The noise generating and suppressing characteristics of bio-inspired rough surfaces

It is hypothesized that the structure of the down covering the flight feathers of larger species of owls contributes to their ability to fly almost silently at frequencies above 1.6kHz. Microscope photographs of the down show that it consists of hairs that form a structure similar to that of a forest. The hairs initially rise almost perpendicular to the feather surface but then bend over in the flow direction to form a canopy with an open area ratio of about 70%. Experiments have been performed to examine the noise radiated by vertical filaments and by a large open area ratio canopy suspended above a surface. The canopy is found to dramatically reduce pressure fluctuations on the underlying surface, in a manner that is found to be consistent with the theory of flows over and through vegetation. While the canopy can produce its own sound, particularly at high frequencies, the reduction in surface pressure fluctuations can reduce the noise scattered from an underlying rough surface at lower frequencies. Theoretical studies are also being performed to look at the noise radiating/suppressing characteristics associated with the flexibility of the hairs, and to examine the extent to which the aeroacoustics of the down can be modeled by treating it as a porous layer.

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