Evaluation of advanced acoustic imaging methods for microphone-array measurements in closed-section wind tunnels

Aeroacoustic measurements with microphones in closed-section wind tunnels are typically hampered by the pressure fluctuations of the turbulent boundary layer (TBL) on the tunnel's walls, ceiling, or floor, where phased microphone arrays are usually mounted. This paper evaluates the performance of several advanced acoustic imaging methods that aim at overcoming this limitation for localizing and quantifying sound sources in closed-section wind-tunnel measurements. The acoustic data employed was obtained using a phased microphone array installed in two different configurations: flush-mounted on the wind-tunnel wall and recessed within cavities behind an acoustically transparent covering. The acoustic imaging methods considered are conventional frequency domain beamforming (CFDBF, as a baseline), functional (projection) beamforming (FUNBF), orthogonal beamforming (OB), CLEAN-SC, and the deconvolution approach for the mapping of acoustic sources (DAMAS). Two sound sources are analyzed: (1) a single speaker emitting broadband sound as a reference signal, and (2) a flat plate inside of the flow as a distributed aeroacoustic source. In general, it is observed that the array with cavities provides considerably better results as it benefits from a higher signal-to-noise ratio. In addition, removing the main diagonal from the cross-spectral matrix also helps obtaining clearer acoustic source maps and more accurate quantitative sound spectra estimations. Overall, DAMAS and OB are the best performing methods for the case with a single speaker. CFDBF and FUNBF are the most suitable methods for the case with the distributed sound source of the trailing edge of the flat plate, whereas the other techniques fail to properly identify it.