Aeroacoustics of porous trailing edges

This presentation concerns the sound generated by a vortex ring passing near the edge of a non-compact porous plate. Theoretical predictions of the radiated sound level and directivity are compared against measurements performed in the ARL Penn State anechoic chamber for a series of plates, each with a different porosity. A shock tube produces the rectilinear motion of the vortex rings, whose speeds range from 39 m/s to 86 m/s, as estimated from high-speed Schlieren imaging. A ring of twelve microphones centered at the plate edge measures the far-field sound and its directivity in synchrony with video capture. These measurements are used to estimate a scaling law for the radiated acoustic power $\Pi \sim U^n L^m$ on the ring speed $U$ and minimum ring-to-edge distance $L$. Predicted variations in these exponents $n$ and $m$, in far-field sound directivity, and in source waveforms on changes in porosity compare favorably to the experimental results.