An Experimental Investigation of Cavity Flow Oscillations and Tones of an In-Flow Microphone

Abstract

An experiment was conducted to investigate the flow-induced nose-cone cavity oscillations of a B&K in-flow microphone. A 38.1-mm-diameter scale model of the B&K 12.7-mm UA 0386 nose cone was constructed and tested in a 177.8-mm-round free-jet in the Anechoic Chamber Wind Tunnel at NASA Ames Research Center at Re_D up to 1.1 × 10⁵. The results indicated that the B&K nose-cone cavity behaves much like a cavity without a screen. Detailed flow velocity measurements over the cavity screen have shown inflection points in the mean velocity profiles and high disturbance intensities in the vicinity of the cavity trailing edge. These results and the high coherence between the strong velocity and acoustic pressure oscillations at the cavity trailing edge implied that the cavity acoustic tones are the consequence of the cavity shear-layer impingement on the cavity trailing edge. The effect of cavity screen appears to be secondary in the velocity range tested. The screen seems mainly to modulate the Strouhal numbers of the higher acoustic modes.

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