Detection of Non-Stationary Aeroacoustic Sources by Time-Domain Imaging Methods

Imaging methods such as the beamforming technique are widely used to localize and identify aeroacoustic sources. However, so far, existing applications in aeroacoustics have mostly been performed in the frequency domain. To tackle the characterization of nonstationary sources (for example, intermittent sources), time-domain imaging methods are more appropriate. Indeed, the spatio-temporal reconstruction of the acoustic fields allows studying the source structure in “real-time”. In this paper, two aeroacoustic problems are investigated with the help of time-domain inverse methods. First, numerical acoustic data obtained from the simulation of the radiation of a 2D mixing layer are studied through a numerical time-reversal method based on the Linearized Euler Equations. The spatio-temporal maxima of the acoustic energy are then detected by observing successive snapshots of the reconstructed acoustic field. These are assumed to correspond to wave focusing and, hence, to be related to the presence of a source. Finally, vorticity field snapshots are observed at the times at which spatio-temporal maxima are found. A conditional average of the flow fields, assuming large acoustic emission, is thus possible in principle. The global structure of the source is found to be quadripolar and each kind of detected maxima corresponds to a fixed vortical structure. Second, experimental data of the noise produced by a forward-facing step in a wind-tunnel flow are analysed by using the timedomain beamforming technique. The detection of spatio-temporal maxima highlights that the broadband noise source produced by the step can be seen as a succession of short duration events scattered around the step edge.