On the Passive Noise Control of the Flow-Induced Noise Using Porous Materials

Machines that work with airflows face the issue of noise that occurs when air flows around a body. In particular, aerodynamic noise in low Mach number flows is usually caused by surface pressure fluctuations due to vortex formation near the object. If the object size is sufficiently smaller than wavelength of the radiated noise, the sound properties may be approximated by a dipole sound source.

Aerodynamic noise reduction methods are roughly divided into two types. One is active control to reduce noise by controlling the flow using an external force supplied by an actuator or the like, and the other is passive control that involves modifying the shape of the object or manipulating the acoustic characteristics of the surface using a porous material. Active control can reduce the highly periodic noise generated by the coherent structure in the fluid to a largest extent, but it requires an external force and cannot mitigate the low-periodicity aerodynamic noise caused by turbulence. Passive control is easy to implement, but its reduction effect is limited by frequency or sound pressure, and it is difficult to obtain a large noise reduction effect. Therefore, the objective of this study was to reduce aerodynamic noise without changing the shape of the object but by substituting the portion of the object exposed to fluid with porous material in order to realize a passive control strategy with a high noise reduction effect.

Currently, numerous studies are underway on the reduction of aerodynamic noise by incorporating porous media, primarily in the field of aviation. However, no detailed analysis of the mechanism of noise reduction by porous media has not been examined. This study involves the experimental examination of a passive noise control technique that uses porous media for reducing flow-induced noise. The objective of this study is to decrease aerodynamic sounds by employing porous media that only permeate sound and to analyze the associated sound reduction mechanism.

In the proposed experiment, flow-induced noise emitted from two types of rectangular cylinders was measured in a low-noise wind tunnel. One cylinder comprised four aluminum plates and the other cylinder comprised two aluminum and porous media plates each. Obtained measurement results indicate that the frequency of the distinct tonal noise is found to be different for the two cylinders, and the frequency is found to be higher while using porous media.  It is also observed that the sound pressure level of the noise is different for both cylinders. Compared to the other cylinder, the maximum sound pressure level of the cylinder incorporating porous media plates is found to be lower by 25 dB.  Furthermore, thickness of the porous media plate has marginal effect on the measurement results. The velocity field around the cylinders was analyzed through particle image velocimetry measurement, and the results indicate that time and space scale of the separation vortex street around the cylinders are smaller while using porous media. Such variations in aerodynamic sounds can be attributed to changes in the velocity field.