Sound directivity pattern radiated from small airfoils

Abstract

In a recent paper, Clark and Ribner [J. Acoust. Soc. Amer. 46, 802–805 (1969)] show that the instantaneous sound radiated by a small airfoil (in comparison with a typical wavelength) and its fluctuating lift are connected by Curle's point dipole equation which describes sound radiated from rigid surfaces in flow. In the present investigation, we provide an additional check on Curle's theory by measuring and comparing with his predictions the sound directivity pattern radiated by a small airfoil in flow. The results of the investigation provide some evidence that, indeed, small airfoils in flow radiate like point dipoles in apparent support of that aspect of Curle's treatment.