Beamforming of aeroacoustic sources in the time domain

A classical array processing technique used for the analysis of aeroacoustic sources is the frequency-domain beamforming technique. The use of this technique requires an assumption on the stationarity of the sources as it works with a time-averaged estimate of the cross-spectral matrix. As a consequence this technique provides an estimation of the average position (in space and time) of an aeroacoustic source. However, some studies, focusing in particular on jet noise, have shown the intermittent nature of some aeroacoustic sources. The aim of this paper is then to show that the time-domain beamforming technique allows assessing the intermittent nature of aeroacoustic sources, and makes possible a space-time tracking of short duration acoustic events occurring in the flow. The method is applied to experimental wind-tunnel measurements. The noise produced by the flow over a forward-facing step is analyzed by using both frequency-domain and time-domain beamforming, and the intermittent structure of such an aeroacoustic source is investigated.