Abstract
A fluid dynamic body having a trailing edge with a pattern formed thereon, the pattern can include a plurality of smoothly surfaced adjacent members with respective interstices therebetween, wherein at least one of the interstices completely contains a porous barrier. In some embodiments, the porous barrier can obstruct fluid flow through the respective interstice between a first surface of the fluid dynamic body on a first side of the trailing edge and a second surface of the fluid dynamic body on a second side of the trailing edge. This helps to reduce noise produced at the trailing edge. In some embodiments, the fluid dynamic body is a wind turbine blade or an air-engine blade.
Claims
1. A fluid dynamic body comprising a trailing edge with a pattern formed thereon, said pattern comprising a plurality of smoothly surfaced adjacent members with respective interstices therebetween, wherein at least one of said interstices completely contains a porous barrier fully occupying a width of said respective interstice and obstructing fluid flow through said respective interstice between a first surface of said fluid dynamic body on a first side of said trailing edge and a second surface of said fluid dynamic body on a second side of said trailing edge.
2. The fluid dynamic body of claim 1 wherein said porous barrier has a solidity ratio of from 4% to 96%.
3. The fluid dynamic body of claim 1 wherein said porous barrier occupies the entire volume of said respective interstice.
4. The fluid dynamic body of claim 1 wherein said porous barrier comprises a foam.
5. The fluid dynamic body of claim 1 wherein said porous barrier is made of foamed metal.
6. The fluid dynamic body of claim 1 wherein said porous barrier comprises a plurality of perforations.
7. The fluid dynamic body of claim 1 wherein said porous barrier comprises a plurality of bristles.
8. The fluid dynamic body of claim 1 wherein said pattern on said trailing edge is part of a contour of an airfoil or hydrofoil of said fluid dynamic body.
9. The fluid dynamic body of claim 1 wherein said porous barrier is part of a contour of an airfoil or hydrofoil of said fluid dynamic body.
10. The fluid dynamic body of claim 1 wherein said pattern is periodic.
11. The fluid dynamic body of claim 10 wherein said pattern comprises a serration.
12. The fluid dynamic body of claim 11 wherein said serration is a sawtooth.
13. The fluid dynamic body of claim 11 wherein said serration is rectangular.
14. The fluid dynamic body of claim 1 wherein said fluid dynamic body is a wind turbine blade or an air-engine blade.
15. The fluid dynamic body of claim 1 wherein said porous barrier has a flow resistivity of from 10.sup.2 Pa.Math.s/m.sup.2 to 10.sup.5 Pa.Math.s/m.sup.2.
16. The fluid dynamic body of claim 15 wherein said flow resistivity is from 10.sup.3 Pa.Math.s/m.sup.2 to 10.sup.4 Pa.Math.s/m.sup.2.
17. The fluid dynamic body of claim 1 wherein said porous barrier is formed of woollen felt, synthetic foam, synthetic felt, elastomer foam, porous glass granulate, melamine resin-foam, metal-foam, PUR-foam or PU-foam.
18. The fluid dynamic body of claim 1 wherein, said porous barrier is formed of a material having a flow resistivity of 700, 1000, 1500, 3600, 4000, 4400, 8200, 9800, 16500, 23100, 40100, 112100, 130200, 164800, 316500, or 506400 Pa.Math.s/m.sup.2.
19. A method of manufacturing a fluid dynamic body, said method comprising: a. forming a pattern on a trailing edge of said fluid dynamic body, wherein said pattern comprises a plurality of smoothly surfaced adjacent members with respective interstices therebetween; b. providing at least one of said interstices with a porous barrier completely contained therein, said porous barrier obstructing fluid flow through said respective interstice between a first surface of said fluid dynamic body on a first side of said trailing edge and a second surface of said fluid dynamic body on a second side of said trailing edge, wherein said porous barrier fully occupies a width of said respective interstice.
20. The method of claim 19, wherein said pattern is formed integrally with a contour of an airfoil or hydrofoil of said fluid dynamic body.
21. The method of claim 19, wherein providing at least one of said interstices with a porous barrier comprises forming said porous barrier integrally with a contour of an airfoil or hydrofoil of the fluid dynamic body.
22. The method of claim 19, wherein forming said pattern on the trailing edge and providing at least one of said interstices with a porous barrier are performed simultaneously, and wherein providing at least one of said interstices with a porous barrier comprises forming said porous barrier integrally with said plurality of smoothly surfaced adjacent members.
23. The method of claim 19, wherein providing at least one of said interstices with a porous barrier comprises micro-drilling said barrier with perforations.
Description
BRIEF DESCRIPTION OF THE DRAWING
[0031] FIG. 1 shows a perspective view of a fluid dynamic body;
[0032] FIG. 2 shows a cross-sectional view through the fluid dynamic body of FIG. 1 in a plane containing the line A-A′ shown in FIG. 1;
[0033] FIG. 3 shows a fluid dynamic body having a trailing edge with a first pattern formed thereon;
[0034] FIG. 4 shows a fluid dynamic body having a trailing edge with a second pattern formed thereon;
[0035] FIG. 5 shows a fluid dynamic body having a trailing edge with a third pattern formed thereon;
[0036] FIG. 6 shows a first embodiment of a fluid dynamic body;
[0037] FIG. 7 shows a second embodiment of a fluid dynamic body;
[0038] FIG. 8 shows a third embodiment of a fluid dynamic body;
[0039] FIG. 9 shows a fourth embodiment of a fluid dynamic body;
[0040] FIG. 10 is a graph showing the noise reduction achieved with the first embodiment shown in FIG. 6; and
[0041] FIG. 11 is a graph showing the effect of the flow resistivity of various porous materials on noise reduction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] Referring firstly to FIG. 1, fluid dynamic body 1 has leading edge 8 and trailing edge 10. Fluid dynamic body 1 has first surface 101 on a first side of trailing edge 10 and second surface 102 on a second side of trailing edge 10, as well as side surface 103. Arrow D represents a major direction of fluid flow over the opposing surfaces 101, 102 of fluid dynamic body 1 and off trailing edge 10 thereof. The opposing surfaces 101, 102 of fluid dynamic body 1 can be more clearly seen in FIG. 2, which is a cross-sectional view through fluid dynamic body 1 of FIG. 1 in a plane containing the line labelled A-A′ in FIG. 1. FIG. 2 also shows that in this instance, the opposing surfaces 101, 102 form part of contour 104 of an airfoil of fluid dynamic body 1.
[0043] FIG. 3, shows fluid dynamic body 1a having trailing edge 10 with first pattern 12a formed thereon. Pattern 12a comprises a plurality of adjacent members 14a with respective interstices 16a therebetween. In this case, the plurality of adjacent members 14a are sharply pointed like triangles with respective apices 24a aligned on trailing edge 10. The plurality of adjacent members 14a also abut each other at their respective roots, so that the interstices 16a therebetween each commence in line 20a where the root of one such member 14a abuts another. Line 20a provides a location where vortices can be shed from fluid dynamic body 1a by fluid flowing parallel to line 20a between upper surface 101a and lower surface 102a of fluid dynamic body 1a in the directions indicated in FIG. 3 by the double-headed arrow labelled E, thereby generating noise.
[0044] FIG. 4 shows fluid dynamic body 1b having trailing edge 10 with second pattern 12b formed thereon. Pattern 12b comprises a plurality of adjacent members 14b with respective interstices 16b therebetween. In this case, the plurality of adjacent members 14b are blunt and terminate in line 24b coincident with trailing edge 10 of fluid dynamic body 1b. On the other hand, the plurality of adjacent members 14b still abut each other at their respective roots, so that interstices 16b therebetween each commence in line 20b where the root of one such member 14b abuts another. As in FIG. 3, line 20b provides a location where vortices can be shed from the fluid dynamic body 1b by fluid flowing parallel to line 20b between upper surface 101b and lower surface 102b of fluid dynamic body 1b in the directions indicated in FIG. 4 by a double-headed arrow labelled E′, thereby generating noise.
[0045] FIG. 5 shows different fluid dynamic body 1c having trailing edge 10 with third pattern 12c formed thereon. Pattern 12c comprises a plurality of adjacent members 14c with respective interstices 16ctherebetween. The plurality of adjacent members 14c are sharply pointed like triangles with respective apices 24c aligned on trailing edge 10. In this case, the plurality of adjacent members 14c do not abut each other, but are spaced apart from each other, so that interstices 16c therebetween each commence with face 26c facing trailing edge 10 of fluid dynamic body 1c, face 26c separating one such member 14c from another. Faces 26c provide locations where vortices can be shed from fluid dynamic body 1c by fluid flowing parallel to faces 26c between upper surface 101c and lower surface 102c of fluid dynamic body 1c in the directions indicated in FIG. 5 by a double-headed arrow labelled F, thereby generating noise.
[0046] FIG. 6 shows a first embodiment, wherein fluid dynamic body 1 has trailing edge 10 with pattern 12 formed thereon. Pattern 12 comprises a plurality of adjacent members 14 with respective interstices 16 therebetween. The plurality of adjacent members 14 are sharply pointed like triangles with respective apices 24 aligned on the trailing edge 10. The plurality of adjacent members 14 also abut each other at their respective roots, so that interstices 16 therebetween each commence in line 20 where the root of one such member 14 abuts another. Thus far, this embodiment is similar to fluid dynamic body 1a shown in FIG. 3. It differs from FIG. 3, in at least the fact that each interstice 16 contains porous barrier 18a, which obstructs fluid flow through respective interstice 16 between first surface 101 of fluid dynamic body 1 on a first side of trailing edge 10 and second surface 102 of fluid dynamic body 1 on a second side of trailing edge 10. On the other hand, since barrier 18a is porous, fluid is still able to flow in a major direction indicated in FIG. 6 by arrow D over the opposing surfaces 101, 102 of fluid dynamic body 1 and off trailing edge 10 thereof through interstices 16 between adjacent members 14 of pattern 12, thereby not inhibiting the beneficial noise reducing effects provided by pattern 12 in the first place.
[0047] In this case, porous barrier 18a comprises a foam. More specifically, in this case, it is made of foamed metal, although it could instead be, among other things, a set polymer foam. Either can obstruct fluid from flowing in the directions previously indicated in FIG. 3 by the double-headed arrow labelled E, thereby reducing noise. On the other hand, since barrier 18a is also porous, it can still allow fluid to flow through the interstices 16 in the major direction D.
[0048] FIG. 7 shows an alternative, second embodiment of fluid dynamic body 1. Fluid dynamic body 1 again has a basic geometry similar to that of FIG. 3, but wherein interstices 16 each contain porous barrier 18b comprising a plurality of perforations 80. In the embodiment shown in FIG. 7, barrier 18b is formed of upper plate 181 which follows the contour of upper surface 101 and lower plate 182 which follows the contour of lower surface 102. Both of plates 181, 182 are provided with perforations 80 and the rest of interstice 16 between plates 181, 182 is a void. However, barrier 18b could instead be formed from a solid block integral with fluid dynamic body 1, in which perforations passing all the way through from upper surface 101 to lower surface 102 could be formed by a micro-drilling process. Barrier 18b has the same beneficial noise reducing effects noted above as for barrier 18a, for the same reasons.
[0049] FIG. 8 shows another alternative, third embodiment of fluid dynamic body 1 having a basic geometry similar to that of FIG. 3, but wherein interstices 16 each contain porous barrier 18c comprising a plurality of bristles 82, in the manner of a brush. In this embodiment, bristles 82 are mounted in interstice 16 in a direction substantially parallel to the major direction of fluid flow over the opposing surfaces 101, 102 of fluid dynamic body 1 indicated in FIG. 8 by arrow D. This encourages laminar fluid flow between the plurality of adjacent members 14 in the direction of arrow D and off trailing edge 10 of fluid dynamic body 1, while inhibiting the flow of fluid in the directions previously indicated in FIG. 3 by double-headed arrow E, thereby reducing noise.
[0050] Whereas FIGS. 6, 7 and 8 show embodiments wherein different types of porous barrier 18a, 18b, 18c have been applied to a fluid dynamic body with pattern 12 formed thereon similar to that of FIG. 3, the different types of porous barrier 18a, 18b, 18c could equally well be applied instead to a fluid dynamic body with pattern 12 formed thereon similar to that of FIG. 4 or FIG. 5 or to a fluid dynamic body having another different pattern formed thereon. For example, FIG. 9 shows an alternative, fourth embodiment, wherein fluid dynamic body 1d has trailing edge 10 with pattern 12d formed thereon. Pattern 12d comprises a plurality of adjacent members 14d shaped like tabs or rectangles with respective edges 24d coincident with trailing edge 10 and respective interstices 16d therebetween. The plurality of adjacent members 14d are also spaced apart from each other, so that interstices 16d therebetween each commence with face 26d facing trailing edge 10 of fluid dynamic body 1d, face 26d separating one such member 14d from another. Faces 26d provide locations where vortices could otherwise be shed from the fluid dynamic body 1d by fluid flowing parallel to faces 26d between upper surface 101d and lower surface 102d of fluid dynamic body 1d. However, in order to obstruct such fluid flow, interstices 16d each contain porous barrier 18d comprising a plurality of perforations 80, thereby reducing noise. In the embodiment shown in FIG. 9, barrier 18d is formed of upper plate 181 which follows the contour of upper surface 101d and lower plate 182 which follows the contour of lower surface 102d. Both of plates 181, 182 are provided with perforations 80 and the rest of interstice 16d between plates 181, 182 is a void. However, barrier 18d could instead be formed from a solid block integral with fluid dynamic body 1d, in which perforations passing through from upper surface 101d to lower surface 102d could be formed by a micro-drilling process.
[0051] Moreover, whereas the different types of porous barrier 18a, 18b, 18c, 18d have been shown respectively comprising only a foam (and more specifically a foamed metal), only a plurality of perforations and only a plurality of bristles, yet another different type of porous barrier according to another embodiment and providing similarly beneficial noise reduction effects could also comprise combination of some or all of a foam, such as a foamed metal, a plurality of perforations and a plurality of bristles.
[0052] Moreover, in FIGS. 3, 4, 5 and 9, patterns 12a, 12b, 12c, 12d on trailing edge 10 are shown to be part of contour 104 of an airfoil or hydrofoil of the respective fluid dynamic bodies 1a, 1b, 1c, 1d and in FIGS. 6, 7, 8 and 9. Porous barriers 18a, 18b, 18c, 18d are also shown to be part of contour 104 of an airfoil or hydrofoil of the respective fluid dynamic body 1, 1d. However, the pattern does not have to be continuous or integral with either one or both of the upper and lower surfaces of the fluid dynamic body, but can instead have a cross-section which is only partially or, in some embodiments, not at all continuous with the upper and lower surfaces of the fluid dynamic body, and can also be a discrete component of the fluid dynamic body, which is bolted-on to it for example Similarly, the porous barrier does not have to be continuous or integral with either one or both of the upper and lower surfaces of the fluid dynamic body, but can instead have a cross-section which is only partially or, in some embodiments, not at all continuous with the upper and lower surfaces of the fluid dynamic body, and can either be formed integrally with the fluid dynamic body during manufacture, or formed integrally with the pattern if the latter is a discrete component of the fluid dynamic body added to it later, or can itself be added to the pattern later in a retrofitting operation. In some preferred embodiments, the porous barrier can be part of a contour of an airfoil or hydrofoil of the fluid dynamic body, as this optimizes, or at least increases, the noise reducing effect of the porous barrier.
[0053] Furthermore, whereas in FIGS. 3 to 9, the pattern is shown to be periodic, similar principles can be applied to aperiodic patterns resulting in similar beneficial noise reduction effects by providing a plurality of porous barriers of different sizes and shapes adapted to the different sizes and shapes of the interstices of an aperiodic pattern. Moreover, whereas the porous barriers 18a, 18b, 18c, 18d in FIGS. 6, 7, 8 and 9 are shown to occupy the respective interstices of the pattern therein completely, in other alternative embodiments, the porous barrier can only partially occupy one or more of the respective interstices of the pattern to have the desired noise reducing effect, provided that the porous barrier obstructs fluid flowing through the respective interstice between a first surface of the fluid dynamic body on a first side of the trailing edge and a second surface of the fluid dynamic body on a second side of the trailing edge. However, since the noise reducing effect increases as the interstice becomes progressively more filled by the porous barrier, in some preferred embodiments, the porous barrier should occupy at least 20%, preferably 40%, more preferably 60%, more preferably still 80%, and most preferably 100% (all) of the volume of the respective interstice. If the porous barrier does occupy all of the volume of the respective interstice, it has the added advantages of making the fluid dynamic body more structurally stable, safer to use and more able to retain an airfoil or hydrofoil shape when fluid flows in a major direction over the opposing surfaces of the fluid dynamic body and off the trailing edge thereof.
[0054] In some preferred embodiments, the porous barrier should fully occupy a width of the respective interstice, even if it does not occupy the full volume of the interstice. By the “width” of the interstice is meant a dimension of the interstice substantially parallel to the trailing edge. A porous barrier which occupies the full width of the interstice is effective in obstructing fluid flow between the first surface of the fluid dynamic body on a first side of the trailing edge and the second surface of the fluid dynamic body on a second side of the trailing edge. For example, therefore, the porous barrier could comprise a single row of bristles occupying the full width of the respective interstice, which would have the desired noise reducing effect, while only occupying less than 10% of the whole volume of the respective interstice.
[0055] FIG. 10 is a graph showing the noise reduction effect achieved with a first embodiment shown in FIG. 6. FIG. 10 is a log-linear plot of sound power level measured in decibels for a free fluid flow velocity of 40 ms.sup.−1 over a fluid dynamic body. This sound power level is represented on a linear scale on the ordinate (y-axis) of FIG. 10, plotted against frequency measured in hertz and represented on a logarithmic scale on the abscissa (x-axis) of FIG. 10. The solid line labelled B shows the sound power level measured for fluid dynamic body 1 with sharp trailing edge 10 as shown in FIGS. 1 and 2, without a pattern formed thereon and without porous barriers, while the dotted line labelled C shows the sound power level measured for a similarly shaped fluid dynamic body, but with pattern 12 and with porous barriers 18a formed thereon, as shown in FIG. 6. As shown, the porous barriers achieve a broadband noise reduction of up to 7 dB relative to the sharp trailing edge, across a frequency range from around 200 Hz up to around 5 kHz, with only a negligible increase in high frequency noise above that level, where the dotted line labelled C firstly crosses and then rises only very slightly above the solid line labelled B Similar measurements of the same embodiment at free fluid flow velocities of from 20 to 60 ms.sup.−1 are found to give the same levels of noise reduction. Other embodiments also achieve the same type of broadband noise reduction with only a negligible increase in high frequency noise as that shown in FIG. 10.
[0056] FIG. 11 shows the results of an experiment to measure the effect of the flow resistivity of various porous materials on noise reduction. The airfoil under investigation was an airfoil with a sawtooth serration cut directly into the main body of the airfoil. The chord length (C) of the airfoil was 150 mm, and the width is 450 mm. Between the leading edge x/C=0, and x/C=0.79, the original airfoil profile was unmodified, where x is the streamwise direction. Further downstream, 0.79<x/C<1.0, is a section that can be removed and replaced by a serration profile. Once attached the serrations form a continuous profile giving the appearance that the serrations are cut into the main body of the airfoil. The porous materials of different flow resistivities were cut to match the shape of the interstices, so that the airfoil would have a continuous profile throughout the chord length. Free field measurements of the airfoil self noise were conducted in an aeroacoustic wind tunnel situated inside a 4 m×5 m×3.4 m anechoic chamber. The nozzle exit of the open jet wind tunnel was rectangular with dimensions of 0.10 m (height)×0.30 m (width). The airfoil was held by side plates and attached flushed to the nozzle lips. Far field noise measurements were made by a single condenser microphone at polar angles of Q=90° at a distance of 1.0 m from the airfoil trailing edge at mid span. Noise data was acquired at a sampling frequency of 44 kHz for 10 seconds by a 16-bit Analogue-Digital card from National Instrument. The data was then windowed and the Power Spectral Density (PSD) of 1 Hz bandwidth computed from a 1024 point FFT.
[0057] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.
