Aerodynamic noise radiated from bodies in subsonic flow is caused by the generation, acceleration and interaction of vortices. Therefore, in order to reduce the aerodynamic noise, we need skillful turbulence management or control. In other words, it is most important to alleviate such vertical motions.In this paper, we examine noise reducing effects of pile-fabrics, namely cloth materials with fine and high density fibers like fur or down, that are used to cover the body surface. Simple tests show that pile-fabrics are very useful for reducing the vortex shedding noise, the turbulent boundary layer noise and the vortex interaction noise.

Aeroacoustic wind tunnels, which consist of large measuring sections, such as anechoic rooms, and open jets, are generally applied as test facilities to measure aerodynamic noise generated around vehicles. In aeroacoustic wind tunnels, it is easy to generate flow-induced low-frequency fluctuations resulting in pressure fluctuations or velocity fluctuations associated with vortices or turbulence in the open jet. These low-frequency fluctuations are generated by various flow-induced phenomena, such as swaying that occurs in various elements of the circuit, fluctuations propagated from the shear layers or mixing layers of the open jet, and the acoustic resonances of the whole circuit, which often become serious problems, generally called pulsations, at particular wind speeds. Because these low-frequency fluctuations may affect the measured pressure and velocity of a flow around vehicles in measuring sections, we should aim to reduce or control them when designing aeroacoustic wind tunnels to ensure measurement with high reliability. In this study, we investigate the generating mechanisms and techniques for reducing these low-frequency fluctuations in aeroacoustic wind tunnels by performing experiments on a model wind tunnel. In particular, new techniques for reducing acoustic resonance components such as by installing an acoustic opening in the circuit are proposed and verified by simulations and experiments. Using these techniques for reducing low-frequency fluctuations, the fluctuations of pressure and velocity in the measuring section are markedly decreased over the entire wind speed range.

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