Described herein are a viscous drag reduction apparatus and a method. The apparatus includes a pair of rollers connected to a supporting surface on a roof of the vehicle, a belt having a frictional surface and partially wrapped around the pair of rollers, such that the pair of rollers allow the belt to rotate in response to an air flow generated around the vehicle when the vehicle is in motion, the pair of rollers having a length in an axial direction that is at least as long as a width of the belt, an assembly of the pair of rollers and the belt being at least partially recessed with respect to a top line of the roof, and a reverse flow cover connected to the front end of the roof of the vehicle to block an air back flow generated by the belt when rotating.

Described herein are a viscous drag reduction apparatus and a method. The apparatus includes a pair of rollers connected to a supporting surface on a roof of the vehicle, a belt having a frictional surface and partially wrapped around the pair of rollers, such that the pair of rollers allow the belt to rotate in response to an air flow generated around the vehicle when the vehicle is in motion, the pair of rollers having a length in an axial direction that is at least as long as a width of the belt, an assembly of the pair of rollers and the belt being at least partially recessed with respect to a top line of the roof, and a reverse flow cover connected to the front end of the roof of the vehicle to block an air back flow generated by the belt when rotating.

Aircraft, air conditioning systems, and air diffusers that may be used to create a quiet, comfortable environment within an aircraft cabin are disclosed. For example, an aircraft cabin air diffuser, which may be used in an air conditioning system and/or an aircraft, may include an inlet section, a neck section downstream of the inlet section, an outlet section downstream of the neck section, and a flow controller within the neck section. The flow controller may be a passive pressure controller (configured to affect air flow through the passive pressure controller in response to an air pressure differential across the passive pressure controller) and/or a vortex inducer (configured to create a vortex of air downstream of the vortex inducer).

Actuatable microstructures and methods of making the same are disclosed. An example a sheet includes a first side including an elastomeric material and a second side opposite the first side. The sheet defines sealed channels. In response to a pressure differential across the elastomeric material, the elastomeric material is to be in a deformed position relative to the sealed channels to define microstructures.

A phononic material and a method of using a phononic material for use in interacting with a fluid or solid flow are provided. The phononic material includes an interface surface and a subsurface feature. The interface surface is adapted to move in response to a pressure, and/or velocity gradients, associated with complex motion of a turbulent flow exhibiting a polarity of frequencies exerted on the interface surface. The subsurface feature extends from the interface surface. The subsurface feature comprises a phononic crystal or locally resonant metamaterial adapted to receive the pressure, and/or velocity gradients, from the turbulent flow via the interface surface and alter the phase and amplitude of a polarity of frequency components of the turbulent flow in order to reduce or increase the kinetic energy of the turbulent flow. The interface surface is adapted to vibrate at a polarity of frequencies, phases and amplitudes in response to the frequency, phase and amplitude of at least one component of the turbulent flow.

A pressure reducing device is disclosed for use with a gaseous fuel system. The pressure reducing device may include a body defining an inlet and an outlet, and a converging-diverging nozzle formed between the inlet and the outlet. The pressure reducing device may further include a shockwave inducing element disposed within the body between the venture and the outlet, and an airfoil located inside the body upstream of the shockwave inducing element and connected to move the shockwave inducing element.

A multi-input, multi-output phononic system including a first interface surface and a second interface surface that respond to at least one of a pressure gradient or a velocity gradient in a wave of a turbulent fluid flow or a laminar fluid flow, the pressure gradient or the velocity gradient associated with complex motion of the flow exhibiting a plurality of frequencies exerted on one or more of the interface surfaces; and a subsurface feature extending from the interface surfaces, the subsurface feature comprising a phononic crystal or locally resonant metamaterial adapted to receive one or more of the pressure gradient or the velocity gradient from the fluid flow via the interface surfaces and to alter one or more of a phase and an amplitude of a plurality of frequency components of the fluid flow.