Trailing edge noise modification due to edge porosity

It is believed that trailing edge porosity characteristics of large owl wings is the cause of their observed quiet flight. This hypothesis was tested in the ARL Penn State anechoic chamber by measuring sound radiated by the interaction of a vortex ring with a single fixed edge. Both porous and rigid trailing edges were tested. Vortex ring motion was characterized by Schleiren. Typical rings measured 9 mm in diameter, and convected at speeds ranging from 30 m/s to 90 m/s. Rigid and porous edge radiated noise was then compared across this range of vortex ring speeds to show degree of attenuation due to edge porosity.