to the one named.
[0039] Also, in describing the disclosed technology, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
[0040] Ranges may be expressed herein as from about or approximately or substantially one particular value and/or to about or approximately or substantially another particular value. When such a range is expressed, the disclosed technology can include from the one particular value and/or to the other particular value. Further, ranges described as being between a first value and a second value are inclusive of the first and second values. Likewise, ranges described as being from a first value and to a second value are inclusive of the first and second values.
[0041] Herein, the use of terms such as having, has, including, or includes are open-ended and are intended to have the same meaning as terms such as comprising or comprises and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as can or may are intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such.
[0042] The components described hereinafter as making up various elements of the disclosed technology are intended to be illustrative and not restrictive. Many suitable components that would perform the same or similar functions as the components described herein are intended to be embraced within the scope of the disclosed technology. Such other components not described herein can include, but are not limited to, similar components that are developed after development of the presently disclosed subject matter.
[0043] Referring now to the drawings, in which like numerals represent like elements, the present disclosure is herein described.
[0044] Aerodynamic bleed, or peripheral bleed, in the context of the present disclosure can be understood as altering fluid flow based on a difference in pressure. More specifically, aerodynamic bleed can be defined as flow driven solely by pressure differences across aerodynamic surfaces in flight. That is, a pressure external to one segment of the outer shell 110 and a pressure external to a different segment of the outer shell 110 may differ, causing a fluid to flow into the outer shell 110 through a segment of higher pressure and out of the outer shell 110 through a segment of lower pressure. In some embodiments, as can be appreciated, the segment of lower pressure can be downstream from the segment of higher pressure. Consequently, the flow into and out of the outer shell 110 as discussed herein can occur regardless of the rotational alignment of the inner shell 120 and the outer shell 110. In some embodiments, the system 100 can further include a gap disposed between the inner shell 120 and the outer shell 110. That is, the fluid can flow into the segment of higher pressure, through the gap, and out of the segment of lower pressure of the outer shell 110. In this way, flow characteristics of the flow into and out of the outer shell 110 can be altered based at least in part on the rotational alignment of the inner shell 120 and the outer shell 110. Thus, in discussion of aerodynamic bleed being altered by a rotational alignment of the outer shell 110 and the inner shell 120, what can be appreciated is that the rotational alignment of the outer shell 110 and the inner shell 120 can affect the fluid flow about the outer shell 110. As will be discussed in greater detail herein, the inner shell 120 can include an aperture therethrough, in which the aperture can extend azimuthally around a portion of the inner shell 120. As is understood by those skilled in the art, flow characteristics of the fluid flowing in and out of the outer shell 110 can be altered based at least in part on whether the fluid flow is aligned with the inner shell 120, and thus flows through at least the gap between the outer shell 110 and the inner shell 120, or the aperture of the inner shell 120. In this way, in the instance of side forces and yawing moments, fluid may flow through the outer shell 110 and the inner shell 120 based on the alignment of an aperture of the inner shell 120 and the aperture 112 of the outer shell, as will be discussed in greater detail herein. Further, altering of the flow characteristics of the fluid flowing about an aerostructure, as can be appreciated, can be manually adjusted in order to induce a force on the aerostructure, or particularly, to steer the aerostructure. Thus, altering aerodynamic bleed as discussed herein can include altering the rotational alignment of the inner shell 120 and the outer shell 110, or any embodiments of one or more inner shells and an outer shell discussed herein.
[0045]
[0046] The inner shell 120, as discussed for
[0047]
[0048] As shown in
[0049] Any of embodiments of inner shell discussed herein may be substantially conical. In this way, the inner shell 120, and other embodiments of inner shell discussed herein, may be at least partially disposed within the outer shell 110. In some embodiments, there may exist a gap between the outer shell 110 and the inner shell 120. As will be appreciated, the gap between the outer shell 110 and the inner shell 120 can be configured to improve changes in aerodynamic bleed incurred by changing the rotational alignment of the outer shell 110 and the inner shell 120. Additionally, the inner shell 120, and any embodiments of inner shell discussed herein, may be at least substantially hollow. In this way, the inner shell 120, or the one or more inner shells, may be configured to house a compartment within. That is, the aerostructure 200 may further include a compartment disposed at least partially within the inner shell 120. The compartment, as can be appreciated, can be utilized to store different components, subsystems, electronics, power sources, and similar elements known in the art for aerostructures. Furthermore, the compartment may be empty so as to reduce a weight of the aerostructure 200.
[0050]
[0051] As shown in
[0052] As shown in
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[0055] As shown in
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[0057] As can be understood by those skilled in the art, altering aerodynamic bleed of an aerostructure may be configured to steer the aerostructure. In this way, axially rotating one or more inner shells within an outer shell of the aerostructure may be configured to steer the aerostructure.
[0058]
EXAMPLES
[0059] The following sections illustrate example implementations of the present disclosure.
Technical Background
[0060] Investigations of the aerodynamic characteristics of axisymmetric slender bodies at moderate and high incidence angles since the 1950s have been largely motivated by the flight dynamics of missiles, munitions, and fighter aircraft. These flight platforms encounter complex, unsteady aerodynamic loads that are usually far less significant at lower angles of attack and are associated with the appearance and evolution of trains of spatially-and temporally-varying vortical structures over the body and in its near wake. The earlier studies showed that these vortical structures are spearheaded by the formation and asymmetries of counter-rotating vortex pairs near the upstream end of the forebody. The dynamics and asymmetries of these forebody vortices and their interactions with vorticity concentrations within the oblique shear layers that bound each side of the near wake along the main cylinder and its aft segment can contribute to strong unsteady side-and cross-stream forces and yawing and pitching moments that may be used for attitude control.
[0061] In one of the early investigations of the forebody vortices, Jorgensen and Perkins (1958) tracked the forebody vortex cores over an inclined ogive-cylinder body (L/D=3, M=1.98). These authors showed that the detachment of the vortices from the surface of the cylinder moves rapidly upstream with increasing inclination angle, and that for a given angle the strength of the vortices increases along their axes apparently indicating merging with the adjacent azimuthal vorticity layer along the surface of the cylinder. Nelson and Fleeman (1975) attributed the induced changes in side force and yawing moment on the cylinder to asymmetric shedding of vortices from its leeward side. Yanta and Wardlaw (1977) noted the asymmetry of the forebody vortices and flow at high inclination angles can be caused by minor variations of the nominally axisymmetric forebody, and in a subsequent investigation (Yanta and Wardlaw, 1981) attributed the side force that occurs when one of the forebody vortices detaches from the body to the opposite sense vortex that remains attached. Subsequently, these authors found that the asymmetric vortex pattern (=45) is formed as a result of secondary vortices that develop adjacent to the primary forebody vortices, causing one of the primary vortices to become detached from the surface (Yanta and Wardlaw 1982; Wardlaw and Yanta 1982). In an investigation of Re effects on the flow over an ogive-cylinder (0.2.10.sup.6<Re.sub.D<4.10.sup.6, 20<<90) Lamont (1982) noted that while large side forces due to asymmetric vortex shedding were present during laminar and turbulent leeward surface separation, force diminution during transition was attributed to separation asymmetries owing to different transitional states on opposite sides of the body.
[0062] The details of the forebody geometry strongly affect the aerodynamic loads induced by asymmetric vortex shedding. For example, for ogive cylinder bodies the asymmetric side force increases with the fineness ratio (Pick 1972; Jorgensen and Nelson 1974; Keener and Chapman 1974; Dahlem et al., 1980). Furthermore, Keener and Chapman (1974) reported that for a given forebody fineness ratio the side force first decreases and then increases with increasing the tip bluntness (ratio of tip radius to body radius) and for some tip bluntness can result in almost negligible side force (at M=0.25) past which the side force increases along with significant flow unsteadiness. More recently, Kumar et al. (2005) assessed the effect tip bluntness of inclined slender cones on vortex asymmetry and showed that the vortices tend to become more symmetric as bluntness increased and the resulting side forces on the model tend to decrease. The sensitivity to the shape of the forebody shape was also investigated for ogive and elliptical forebodies by Luo et al. (1998) and for hemispheric, elliptic and ogive forebodies by Sirangu and Ng (2010) who reported that the ogive bodies produced the least vortex asymmetry and side forces. In a recent experimental investigation of the vortex asymmetry on a conical forebody, Taligoski et al. (2014) showed that the bi-stable side-force and moment characteristics with varying roll angle at high angles of inclination resulted from the formation of a stronger vortex that was advected away from the surface and its initiation was attributed to micro-scale surface imperfections close to the forebody tip.
[0063] Based on simulations and flow visualization studies of the forebody vortex flow over a range of angles of inclination in various studies (e.g., Wu et al., 1986, Ward and Katz 1989a, 1989b, 1989c, Zilliac et al. 1991, and Deng et al. 2003), the topology of the forebody vortices over a range of inclination angles can be divided into three primary regimes. These regimes include: symmetric vortices that are mostly jointly located adjacent to the surface of the cylinder or become jointly detached from the surface (<30), asymmetric vortices where one of the counter-rotating vortices become detached first, leading to mutual roll (30<<60) and to significant side force and yawing moment, and unsteady wake-like flow when the vortices couple to the oblique shear layer and Krmn shedding off the cylinder section (60<<90).
[0064] In an effort to mitigate asymmetric vortex formation and the associated increase in side forces and yawing moments, Rao (1979) used helical trips to modify the forebody surface of an axisymmetric cylinder (L/D=6-9, <55) and showed that induced nonuniform separation over the forebody disrupted the coherence of the shed streamwise vortices. Similarly, Modi et al. (1984) showed that passive modifications (including surface grit and helical trips) on conical forebodies led to reduction of up to 60% in the side forces. Further improvements were achieved by adding a rotating tip and a nose boom, resulting in a remarkable reduction of 98% in side force at =30, presumably due to complete suppression of the vortex pair. The utility of movable and/or deployable mechanical protrusions for reduction in aerodynamic side forces and moments has also been investigated. Rao et al. (1987) tested deployable strakes on an isolated forebody (L/D5; =50) and reported large changes in the side forces with the strakes azimuthal angle that were associated with the formation of a strake vortex that remained close to the forebody, or a larger-scale detached spoiler vortex. Leu et al. (2005) utilized an array of inflatable micro-balloon actuators fixed to the surface of a conical forebody (L/D=5) to induce the formation of asymmetric vortices and side forces of a desired direction. Stucke (2006) manipulated the forces, pitch and yaw moments, and roll angle of an inclined axisymmetric body (L/D=4, =50) using spoilers and strakes near the leading edge, and, in a follow-up work, Lopera (2007) used a deployable version of the strake as part of a closed-loop flow control system for maintaining a specified angle of inclination. More recently, Mahadevan et al. (2018) triggered and managed the asymmetry of forebody vortices using boundary layer scale hemispherical protrusions on a highly-polished conical forebody.
[0065] A number of investigations employed fluidic actuation (steady and unsteady blowing and suction) and limited plasma actuation near the tip of inclined forebodies to manipulate the shedding of the vortices from the leeward surface and thereby effect changes in the side forces and yawing moments. Steady jets have been used over a range of subsonic and transonic speeds and momentum coefficients (e.g., Peake et al., 1980, cone, L/D=5.7, C.sub.0.005, Almosnino and Rom, 1981, cone, L/D=6, =35-55, C.sub.<0.002, and Skow et al., 1982, ogive, L/D=3.5, =35-55). More recently, Kumar et al. (2008) used a jet blowing upstream along the axis of and through the tip of an inclined cone to mimic fluidic nose blunting and reported a reduction of up to 80% in side forces at <45. Unsteady blowing using a linear array of synthetic jets along the leeward stagnation line of a conical forebody was used by Williams et al. (1989) and Williams and Papazian (1991) to form pneumatic splitter plate and effect flow symmetry at =55. In a related investigation, Williams and Bernhardt (1990) used synthetic jets for variable control of the forebody vortices and of the sectional side force at <50. Similarly, Kalyankar et al. (2018) used unsteady sweeping jets on the side of an inclined cylinder (L/D=9, =60) to alter the separation line on the surface and generate yaw moment as large as C.sub.LN0.8 with C.sub.=2.7%. The phantom yaw effects associated with asymmetric vortex shedding over a pitching axisymmetric body (L/D=20) were characterized in the recent simulations of Schnepf and Schlein (2018), who used steady blowing from a slot along the side of the body to mimic an aerostrake and to mitigate asymmetric vortex shedding and reduce the aerodynamic side force by 25%. In a noteworthy approach, Sato et al. (2016) were able to reduce the side force and yawing moment on a cone-body (L/D=5.7, <90) by up to 50% by using autonomous bleed driven through internal passages within a forebody cone by the external pressure differences. Plasma actuation was used by Fagley et al. (2012) to manipulate the asymmetric aerodynamic side force on an inclined forebody (Krmn ogive, L/D=3.5, 40<<60) by up to C.sub.y=1. Considering the effectiveness of active actuation, a number of investigations have demonstrated closed-loop feedback control of the aerodynamic side forces induced by the forebody vortices. For example, the methodology of Porter et al. (2014) was recently adopted by Seidel et al. (2018) in a simulated closed loop feedback controller which could effect specified side forces.
Experimental Setup and Wind Tunnel Model
[0066] The present investigations were conducted in an open-return wind tunnel at Georgia Tech in a square test section measuring 91 cm on the side with a free stream velocity of U.sub.o=40 m/s. The axisymmetric model is wire-supported in an open-return wind tunnel (test section measuring 91 cm on the side) by a dynamic 6-DOF eight-wire (1.2 mm dia.) traversing mechanism described in detail by Lambert et al. (2016). Each support wire is controlled by an independent servo motor, with an in-line load cell, and electrical connection for the flow control actuators is provided by thin wires weaved along the back four support wires while the support wires provide electrical ground. The forces and moments on the model are calculated from the measured wire tensions projected onto the model (the resultant aerodynamic loads on the model are calculated relative to the loads in the absence of cross flow, and accounting for wire drag). The attitude of the model is commanded by a Matlab Simulink controller, which feedback utilizes inputs from VICON motion-capture camera system at an update rate of 500 Hz. Besides providing the feedback signal, the six-camera motion capture system resolves the spatial and temporal position of the model at any instant in time. In an alternate configuration, the feedback loop can be disconnected and the model locked in the desired attitude. Either configuration is utilized, depending on the objectives of the studies.
[0067] The information regarding the model position/orientation is used to extract the wire orientation and accurately decompose the forces measured on each load cell into x, y, and z components in real time.
[0068] Two different slender axisymmetric models were utilized in the present studies. Initially, flow control efforts were centered about the synthetic jet control of the model aerodynamic loads, having the model diameter D=40 mm and L/D=11, including the tangent ogive forebody of the length l/D=2. Since the prior studies indicated that the ogive body geometry generally induces more prominent side forces when compared to the conical forebodies (e.g., Chapman et al, 1976), the ogive geometry is utilized in the present investigation. The investigations were focused on control of autonomously-formed forebody vortices over a range of angles of inclination (25<<65), while the wind tunnel was operated with uniform wind speed of U.sub.o=30 m/s, while the emphasis is placed on the high angle range 45<<65.
[0069] The axisymmetric body is comprised of three major modules: the ogive forebody, synthetic jet actuator module, and the central cylindrical body. Both the forebody and the jet module are designed such that can be rotated by the full azimuthal period. The azimuthal orientation of the forebody and the jet are referenced to the top vertical point, with the angles increasing clockwise, in the upstream view. The jet module incorporates a single azimuthal orifice measuring 0.615.7 mm, imparting the jet momentum coefficient C.sub. while issuing normal to the surface at the frequency of about 2.3 kHz. To illustrate the forebody vortices that the control jet is designed to affect, the forebody vortex pair is visualized over the default l/D=2 ogive forebody at the angle of attack =60. For that purpose, a mixture of a titanium-oxide paint and the linseed oil is applied over the forebody, where its ratio is iteratively adjusted such that the oil mixture does not shear before the operating flow condition is attained. After the test section speed reached U.sub.o=30 m/s, the oil is sheared for about 20 minutes. The two vortical traces are clearly seen, forming off the forebody tip and evolving along the forebody surface. The strong traces along each line where the vortex lifts the flow away from the surface indicates that these vortices remain in the proximity of the surface over the full forebody extent.
[0070] The second set of experimental studies was focused on the aerodynamic bleed control of slender axisymmetric bodies at high angles of attack, and this model had a cylindrical body (diameter D=50 mm, length L=7 D) with a tangent 2 D long ogive forebody that can be rotated azimuthally, resulting in the overall L/D=9. These investigations focused on aerodynamic-bleed control of the forebody vortices that form autonomously over a range of angles of incidence (35<<55) at cross stream speeds of up to U.sub.o=30 m/s (Re.sub.D=9.9.Math.10.sup.4).
[0071] In addition to measurements of the aerodynamic loads, characteristic flow fields about the models were measured by the particle image velocimetry (PIV) techniques. A stereo PIV (SPIV) system is used to characterize the 40 mm dia. model's wake dynamics using two CCD cameras that are each placed at an angle of 20 relative to an image plane normal to the oncoming flow at x/D=2-9 from the tip of the model. Initial PIV measurements of the 50 mm dia. model focused on the vortex-dominant, wake side of the flow about the body, and this PIV configuration. A planar PIV system is used to characterize the model's forebody vortices and wake dynamics using a CCD camera aligned with the model axis of the model and placed downstream of the aft end of the model, imaging 532 nm Nd:YAG laser sheet such that the measurement plane is normal to the model's axis at several axial stations. Such PIV orientation enables investigations of the near-surface flow and the advection of streamwise vorticity concentrations into the near wake. Taligoski et al. (2015) demonstrated that, with a similar optical setup, it was possible to capture cross flow velocity field near the surface of the model. In the present investigations, 1,000 instantaneous PIV images are acquired at 1 kHz at x/D=1.6, 2.2, 4.0, 6.0, and 8.4 from the tip of the model's forebody at inclination of =50. Schematics also illustrate orientations of the six motion-capture cameras that are distributed evenly on both sides of the test section. Final, detailed PIV investigation of the flow about the slender axisymmetric body at high angles of attack is designed such that the measurements can be done over both the windward and leeward sides of the body's cross-section. For that purpose, the laser-sheet optical paths were assembled on either side of the wind tunnel test section. Laser beam is initially directed to a linear traverse on the top of the test section carrying two 45 mirrors; one mirror directs the beam to the front and the other to the back of the test section. Both sides of the wind tunnel have identical optical paths that first direct the beam to another mirror atop the vertical rail attached to the computer-controlled traverse. After being directed downward and passing through a spherical and cylindrical lens, the laser sheet is finally directed horizontally intersecting the model's cross section and illuminating either the windward or leeward side of the flow about the model. Each PIV camera is mounted on its own linear traverse, aligned with the model axis. All the PIV measurements are done in the following procedure. The PIV camera is positioned at the starting distance from the model, at the focal length of the first measurement plane. Then, the laser beam is steered to the front forming the illuminated sheet on the vortex pair side. Once the PIV measurements are taken on this side, the laser beam is then steered to the back by the second mirror on the top and the paired PIV measurements are taken on the flow stagnation-point side of the model. Once such a pair of measurements is taken, both the laser sheet (still directed from the back) and the PIV camera are axially incremented to the next plane, and the first set of PIV measurements are taken. Next, its complementary PIV measurements on the vortices'side are taken after the top mirrors are switched again. Clearly, this procedure is thereafter repeated until the last measurement plane.
The Base Flow
[0072] Initially, the aerodynamic loads on the cylindrical model were characterized in the absence of actuation over a range of inclination angles (25<<65, Re.sub.D=7.9.Math.10.sup.4) using the three ogive forebodies of l/D=1, 2, and 3. The inclination angle of each model was increased monotonically from the same base angle to avoid hysteresis effects. In order to enable meaningful comparison between the models with the different ogive forebodies, the variation of the force coefficients C.sub.D, C.sub.L, and C.sub.S were computed based on the model's planform area (including the forebody). Hoerner and Borst (1985) characterized the lift and drag on an inclined cylinder in the absence of a forebody. These authors noted that at low inclination (15), the flow over the cylinder is predominantly oriented along its axis and it may be thought of as low-aspect-ratio wing with a pair of counterrotating tip vortices that form over the suction surface resulting in lift-induced drag. At higher angles of attack, the flow from the windward to the leeward face of the cylinder separates on its leeward face and generates a normal force on the body whose respective cross-stream and streamwise projections are the lift and drag. Both lift and drag forces are small for 15, and as a increases, the drag increases monotonically and reaches a maximum at =90, while the lift has a local maximum (around 55), and then decreases monotonically and vanishes =90.
[0073] On the present model, the drag increases monotonically over the entire range of a while the rate of increase of the lift begins to diminish for >35 ostensibly as the forebody vortices begin to lift off the cylinder and the flow from the windward separates on its leeward surface. Similar to the observations of Hoerner and Borst (1985), while the drag force continues to increase monotonically, the lift force has a maximum around =55, and then decreases at higher angles of incidence. While the drag coefficients for the three ogive forebodies are nearly identical through a 60 the drag coefficient of the l/D=3 ogive is lower at higher a and appears to reach a local maximum that is lower than the corresponding drag coefficients of the l/D=1 and 2 ogives. However, the corresponding peak lift coefficients of the l/D=1 ogive is lower than the coefficients of the other ogives. It is apparent that these changes are associated the changes in the side forces that remain nominally symmetric about the cylinder's vertical (y-z) plane of symmetry up to 50. According to earlier work (e.g., Keener and Chapman, 1974), the onset of the vortex asymmetry approximately scales with the forebody tip angle, which is 58 for the present model. As is evident from the variations of the side force coefficients, the onset of forebody vortex asymmetry which is affected by small variations in the ogive surfaces, varies between the different forebody models, and also affects both the lift and drag forces. For the remainder of the current study, the forebody is fixed at l/D=2. In addition, it is observed during the pitch sweeps that the model, once in the non-zero side force domain, can undergo unstable motion.
[0074] To gain a better understanding of the base flow features at high angles of attack, preliminary sPIV measurements are taken at three streamwise positions, measured from the forebody tip, x/D=2, 5, and 9 while the body is oriented at =60. These locations are selected such to characterize the flow state just downstream from the location of the control jet, far over the body, and finally off the body, in the wake. Due to the wake spreading, the measurement resolution is adjusted with the downstream distance, such to capture the wake extent. For the same reason, measurement s on the wake are taken over two measurement planes that are merged into a single composite flow field. The resulting captured flow field can illustrate the dominant vortical composition of the flow. As it could be expected, the initial vortex pair, formed at the forebody, lifts off the surface shortly downstream from the forebody, due to the high incidence. Although still at the surface at x/D=2, this pair evolves into a highly asymmetric pair at x/D=5, where the CW vortex remains closer to the body, while the CCW vortex, rotated in pair with the CW one, moves away and nearly atop its CW pair. This relative orientation remains preserved into the wake at x/D=9. Once the initial vortex pair is peeled off, the successive folding of the flow over the cylindrical body results in the secondary vortex pair formation, which is just barely captured at the bottom end of the measurement plane at x/D=5, and fully seen at x/D=9 underneath the primary vortex pair, and assuming a nearly identical relative orientation between the CW and CCW vortices. Besides these two pairs of streamwise vortices, additional vortical concentrations are seen in the wake, as it is expected that the shear layers of the flow separating of the cylinder body partially contribute to the streamwise vortical components. Besides, each vortex can induce a neighboring lesser vortical motion of the opposite sense, which is likely manifested just below the lowest CW vortex at x/D=9.
[0075] The uneven liftoff of the vortex pair and its subsequent tilt about the common axis signalizes disruption in the side force balance and induces a net non-zero side force. Such a liftoff of one of the forebody vortices from the surface was documented in detail in a number of earlier studies. Lamont and Kennaugh (1989) showed a nearly periodic switching in the direction of the side force as the forebody is rotated azimuthally about the axis of the cylinder over a range of incidence angles, which reflects the switching vortex asymmetry in the flow. As shown by Mahadeven et al. (2018) even fine polishing of the forebody surface was insufficient to fully suppress the vortex asymmetry and the direction switch of the induced side force. It should be noted that this sensitivity of the forebody vortices to small perturbations indicates their potential receptivity to flow control actuation as well.
[0076] The asymmetry in the evolution of the forebody vortices and the resulting side forces is investigated at =60 (Re.sub.D=7.9.Math.10.sup.4) over a full azimuthal rotation of the l/D=2 forebody. The resulting drag, lift, and side force coefficients (each normalized by the cylinder's cross sectional area Ab). In concert with the earlier investigations, the side force exhibits azimuthally-periodic switching. However, C.sub.S>0 for most of the azimuthal orientations 100<<220, 275<<50 and switches direction C.sub.S<0 only within narrow azimuthal domain centered about =70 and 220. One notable exception is a sudden drop of the side force at =330, which is caused by the body undergoing instability for that particular forebody orientation. It is interesting to note that the induced side force does not change its direction when the forebody is rotated at 180 relative to some given azimuthal position (i.e., C.sub.s>0 or <0 at both domains although the nominal magnitudes of the opposite side forces are not necessarily of the same). That the sense of the side forces does not change when the forebody is rotated at 180 indicates that the flow asymmetry is likely not brought about by a random surface imperfection, because a strong periodic behavior of the present data (and a number of the earlier studies) suggest that the origin of such behavior is likely in the regular geometry deviation with a preferential axis. The most obvious source of such deviation would be an imperfect tip of the forebody, particularly since many investigations indicated extreme flow sensitivity to small geometrical perturbations at the forebody tip. Examination of the tip of the current forebody model indicated small oval deviation from the perfectly circular termination of the tip, and it is argued that such an oval shape with the dominant axis would be sufficient to induce preferential vortex asymmetry, depending on the dominant axis azimuthal orientation. Moreover, as the forebody is rotated by 180, the oval orientation would assume the same orientation of its dominant axis, which would explain periodicity is the side force formation. As the oval manufacturing perturbation is not perfect, this would also explain that asymmetry in the magnitudes of the excursions and the disparity in their azimuthal extents are associated with the randomness of this deviation. It is remarkable that for fixed angles of incidence and yaw the lift and the drag are nearly invariant with q$ even though the side force undergoes significant variations which are associated with topological changes in the trajectories of the streamwise forebody vortices. This indicates that once the vortices separate and migrate off the cylindrical body, their effects on the lift and drag diminish.
[0077] The changes in the topology of the vortex pair associated with the changes in the direction and magnitude of the side force can be illustrated in color raster plots of the time-averaged streamwise vorticity superposed with vectors of the cross-stream velocity field captured using stereo PIV within the domain 6<y/R<6, 6.5<z/R<6.5 (x/D=9) for =90 and 180. The data shows a dominant pair of counter-rotating streamwise vortices where the CW vortices in this view are associated with the rollup at the left side of the forebody (in this upstream view), along with additional, weaker streamwise vortices that would be shed within the cylinder's wake (cf. the simulations of DeSpirito 2017). As is evident from the vorticity concentrations, the major axis of the dominant vortex pair (i.e., the axis centered between the vortices, nominally normal to a line through the centers of their cores and pointing in the direction of the induced flow) can be rotated by 128 (from 26 to 154). The directions of the major axes are asymmetric (i.e., the major axes can be pointing to the right and the left, respectively) and commensurate with the changes in the directions of the side forces namely Cs<0 at =90 and C.sub.s>0 at =180. It is also noteworthy that the change in the directions of the major axes of the primary vortex pair is also accompanied by changes in the sense of the accompanying streamwise vortical traces that are captured within the field of view, ostensibly by reversal of the induced cross flow by the dominant vortex pair.
[0078] To summarize the base flow topology over the slender axisymmetric model at high angle of attack, the color raster plots of the streamwise vorticity relative to the body coordinate system (x) at five cross stream planes along the model for the pitch angle =50. The initial formation of the forebody vortex pair at x/D=1.6 is reasonably symmetric and has nearly identical magnitudes of the vortex circulations. The symmetry appears to be preserved at x/D=2.2, where only the entrainment-driven growth of the vortices is measured. Some disruption in symmetry is noted at x/D=4, where both vortices begin to deflect away from the surface. By x/D=6, the two forebody vortices appear to have been displaced away from the surface and their streamwise vorticity is weakened as a result of spreading, while the CW and CCW vortex sheets on both sides of the body's wake begin to roll up underneath the primary vortex pair. The asymmetry in the shedding of the forebody, as is evidenced by their trajectories, is manifested by an induced net right side force (along y<0) as a result of the earlier displacement of the CW vortex. The topology of the streamwise vorticity at x/D=8.4 indicates that the asymmetry propagates downstream, as the CW is seen displaced further into the wake than its corresponding CCW pair. Furthermore, formation of the secondary vortex pair about the cylindrical body continues underneath the displaced primary vortex pair, and their asymmetry further contributes to the net side force.
The Controlled Flow
Azimuthal Control of the Forebody Flow Using Segmented Forebody Aerodynamic Bleed
[0079] The prior studies that indicated the sensitivity of the evolution the forebody vortex pair to small surface perturbations also pointed out to a path of exploiting this sensitivity for controlling the aerodynamic loads with minimal control input. An approach to delay the side force, relevant to the present study, was tested on a forebody model by progressively perforating the increasing axial extent of the forebody and utilizing the aerodynamic bleed driven by pressure differences to enforce flow symmetry (Bauer and Hemsch, 1994). They showed that the increase in incidence of the onset of a nonzero side force was proportional to the axial extent of the bleed. In a similar application, Fears (1995) reported comparable results when replacing the solid forebody of a fighter aircraft model with a porous forebody as an alternative to control by strakes for generating yawing moment.
[0080] The present investigation is motivated by the findings of Bauer and Hemsch (1994) and focuses on exploiting segmented porosity over the forebody of a cylindrical platform for suppression of asymmetric baseline side forces that arise from flow sensitivity to minor imperfections of the body surface. Furthermore, configurations of segmented bleed actuation are also used for prescribing desired side forces and yawing moment for bi-directional aerodynamic control.
[0081] The axisymmetric body is comprised of three major modules: The ogive forebody, the central cylindrical body, and the aft control module, which is kept inactive in the current investigation. The prior work (Lee et al. 2021a and 2021b) focused on direct control of the separating flow on the leeward side of the body, which, in turn, altered the dominant wake vortical composition through its inherent coupling. During the present investigation, the aerodynamic loads on the model are controlled by manipulation of the symmetry of the forebody vortices and, consequently, their interactions with the cylinder's wake using the aerodynamic bleed over the forebody. Therefore, while the earlier approach affected the dominant streamwise vortices, and consequently the aerodynamic loads, indirectly by focusing on their coupling to the wake along the cylindrical body, the present investigations focus on the direct control of the forebody vortices over their formation domain along the forebody. For that purpose, instead of the forebody with integrated fluidic actuation, a forebody with azimuthally distributed bleed ports is designed and manufactured, following the forebody bleed study of Bauer and Hemsch (1994). Initially, three different azimuthal extents of the bleed surface over the forebody are tested, spanning 45, 90, and 180 azimuth of the forebody. Each of these three configurations is tested in two orientations relative to the flow, by promoting the flow symmetry in the top-central orientation of each segment, and by promoting the flow asymmetry by orienting each segment to be centered on one side of the body. These preliminary results indicate that base flow suppression of asymmetry, expressed through the lowering of the resulting side force, increases with the azimuthal opening on the top of the forebody. In contrast with this, a minimal effect on the asymmetry is measured, as all the three configurations exerted nearly the same magnitude of the side force. Based on these preliminary findings, in spite of slightly better results achieved by half-the-circumference bleed application, it is decided to conduct a full investigation by utilizing a 90 bleed opening due to the limited number of orientations that the 180 bleed section would allow.
[0082] The bleed forebody model can comprising the outer and inner shell. Each azimuthal half of the outer shell is populated by the ten bleed rows, having the equal angular distribution, while each row comprises seven streamwise narrow bleed slots. Different bleed configurations are formed by closing inactive ports, where the closed ports can be seen in light color. Given the primary targeting of the two forebody vortices with an objective of both symmetric and asymmetric alterations of that vortex pair, a total of eight different bleed configurations are tested. Dark azimuthal sections represent the active bleed segments, where most configurations represent different azimuthal orientations of the 90 bleed segment, except for a configuration having fully open bleed ports across the whole forebody surface. Therefore, the tested configurations, from left to right, included the fully closed (i.e., the base flow) and fully open configurations, top right, top and bottom left 90 segments, and the 90 segments centered at the left, top, and bottom.
[0083] The present work particularly focuses on the forebody vortex pair evolution over the body, while the prior work (Lee et al., 2021a and 2021b) placed emphasis on the wake interactions of these vortices, both in the presence and absence of flow control. To illustrate the base flow over the model with all the bleed ports closed, the color raster plots of the streamwise vorticity relative to the body coordinate system (x) at five cross stream planes along the model for the pitch angle =50. The initial formation of the forebody vortex pair at x/D=1.6 is reasonably symmetric and vortices have nearly identical magnitudes of vorticity (and its integral measure of circulation). The symmetry appears to be still preserved at x/D=2.2, where only the entrainment-driven growth of the vortices is measured. Some disruption in symmetry is noted at x/D=4, where both vortices begin to deflect away from the surface. By x/D=6, the two forebody vortices appear to have been grown away from the surface while still remaining attached to it, while the CW and CCW vortex sheets on both sides of the body's surface begin to roll up underneath the primary vortex pair. The asymmetry in the shedding of the forebody vortices downstream from x/D=6, as is evidenced by their trajectories, is manifested by an induced net right side force (along y<0) as a result of the earlier displacement of the CW vortex. The topology of the streamwise vorticity at x/D=8.4 indicates that the asymmetry propagates downstream, as the CW is seen displaced further into the wake than its corresponding CCW pair. Furthermore, formation of the secondary vortex pair about the cylindrical body continues underneath the displaced primary vortex pair, and their asymmetry further contributes to the net side force.
[0084] As the first PIV measurement plane (x/D=1.6) is set right at the axial termination of the bleed, the corresponding flow fields are utilized for an initial assessment of the flow control effects for the fixed pitch angle =50. Hence, all eight flow fields, corresponding to the eight bleed configurations can be shown in terms of the mean velocity field in the y-z plane. The base flow, in the absence of bleed. The overall flow symmetry is emphasized by the symmetric vortices of nearly matched circulation magnitudes. This base flow is first contrasted with the case when the aerodynamic bleed is fully opened across all of the bleed ports. As it could be expected, the net bleed is driven from the pressure side of the surface along the bottom to the upper surface, having a rather dramatic effect on the vortex pair evolution. Consequently, the vortex cores are displaced away from the surface and weakened. It is interesting that they still preserve the symmetry and remain attached to the body through the folding vortex sheet off the lower section of the forebody. The localized bleed segments that target only one of the forebody vortices induce asymmetric vortical composition already in the first measurement plane. For instance, symmetrically switched bleed configurations at the top right and left side promote displacement of the vortex on that side, already inducing the vortex/side force imbalance over the forebody. Although not as pronounced as when the bleed segment is directly underneath the evolving vortex on the top left side, even in the case when the bleed segment is along the pressure side, the aerodynamic bleed induces imbalance in the vortex pair growth, by promoting the same-side vortex. Knowing the outcomes of the aerodynamic bleed across the top and bottom left segments, it is not surprising that by orienting the bleed segment such that half is in the upper and the other half in the lower hemisphere on the left, the primary effect on the growth of the same-side vortex is approximately between the effects of the individual top and bottom segmentsthe bleed enhances the growth (and asymmetry) of that side vortex, but not as significantly as in the case of the top-segment flow manipulation. In a summary of the three same-side azimuthal orientations of the 90 bleed segment, it appears that the most effective approach in inducing the vortex pair asymmetry is the bleed manipulation directly underneath the evolution of the targeted vortex. Lastly, two additional bleed configurations targeting symmetrically both the left and right forebody vortices are done by centering the 90 bleed segment directly and the top and bottom. Not surprisingly, symmetric application of bleed preserves the overall flow symmetry in both cases. It is interesting that in either case there is actually a very little change even in the vortex structure, where at most the vortex sheet along the surface becomes somewhat enhanced in the case of the top centering, while there is no discernable difference relative to the base flow when the bleed segment is centered at the bottom, symmetrically along the pressure surface.
[0085] The resulting global effect of these different bleed configurations is assessed through a cumulative, net change of the aerodynamic forces and moments, which can be shown with respect to the fixed coordinate system (x, y, z,), where x-axis is aligned with the flow direction. As expected, the bleed-induced alteration of the vortex symmetry, which couples to the wake asymmetry, is reflected in the strongest effect on the side force coefficient C.sub.S. Cumulative asymmetry in the base flow results in the net side force coefficient of nearly C.sub.S=1, along with the net baseline lift and drag forces. It should be noted that the normal force relative to the model orientation (Fz) decomposes into the lift (Fz) and drag (Fx) forces shown in terms of the coefficients C.sub.L and C.sub.D. As the bleed control is applied over the full circumference of the forebody, there is a clear drop in the net side force, accompanied by the increase in both the drag and lift coefficients C.sub.D and C.sub.L, which implies that the lift force normal to the body Fz increases as well. As suggested by the altered flow field can be seen, premature displacement of the CCW vortex by the bleed from the top right segment would contribute to the net negative side force. Furthermore, there is an additional increase in both C.sub.D and C.sub.L, which also implies additional increase in the body-normal Fz force. In principle, a switch to the top left bleed segment is expected to generate the same and opposite effect from the top right segment. Indeed, there is a sharp switch in the side force to the net positive side, but some imbalance in magnitudes is attributed to the non-zero base flow, which is already biased towards the net positive side force. It is interesting that the bottom-left bleed case is closer to an inverted top-right case in terms of all the three resulting aerodynamic forces. When the left-side bleed segment is azimuthally centered, the net side force remains positive but with a further reduction in magnitude, while the drag and lift forces remain practically unchanged. Finally, while the top-centered bleed segment reverses the net side force sign back to negative, the bottom-centered segment bleed does not effect much difference relative to the base flow. When assessing the bleed control effect on the moments, it can be expected that the major changes seen in the side force Fr/C.sub.S would translate in the corresponding dominant changes in the yaw moment C.sub.Y. Indeed, there may be negligible change in pitch C.sub.M, while the most pronounced change is in the yaw moment. All the left-centered bleed control tilts the yaw moment to the net negative, while the opposite sense and a similar to the magnitude in pitch is created by the top-right centered bleed control.
[0086] Evolution of the flow fields for the two presumably most interesting control cases that provide the opposing net side forces and pitching moments can be examined, where the flow fields in all the five measurement planes can be shown for both the segmented bleed control over the top quarter of the forebody on the right side and on the left side. In addition, the base flow field evolution, having the solid forebody, is included for reference. Besides illustrating how the surface vorticity rolls into two forebody vortices in the base flow, along with the cross section at x/D=2.2 indicates a nearly perfect symmetry in the initial formation of both vortices. By x/D=4, vortices mainly grow in size and become displaced away from the surface, also indicating a possible onset of asymmetry. By the next measurement plane, the vortices become notably elongated and lose some coherencea possible indication of the increased unsteadiness. Finally, it becomes clear at the last measurement plane that the CW vortex separated off the body first, while the CCW vortex lags in the detachment. Another notable feature is that as soon as the primary vortex cores become sufficiently displaced off the surface, the continuing supply of the surface vorticity along the cylindrical body from below rolls into a secondary vortex pair. As suggested by the difference in the net side force magnitudes (of opposing signs), it is expected that the flow field evolution for the top-right and top-left segmented bleed control would not be perfectly antisymmetric. Still, the dominant features are clearly antisymmetric. Right at the axial termination of the bleed control the vortex on the side of the activated bleed grows disproportionally and by x/D=2.2, it already begins to separate off the surface, while its counterpart's growth appears even inhibited relative to the base flow. The flow fields remain nearly antisymmetric even at the next measurement plane, where the already detached vortices still remain within the measured field of view. It is interesting to note that both counterparts of the detached vortices shift azimuthally along the top surface across the vertical plane of symmetry and affect the initial rollup of the secondary vortices underneath the detached primary ones. By the next measurement plane at x/D=6, the second primary vortex also becomes detached in either case, while their counterparts are already outside of the measurement field of view. Some discrepancy in the overall anti-symmetry of the flow is seen in the pair of secondary vortex pairs that form along the body surface. However, the main feature of the azimuthal tilt of the remaining surface-bound vortices is seen in either case. As it was observed for the primary vortex detachment, the surface-bound vortex pair tilts azimuthally to the side of preceding vortex detachment. By the last measurement plane, there is another strong switch in the vertical directionality of the wake above the body, having a stark contrast to must a weak disruption in the wake symmetry observed in the corresponding base flow. Aside from the vortical composition of the flow, there is additional interest in assessing the level of the flow unsteadiness, with and without the flow control. For that purpose, the in-plane contribution to the turbulent kinetic energy TKE. Overall, relatively low levels of the velocity fluctuations are measured in the base flow upstream planes, indicating a fairly stable initial rollup of the forebody vortices. While the vortical field at the next plane at x/D=4 only hinted to the onset of asymmetry, the corresponding TKE field clearly indicates the onset of unsteadiness on the CCW side of the vortex pair. The increasing asymmetry in the TKE field is further seen at the next measurement plane, showing a strong peak on the side of the formation of the secondary CCW vortex. As the CW vortex becomes detached and displaced off the body in the last measurement plane, its TKE signature weakens, compared to its still-attached CCW pair. Furthermore, continuing growth of the secondary CCW vortex indicates its formation coupling to the high unsteadiness. It is interesting to note that the secondary formation of the CW vortex actually lags its CCW pair, but appears to form with increased unsteadiness as well. The notable increase in the secondary vortex pair unsteadiness compared to their primary pair counterparts is attributed to their formation within the wake of the cylinder. As previously noted, anti-symmetry in analysis of the flow fields controlled by either the top right or top left 90 bleed segments, is even further supported by comparing the TKE fields over the first three measurement planes. Not surprisingly, the braid of vorticity that feeds into the vortex on the side of the active bleed segment exhibits the elevated TKE. Past the initial vortex detachment from the surface, the remaining flow evolution becomes greatly affected by the surrounding wake, hence rendering the TKE distribution (just as the whole flow field) not exactly anti-symmetric, although the main features remain anti-symmetric throughout the measured planes. Thus, the first detached vortex remains marked by the elongated braid of increased TKE, while the tilted surface-bound vortex remains fairly stable. The overall unsteadiness seems to increase thereafter, after the second vortex detachment, and in particular for the vortices measured in the farthest downstream plane, which is again attributed to the increasing coupling to the growing wake of the cylindrical body, as the vortices progress downstream.
[0087] While the previous section focused on the most efficient flow control configurations for creating a net side force of either direction (and the accompanying yaw moments), the general assessment of the net effects of all eight bleed configurations, points to another configuration that can potentially counter the pre-existing asymmetry in the flow and act to promote the flow symmetry and minimize the resulting net side force. Out of all the testing configurations, the one with the fully azimuthally open bleed over the forebody was the most successful with that respect. Hence, the remaining sections of this paper will focus on the detailed analysis of the representatives of the bleed control configurations that promote the flow symmetry (full azimuthal bleed) and asymmetry (top-right 90 bleed segment), in addition to the base flow, which is used as a reference case.
[0088] Since the resulting flow fields pertinent to the base and the top-right bleed segment to contrast the characteristic three configurations here, only the flow fields in the most upstream and downstream planes for the base flow and the bleed control by the full and 90 segmented azimuthal bleed. Analogous to the flow fields, color raster plots of the mean streamwise vorticity overlaid with velocity vectors are shown at x/D=1.6 and 8.4. Clearly differently-evolving flow fields are seen immediately, in the most upstream plane, among all the three considered configurations. While the promoted asymmetry by the sectional bleed control relative to the base flow was noted earlier, it is seen here that the full azimuthal bleed induces a rather dramatic change in the vortex evolution over the forebody, when compared to either of the other two cases. The full azimuthal bleed prematurely and symmetrically displaces both forebody vortices away from the surface, having the bleed contribution clearly marked by the elongated braids of vorticity connected to the primary vortices. Another important distinction is that the vorticity feed off the surface becomes disrupted, and the primary vortices become weaker, having reduced circulation. The altered topology of the primary vortex pair formation over the forebody is certainly carried through their ultimate detachment, al. It is interesting that at the most downstream measurement plane (x/D=8.4), despite the significant upstream differences, the base flow vortical composition and that in the presence of full azimuthal bleed become reasonably similar, although the circumferential bleed improves the overall wake symmetry, as seen in comparison of the wake flow fields above the body. The similar vortical composition of these secondary vortex pairs is attributed to the fact that the forebody bleed directly affects only the formation of the primary vortex pair and once this pair is detached, the remnants of the forebody flow control can affect the formation of the secondary vortex pair only through their interaction with the altered wake, which is clearly only a secondary effect compared to the secondary vortex pair rollup around the downstream cylindrical body. Interestingly, when the bleed is intentionally asymmetric (i.e., restricted only to the right segment of the forebody), not only that the CCW vortex on the bleed side becomes displaced away from the surface even over the forebody, but the profound change that this asymmetry induced in the upstream wake propagates all the way to the downstream end of the body. Not only that the formation of the secondary vortex pair does not restore to the process seen in the base flow, but both the vortical composition and the whole wake remains biased, which certainly contributes to the net side force attained in this control case, which is a cumulative effect of the whole flow field along the body.
[0089] Initial comparison between the three characteristic flow fields is followed by the more detailed comparisons of the mean velocity distributions along the central vertical plane above the body. Such velocity profiles are extracted for all five measurement planes and for both the mean horizontal Uy and vertical Uz velocity components. Distributions of the horizonal velocity components are a good proxy for the flow symmetry assessment. Thus, it is seen in the base flow that the flow remains nearly perfectly symmetric in the first three measurement planes, as the horizontal velocity component remains virtually zero across the full vertical distribution. While both other velocity distributions return to zero at the upper end of the measurement domain, the symmetry disruption is clear towards the surface, where both distributions change signs, indicating a presence of the rotational motion in the mean. In accord with the prior analysis, these profiles indicate that the base flow develops asymmetry only at or after the initial primary vortex detachment. The overall improvement in the flow symmetry is suggested by the corresponding profiles for the full azimuthal bleed control, although there is some disruption of the initial symmetry already at the third measurement plane. Still, both of the most downstream planes appear to have the horizontal velocity excursions suppressed relative to the base flow. Finally, it is not surprising that analogous horizontal velocity profiles for the 90 top-right segment bleed are dramatically different than either of the other two cases. As the flow vortical composition becomes immediately biased, this is also reflected in the velocity distributions that are initially predominantly biased such that the flow is towards the displaced vortex, having a positive horizontal velocity component. The switch in the vortex detachment from the third to the forth, and then again to the fifth plane is reflected in the switch of the horizontal velocity bias in, where the velocity profiles at x/D=4 and 8.4 are biased to the positive, while the profile at x/D=6 is biased to the negative horizontal velocity. Just as the central horizontal velocity component is shown to be a good indicator of the flow symmetry, and consequently a good predictor for the generation of the aerodynamic side force, the accompanying vertical velocity distribution suggests how the applied flow control may affect the bulk wake evolution, towards or away from the body. The two clear trends are seen in the vertical velocity profiles of the base flow. First, although the near surface negative velocity points to the prevailing flow component towards the surface, it initially strengthens through the first three planes, and then relaxes. This is attributed to the initial vortex pair strengthening (in circulation) as they continuously feed off the surface vorticity until they begin to displace away from the surface and their contribution to the near surface vertical velocity component begins to weaken. Secondly, as expected, a clear opening of the wake away from the surface is seen in the progressive stretching of the velocity profile with distance, while the velocity sign points to the direction away from the body. As seen in the corresponding profiles for the full azimuthal bleed case, only a minor modification of the vertical velocity profile distributions is attained relative to the base flow case, which suggests that only a secondary modification of the bulk wake is attained by the full azimuthal control. Finally, just as in the case of the comparison of the horizontal velocity component, the vertical velocity distributions for the 90 azimuthal segment bleed is substantially different than in the other two cases. The local extent of the flow towards the surface becomes significantly suppressed in this case over the first two planes, followed by its complete suppression. Furthermore, the velocity sign switch is followed by a swift increase in the vertical velocity component away from the body.
[0090] Spanwise extent of the wake above the body is analyzed in, where both the mean horizontal Uy and vertical Uz velocity profiles are shown as waterfall plots away from the body for the same three characteristic bleed configurations discussed. All the data are shown at x/D=2.2, which is the most downstream plane at which the full extent of the wake is captured within the measured field of view. Horizontal velocity component in the base flow indicates a fairly antisymmetric perturbation of this velocity due to the presence of forebody vortices, with the velocity slowly relaxing back to zero only at the outskirts of the measurement domain, as marked by the dashed lines. The corresponding vertical velocity distribution exhibits a typical wake structure that, instead to zero, relaxes back to the in-plane vertical velocity projection of the free stream, away from the center. A narrow domain of the reversed flow, towards the body, is marked by the dashed lines. As the full azimuthal bleed is activated, the spatial evolution of the mean horizontal velocity remains antisymmetric, but having attenuated excursions relative to the base flow. This attenuation also results in earlier relaxation of this velocity back to zero in the spanwise extent. Although the vertical component of the velocity excursions becomes also attenuated, the bleed control also spreads the reversed flow region away from the body in either direction. As it can be expected, the asymmetric application of the segmented bleed (90 segment at the top right), biases the flow fields, which is particularly seen in the horizontal velocity component. It is interesting that the disturbance to the horizontal velocity component is effected on the side opposite to the bleed actuation. While the horizontal velocity relaxes faster on the bleed side (marked by the dashed line), it practically does not fully relax up to the edge of the measurement domain on the opposite side. This can be attributed to the promoted roll-up and tilting of the vortex on the side opposite to the bleed actuation. Finally, in comparison of the vertical velocity component distribution relative to the base flow, it seems that the reversed region of the wake does not significantly change, except of the whole wake vectoring towards the unactuated side, only not as pronounced as it was the case of the horizontal velocity component.
[0091] Another characterization of the wake evolution through the five measurement planes is based on evolutions of the wake minimum streamwise velocity Uz distributions across the wake in the vertical and horizontal directions, for the three characteristics. At the first measurement plane immediately downstream from the forebody, no significant difference is measured among the three considered cases. Besides a slight increase in the recirculating extent of the near wake (, a typical downstream evolution of the wake is reflected in the increasing minimum velocity. In addition, the minimum velocity remains closely aligned with the axis of symmetry for all the three cases, indicating that even in the case of induced vortical asymmetry, no significant side-tilt of the wake is effected. Even in the second measurement plane, only a moderate expansion of the distributions observed in the first plane are noted, with no fundamental changes. By the third plane, the aerodynamic bleed begins to substantially alter the natural wake evolution. In the vertical evolution of the minimum velocity, the full azimuthal bleed expands the reversed flow extent relative to the base flow, while the top-right bleed application reduces it. While the other two configurations still nearly preserve the minimum velocity evolution along the axis of symmetry, the asymmetric bleed control clearly offsets the minimum velocity/wake to the side of the bleed activation as the CCW vortex on that side is already shed off the surface. By the next-to-last measurement plane, vortical asymmetry is developed in all the three configurations, which is reflected in all of the vertical distributions. The sharp sideways deflections of the minimum velocity are related to the effect of the wake by the detached vortices, which is clearly the most pronounced in the most vertically-driven wake of the top-right bleed configuration. The corresponding vertical wake evolution indicates a nearly eliminated reversed flow for the top-right bleed. Finally, by the last measurement plane, the full azimuthal bleed case does indicate the least departure from the minimum velocity alignment with the axis of symmetry, while the top-right bleed case induces the widest shifts from one side to the other due to the strong shed vortices, while the vertical wake evolution of the minimum velocity indicates much stronger wake growth for the top-right case compared to the other two.
[0092] In addition to the analysis based on the velocity distributions, an integral analysis of the wake flow fields is also undertaken, primarily with an emphasis on the effects of the flow field structure in the wake on the net side force. To this end, the net horizontal momentum flux and vertical momentum deficit are assessed across a control volume depicted by overlaid dashed lines in each of the flow fields. The evolution of the fluxes is assessed by varying the position of the top edge of the control volume (i.e., vertical expansion of the control volume) that captures the effects of the three-dimensional vortical flow structures within the wake on the fluxes. In this analysis, bottom edge of the control volume is taken to be sufficiently far below the model (outside of the field of view) with uniform free stream velocity across it. In the first measurement plane only the top-right bleed leads to horizontal flux imbalance indicating the net flow contribution to the side force even as far upstream as at the tail end of the forebody. Interestingly, the momentum flux switches sign away from the body, a signature of the coherent vortical field switch, resulting in asymptotically small net effect for the sufficiently large control volume extent (z/D3.2). The evolution of the net effect for the top-right bleed at the next plane is only amplified relative to the first plane, having the final net contribution of the same sign. It is interesting to note that by the third plane, both the base flow and full azimuthal bleed begin to develop nonzero contributions to the horizontal flux; however, the net effect still asymptotes to zero far away from the body. Although a significant opposing horizontal flux is measured closer to the body, the net flux effect of the top-right bleed when the control volume extends away from the body is of the same sense as in the first two planes. While the other two configurations still do not indicate a net horizontal flux at the fourth plane, there is a complete switch in the top-right bleed contribution, which reaches the opposing sign contribution at about fourfold magnitude than in the preceding planes. This strong change in the horizontal flux is clearly attributed to the antisymmetric vortical composition of this flow field relative to the three upstream fields. Not surprisingly, another flip in the vortical composition of the flow field in the last measurement plane results in the corresponding flip in the net horizontal flux distribution. Interestingly, only at this last measurement plane, a small net contribution to the horizontal flux is seen for the base flow and for the full azimuthal bleed. Although assessed only over the five flow field snapshots, this analysis indicates that the net horizontal flux that ultimately leads to a net side force is strongly associated with the vortical wake composition, where different segment of the wake can locally impose side forces of opposing signs. Theoretically, if the forebody vortices would be shed in an antisymmetric time-periodic manner (akin to the Karman Vortex Street) along the body, it would be possible to attain a nearly zero net side force on the whole body, while body sections would be exposed to significant switching side forces along its axis. The other suggestion of the present analysis is that the net side force in the weakly asymmetric vortex shedding off the axisymmetric body is predominantly associated with its tail-end wake.
[0093] Another integral measure of the controlled and uncontrolled wake evolution is based on the momentum deficit of the normal component of the freestream flow, which in a 2-D flow over a spanwise cylinder would be associated with drag. Clearly, in the present case the flow is three-dimensional, and the wake grows along the body. As already stated above, the oncoming free stream flow projection from below (outside of the measurement domain) is taken to be uniform, while the momentum flux above the body is based on the momentum variation across the body-normal coordinate z. Thus, momentum deficit distributions with increasing distance away from the body are shown analogous to the flux distributions.
[0094] Regardless of the measured plane or the control configuration, it is clear that no asymptotic deficit is reached even farthest away from the body (z/D3.2) and that, along with the wake growth along the body, each successive downstream wake section exhibits increasing momentum deficit. In addition, each of the distributions of the momentum deficit is strongly affected by the vortex-induced artifacts of the flow; namely, the local peaks of the deficit are associated with the opposing effects that the vortex pair suppresses or even reverses body-normal flow. For instance, the local momentum deficit peak at z/D0.6 in the base flow is associated with the reversed flow induced by the forebody vortex pair. Further away from the body, these local peaks are typically associated with a strong wake sidewise vectoring (that also contributes to the net horizontal momentum change), such as in the base flow at z/D2.1. Overall, not surprisingly, the most momentum deficit through the measured flow fields is associated with the full azimuthal bleed, as suggested by the observed wake evolution. It is interesting to note, though, that since the far wake evolutions, past the initial vortex pair detachment, are fairly similar between the base and the full-azimuthal bleed flows (compare the flow fields), this previous observation is strongly supported by the nearly overlapping distributions.
[0095] An integral measure of the flow control effect on the forebody vortex pair evolution is further sought through the estimates of evolution of each vortex circulation through the five PIV measurement planes. Based on each ensemble-averaged PIV flow field, two primary vortices are first isolated. Once the vortical bounds are set, their total circulation was calculated as an integral measure of the product of the local vorticity component .sub.x and the unit area dA=dydz. The resulting circulation evolution of the forebody vortices in the base flow. As expected, circulations of both the CCW and CW vortices are of the same magnitude up to the third measurement plane. After the primary vortices become detached, its circulation weakens, while the CW circulation still remains higher than its counterpart CCW circulation. However, from the standpoint of the force exerted on the body, detached vortex influence diminishes with distance, and circulation of the secondary vortices becomes more important than the primary ones, once both of the primary vortices are detached.
[0096] Changes in circulation of the primary vortices due to the full and segmented azimuthal bleed configurations, relative to that of the base flow vortices. In either case, the base flow circulations are shown in grey as a reference. The overall effect of the full azimuthal bleed is seen in the closer balancing of the circulation magnitudes of both vortices, which is another indication of the enhanced symmetry of the flow. Although the circulation magnitudes at the first two measurement planes indicate that the total CCW and CW circulation magnitudes are increased relative to the base flow, it should be noted that these total circulations include a rather strong vorticity braids that feed off the bleed ports and are connected to the vortices and therefore become included into the vortical structure detected on either side. It is clear that without the braids, the vortices themselves do not necessarily carry more circulation than their counterparts in the base flow. This is further illuminated in the downstream evolution when the direct feed from the bleed becomes disconnected, and the results at the two most downstream planes indicate that the circulation of the controlled vortices is lower than that of the base flow. Circulation evolution is markedly different when the bleed is applied only over the 90 top-right azimuthal segment. Initially, a small increase in circulation is measured over the CCW vortex on the side of the active bleed, again enhanced by the promoted braid of vorticity from the bleed surface, while a minute reduction in the CW circulation on the opposite side might be associated with some enhanced spreading and tilting of the CW vortex pair on the opposite side. By the third measurement plane at x/D=4, the CCW vortex is already detached and its circulation remains nearly constant to that at x/D=2.2, while the CCW vortex remains attached and builds up circulation. By x/D=6, the CCW vortex is mostly gone from the field of view and its circulation is consequently low, while the CW vortex becomes detached and subsequently its circulation does not get notably changed from the previous one at x/D=4. By the last measurement plane, the CCW vortex is completely out of the field of view, while only a small remnant of the CW vortex within the field of view amounts to the low circulation magnitude. Overall, a strong imbalance in the primary vortex pair circulation around the body clearly induces a net side force, as the forebody vortices have a major contribution to the net side force.
[0097] As already noted, besides the primary forebody vortices, a secondary vortex pair forms over the body, once the primary vortex pair detaches off the surface. From the vortex contributions to the net changes in side forces, it is desired to follow and consider the vortices that are bound or close to the surface, i.e., the forebody vortices up to their lift off, and the secondary vortex pair downstream from that point. To place emphasis on the major vortical contributions to the aerodynamic forces (and moments), isolate only the vortical structures around the body at all five measurement planes, for the base and the flow controlled by the full azimuthal and 90 top-right segmented bleed. In addition, surface-tangential mean velocity profiles with respect to the wall-normal direction that pass through either vortex (marked in dashed lines) are overlaid on the plots. As such dashed lines also represent the azimuthal orientation of the vortices, it is seen that the azimuthal orientation of the forebody vortex pair remains fairly stable, with the weakening of the velocity perturbation near the surface with the vortex stretching and with their approach to detachment. Also, perturbations of both vortices remain antisymmetric and clearly indicate the equal but opposing effect. By x/D=6, both primary vortices appear fairly diffused in the average, attributed to their increased unsteadiness. The peak velocity perturbations are displaced away from the surface, while the near-surface perturbation weakens. Finally, the vortical velocity perturbation near the surface is notably stronger on the CCW vortex side at the last measurement plane, with both vortices remaining spaced much more apart azimuthally than for the forebody vortices. Although structurally fairly similar to the base flow, there are secondary modifications of the forebody vortical perturbation of the near-surface flow when the full azimuthal bleed is applied. Along with the extended outboard reach of the perturbations, their excursions near the surface lessen, in addition to the slightly increased azimuthal spacing of the vortices. There are even more similarities of the vortical perturbations associated with the downstream fields, as the direct flow control effects terminated far upstream from these axial locations, and the convecting changes of the wake from upstream are rather mild. Lastly, as expected, velocity perturbations along the surface become notably asymmetric for the forebody vortex evolution under the segmented 90 top-right bleed. Initially, strong growth of the vortex on the bleed side induces the accordingly strong increase in the velocity along the surface on its side, while the uncontrolled-side vortex contribution does not appear much different from that of the base flow. However, further disbalance in the vortex growth affects not only the bleed-side perturbations, but also that on the opposite side, as the CW vortex begins to roll and tilt at x/D=2.2, such that not only the CCW vortex contribution is increased relative to the base flow, but the CW vortex opposing contribution weakens. By the next measurement plane, not only that there is an imbalance in velocities near the surface but the vortex pair becomes tilted to the bleed side, further increasing the uneven net contribution to the aerodynamic side force. This observed tilt completely changes after detachment of the CW primary vortex, when the perturbation balance switches side. Nonetheless, velocity perturbations near the surface seem to be rather small. Lastly, it appears that the phase delay between the secondary vortex formations on the cylinder sides increases by the last measurement plane and the vortex pair azimuthal switch to the opposite side appears incomplete.
[0098] After the initial study of the bleed flow control effectiveness presented above, the bleed configuration about the forebody was redesigned, keeping the same porosity. To check for the resulting flow control effectiveness, a sweep in the body pitch angles was performed first, ranging from 35<<55. The two configurations inducing the largest excursions in the side force are first tested at each pitch anglea 90 azimuthal segment positioned at the top right and left, i.e., directly affecting only the CCW and CW vortex, respectively. In principle, both the effect on drag and lift should not depend on the side at which the bleed control is applied. While the lift data are practically overlapped for the right-and left-side flow control, there is a small by consistent offset in the drag results. As for the most interesting aerodynamic effectthe side force, progressively increasing side force is achieved with the pitch angle, with the sign defined by the side of the bleed control application. The difference relative to the results shown is that the side force coefficients reach or even exceed C.sub.S=4, which nearly match the lift coefficient at higher angles of attack. Some discrepancy in the attained magnitudes for the opposite-side actuation can be attributed to the non-zero base flow side force, i.e., to the fact that the base flow is biased towards the one-side asymmetry (that results in the net positive C.sub.S in this case) in the absence of the flow control. As almost exclusive change in the aerodynamic forces is in the side force direction, the associated moments change is in the yaw (in the fixed coordinate system). Some discrepancy in the pitch moments that result from the bleed control by the top right and top left orientations are the direct consequence of the offset in drag.
[0099] Similar to the earlier analysis of the effects of the two characteristic bleed configurations, the representatives of the bleed control configurations that promote the flow symmetry (full azimuthal bleed) and asymmetry (top-right 90 bleed segment), are also assessed with the revised bleed configuration, with addition of the base flow, as a reference case. Hence, the corresponding evolution of the lift, drag and side force coefficients with angle of attack can be shown for the baseline flow and the bleed configurations, while the corresponding changes in pitch and yaw moments. As expected, both C.sub.D and C.sub.L increase monotonically over most of the range of a, where the rate of increase in C.sub.L begins to diminish past >45 as the forebody vortices begin to lift off the cylinder surface and the flow from the windward separates on its leeward surface. The side force coefficient C.sub.S in the base flow is nearly zero as long as there is no major asymmetry in the evolution of the forebody vortices, but when the symmetry breaks for >45, there is an increase in the magnitude of the net side force. Since for this forebody the CCW vortex is the first to become detached from the surface, the net side force is negative. The symmetry-enforcing full circumferential azimuthal bleed is therefore expected to alleviate some of the net side force of the base flow, which is particularly evident at these high inclination angles. Along with the reduction in the side force, this bleed actuation also slightly reduces both the lift and drag at these high a. Finally, as expected, the strong flow asymmetry induced by the segmented bleed is accompanied by a strong net side force whose magnitude increases with a, while the lift and drag remain nearly unchanged relative to the base flow. The benefits of the full azimuthal bleed are presumably even better expressed in the net yaw moment, where the increasing yaw moment of the base flow for higher elevation angles becomes diminished back to zero under the full azimuthal bleed control.
Dynamic Azimuthal Forebody Bleed for Regulation of the Side Forces
[0100] After the preliminary studies on utilization of the forebody aerodynamic bleed on the model's aerodynamic loads, were concluded, an active control device was devised that can be controlled remotely and tailor the bleed control in real time. For that purpose, a miniature rotary motor is integrated into the model having an azimuthal step resolution of 0.17. The motor shaft is extended such to connects to an internal conical segment with partition azimuthal opening (hereinafter referred to as a valve) that is inserted in the forebody and it is positioned under the outer shell that is populated with arrays of bleed ports. Two outer shells for the two bleed configurations are designedthe coarser (1) and the finer (0.5) one. In addition, two valve configurations are tested as well, one with the segmented path, which forces both the bleed in and bleed out flows over that single side of the outer shell surface, and the flow-through path that nominally forces a directional bleed path, from the higher-pressure side surface to the opposite. It should be noted that, opposite to the prior bleed configuration that comprised of the outer shell, there is no fully closed configuration (no bleed), as the bleed pattern on the outer shell is fully distributed across the azimuth and there is a segment of the that surface that is always open for bleed, depending on the inner shell valve orientation.
[0101] A typical change in the aerodynamic loads in the body-axial and side directions, C.sub.A and C.sub.S, respectively, for all the four combinations of the forebodies and valves. In each set of experiments, the model is set at the pitch angle =45 and the valve is initially positioned at =0, i.e., at the bottom on the windward side. After the set of measurements is taken, the valve orientation is incremented in q and the next set of measurements is acquired. This process is repeated until the valve central reference point traverses full 360. These full azimuthal changes in these two aerodynamic loads can be shown. While the dominant change is in the side force magnitude, there is typically smaller, pseudo-periodic variation in the axial force as well, approximately alternating every quadrant of the valve motion, having the minima lined with the starting valve orientation. The largest effect is certainly seen in the side force, in accord with the initial discussion of the aerodynamic bleed effect above. When assessing the differences between the coarse (1) and refined (0.5) bleed distributions, for a single path valve, there is somewhat more variations for a finer distribution, although, in principle, aerodynamic behavior is quite similar. This is even more pronounced for the dual-path valve, where the primary modification is expressed through the extended leveling of the peak positive and negative force azimuthal extents for the finer bleed distribution. Lastly, the most pronounced difference is seen in comparison between the single and dual-path inner shells. While the peak magnitudes of the forces do not differ significantly, the period of the side force doubles for the dual-path valve, as reflected in the four angles about which the force changes sign, relative to the single-path configuration that has two such locations.
[0102] These zero-crossings are particularly important from the standpoint of flow control, as they define regions over which the bi-directional control should be centered about. One such region can be highlighted, which is centered about the zero crossing at .sub.o=180.5. The steep slope dCs/d enables a high sensitivity to the valve angular changes, such that the full effect in either direction can be attained by displacement of only about 14 in either direction about the .sub.o. As an example, the peak positive side force (C.sub.S2.05) is attained at .sub.1=193, while the peak negative force (C.sub.S1.77) corresponds to .sub.2=168.
[0103] Another important parameter for a quick control application is the flow response to the control input, expressed through the altered aerodynamic loads. To test how fast the flow responds to the bleed control, the valve is commanded to periodically switch between the two end states, yielding the maximum force in either direction, i.e., between =168 and 193. Initially, the valve switched between the two states after staying at each end state for 5 s, i.e., having the period of 0.1 Hz. Two switches associated between the two extremes, having the valve position in blue and the corresponding side force signal in red. In addition, the two transitions are emphasized by arrows. Clearly, after maintaining a constant side force during each 5 s half-period, the step-on change in the valve angular position is closely followed by the force switch without any discernable delay. It is noted, though that the valve commanded step-on motion is realized as slightly underdamped, having a single overshoot-undershoot before settling at the desired angular position. This initial valve-motion response is also reflected in the side-force response, which becomes more pronounced with an increase in the switch frequency. While the force response for f=0.25 Hz does not significantly differ from that at f=0.1 Hz, increasing the frequency to f=0.5 Hz, indicates that the force response during the 1 s stays at each extreme yields the force component having a clear resonant system frequency of about 7 Hz. This becomes pronounced even stronger when the frequency is further quadrupled and the stay time corresponds to not even the full two periods of the natural frequency of the system.
[0104] Lastly, a sample of the controlled side force without the feedback control loop can be shown, where the pitch angle of the model is varied between =30 and 60 in increment of 5. At each of these pitch angles, the single-path valve is adjusted about the top zero-crossing position such that the resulting controlled side force is kept close to zero. At each of the zero-force states, the corresponding angular position of the valve is recorded as well. The resulting controlled states indicate the controlled states closely following zero up to =50, while being somewhat relaxed at the two highest pitch orientations. Overall, a close following of the targeted zero force state is realized. While examining the bleed-valve orientations that facilitated such states, another close following of the =180 orientation is observed, with most of the departures being within 3 away from the top orientation.
SUMMARY
[0105] These investigations focused on exploiting segmented porosity over the platform's forebody for not only suppression of asymmetric side forces that arise from flow's receptivity to minor imperfections of the body surface but, more importantly for prescribing desired side forces and yawing moment for bi-directional aerodynamic control.
[0106] It was shown that segmented azimuthal bleed actuation over the forebody can lead to significant changes in the evolution and symmetry of the forebody vortex pair that are accompanied by controllable side forces of prescribed sense and magnitude. Bleed control can be used to overrides naturally evolving forebody flow asymmetries and can either amplify the resulting induced loads or suppress them by manipulation of the forebody vortices. This finding is of particular significance considering the receptivity of the base forebody flow to small surface and flow nonuniformities. Depending on the angle of incidence, bleed-induced side force coefficient increments as high as C.sub.S=3.5, depending on the pitch incidence, could be attained where, by comparison, the corresponding vertical (lift) coefficient is C.sub.L4.5.
[0107] The wake flow features associated with the base and controlled realizations of the aerodynamic loads were elucidated using successive PIV measurements that revealed the evolution of streamwise vortical structures along the body. It was shown that weakly asymmetric forebody vortices in the base flow, primarily induce the unbalanced side loads over the downstream end of the body, following the first forebody vortex detachment. Full azimuthal bleed control altered the vortical evolution over the forebody by the equal vortex displacement off the body and thereby severing the off-surface vorticity influx, while maintaining the left-to-right flow symmetry. In contrast, segmented one-sided bleed directly affected the adjacent forebody vortex, inducing an aerodynamic imbalance while the opposite side vortex responded to the wake of the detached vortex by tilting in its direction. The detachment of the primary vortices was followed by successive rollup and interactions of secondary vortices along the cylinder's wake.
[0108] The main implementation of this actuation technology is the development of a unique approach for activation of dynamic, time-dependent segmented bleed actuation that enables tailoring of bleed control in real time. In this configuration, an internal rotary shell underneath a forebody shell with prescribed bleed port pattern prescribed which azimuthal segment of the forebody was open for aerodynamic bleed. The objective of this final stage of the project was to completely bypass any naturally, and unpredictably, evolving state of the flow over the axisymmetric body at high incidence, which would typically result in a net side force (and the yawing moment) of either sign. Instead, the actuation approach was to set to realize side forces of prescribed sense and magnitude over a range of incidence angles including complete suppression of the side force. As part of these concluding investigations two different forebody porosities and two different bleed valves were tested and the interactions of the bleed with the cross flow were characterized through the realized aerodynamic loads and their sensitivity to variations of the azimuthal position of the valve Cs/ as well as the characteristic response time to a step change in the bleed control input. These investigations showed that for one-or two-way bleed configurations, there are either two or four azimuthal bleed orientations about which the side force changes sign demonstrating broad dynamic control authority over of the side forces over a broad range of angles of incidence.
[0109] The disclosed technology can be further understood according to the following clauses:
[0110] Clause 1: A system for controlling aerodynamic loads on an aerostructure, comprising: an outer shell comprising at least one aperture; an inner shell having an aperture therethrough, the aperture extending azimuthally around a portion of the inner shell; and an actuator configured to control a rotational alignment of the inner shell and the outer shell to alter aerodynamic bleed through at least a portion of at least one aperture of the outer shell.
[0111] Clause 2: The system of Clause 1, wherein the actuator comprises a motor configured to axially rotate at least one of the inner shell and the outer shell.
[0112] Clause 3: The system of Clause 2, wherein the outer shell is rotationally fixed to the aerostructure, and wherein the motor is configured to axially rotate the inner shell with respect to the outer shell.
[0113] Clause 4: The system of Clause 1, wherein the outer shell is disposed on an end of the aerostructure.
[0114] Clause 5: The system of Clause 1, wherein at least one aperture of the outer shell is part of an array of apertures azimuthally distributed on the outer shell.
[0115] Clause 6: The system of Clause 5, wherein at least one aperture of the outer shell comprises at least one straight edge, and the aperture of the inner shell comprises at least one straight edge, such that the at least one straight edge of the outer shell is configured to align with the at least one straight edge of the inner shell.
[0116] Clause 7: The system of Clause 6, wherein the array of apertures comprises a first row of apertures and a second row of apertures, wherein at least one aperture of the first row is aligned with at least one aperture of the second row substantially parallel to a longitudinal axis of the aerostructure.
[0117] Clause 8: The system of Clause 1, wherein the actuator is configured to cause the inner shell to transition between a first rotational alignment with the outer shell in which the inner shell blocks aerodynamic bleed through at least a portion of at least one aperture of the outer shell and a second rotational alignment with the outer shell in which the inner shell allows aerodynamic bleed through the at least a portion of the at least one aperture of the outer shell.
[0118] Clause 9: The system of Clause 1, wherein the actuator is configured to control the rotational alignment of the inner shell and the outer shell based at least in part on a side force component from a fluid flow on the aerostructure.
[0119] Clause 10: The system of Clause 1, wherein at least one aperture of the outer shell is part of a plurality of apertures spanning a top azimuthal portion of the outer shell.
[0120] Clause 11: The system of Clause 10, wherein the inner shell is part of a plurality of inner shells comprising a first inner shell and a second inner shell, wherein the first inner shell and the second inner shell overlap thereby forming a relative azimuthal aperture.
[0121] Clause 12: The system of Clause 1, wherein the actuator is selected from a group consisting of: a rotary motor, an inflatable actuator, piezoelectric plates, and an electrostatic actuator.
[0122] Clause 13: The system of Clause 1, wherein the actuator is configured to automatically control the rotational alignment of the inner shell and the outer shell based at least in part on a desired steering direction of the aerostructure.
[0123] Clause 14: An aerostructure comprising: a substantially cylindrical body; and a forebody section comprising: an outer shell comprising an array of apertures; and an inner shell having one or more apertures therethrough, the one or more apertures extending azimuthally around a portion of the inner shell; and an actuator configured to control a rotational alignment of the inner shell and outer shell, such that the rotational alignment is configured to alter aerodynamic bleed through at least a portion of at least one aperture of the outer shell.
[0124] Clause 15: The aerostructure of Clause 14, wherein the forebody section further comprises a gap disposed between the outer shell and the inner shell.
[0125] Clause 16: The aerostructure of Clause 14, wherein the outer shell and the inner shell are hollow, such that the forebody section further comprises a compartment disposed at least partially within the inner shell.
[0126] Clause 17: The aerostructure of Clause 14, wherein the inner shell is part of a plurality of inner shells comprising a first inner shell and a second inner shell, wherein the first inner shell and the second inner shell overlap thereby forming a relative azimuthal aperture.
[0127] Clause 18: A method of controlling aerodynamic loads on an aerostructure, the aerostructure comprising an outer shell, the outer shell comprising at least one aperture, and an inner shell having an aperture therethrough, the aperture extending azimuthally around a portion of the inner shell, the method comprising: placing the aerostructure in motion, such that the aerostructure experiences aerodynamic loads; and altering a rotational alignment of the inner shell with respect to the outer shell to alter aerodynamic bleed through at least one aperture of the outer shell.
[0128] Clause 19: The method of Clause 18, wherein altering the rotational alignment of the inner shell with respect to the outer shell comprises: substantially nullifying effects of side forces and yawing moments via a first rotational alignment adjustment; determining a desired directional change of the aerostructure; and making a second rotational alignment adjustment based at least in part on the desired directional change of the aerostructure.
[0129] Clause 20: The method of Clause 18, wherein altering the rotational alignment of the inner shell with respect to the outer shell to alter aerodynamic bleed through at least one aperture of the outer shell comprises altering an azimuthal position of the aperture of the inner shell.
[0130] It is to be understood that the embodiments and claims disclosed herein are not limited in their application to the details of construction and arrangement of the components set forth in the description and illustrated in the drawings. Rather, the description and the drawings provide examples of the embodiments envisioned. The embodiments and claims disclosed herein are further capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purposes of description and should not be regarded as limiting the claims.
[0131] Accordingly, those skilled in the art will appreciate that the conception upon which the application and claims are based may be readily utilized as a basis for the design of other structures, methods, and systems for carrying out the several purposes of the embodiments and claims presented in this application. It is important, therefore, that the claims be regarded as including such equivalent constructions.
[0132] Furthermore, the purpose of the foregoing Abstract is to enable the United States Patent and Trademark Office and the public generally, and especially including the practitioners in the art who are not familiar with patent and legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the claims of the application, nor is it intended to be limiting to the scope of the claims in any way.