Beamforming of aeroacoustic sources in the time domain: An investigation of the intermittency of the noise radiated by a forward-facing step

The present study investigates the intermittency of the broadband aeroacoustic noise produced by a forward-facing step in a flow. The noise source is viewed as a random succession of the so-called intermittent events of short duration distributed spatially in a source region in the flow. An array processing method based on time-domain beamforming has been developed in order to track systematically the intermittent events, both in the time and space domains. Based on a simulated model of the far-field pressure field, the method is validated in terms of event detection and of performance for recovering the pressure spectrum. The method is then applied to experimental array data taken in an anechoic wind-tunnel at low Mach numbers (not exceeding 0.15) for a forward-facing step of height 30 mm. The results show that some very short intermittent events (with a mean duration of the order of 0.15 ms) can be identified from the array data. The spatial distribution of the intermittent events is found to be in agreement with the frequency domain beamform maps. The probability density functions of the events, in terms of widths and apparition times, are shown to be governed by Gamma laws and indicate random phenomena; it is observed that the statistical distributions vary with the streamwise position downstream and upstream of the step, the trends being in agreement with the source behavior as evidenced by using the frequency-domain beamforming methods. The proposed method is then shown to identify, in terms of emission time, location and temporal width, a succession of short acoustic events that participate to the broadband aeroacoustic noise produced by the step; those random events are likely to be linked to the dynamics of the flow interacting with the step.