Effect of turbulent boundary layer induced coherence loss on beamforming measurements in industrial scale wind tunnel tests

To obtain far field (aero)acoustic radiation characteristics a model is placed in an open test section wind tunnel with microphones positioned outside the flow. However for aerodynamic measurements a closed test section performs better. In stead of far field microphones beamforming can be used for acoustic testing in closed test section wind tunnels. To produce reliable results with a beamforming algorithm it is essential to measure the cross-spectral matrix with good accuracy, which in turn assumes that microphone signals are coherent. The signal coherence of a pair of microphones is however decreased due to the interaction of the acoustic waves with the turbulent boundary layer. Several publications have been made that model coherence loss in random/turbulent media. However, the impact of coherence loss on beamforming noise levels in industrial scale wind tunnel tests has not yet been evaluated. In this work simulations are performed and compared in the context of a real industrial scale test and an estimate is given of the loss of SPL due to coherence loss. Frequencies below 10 kHz show a negligible effect of coherence loss while in case the frequency is as high as 50 kHz the coherence loss results in a 4.5 dB reduction in peak SPL.