Acoustic particle velocity investigations in aeroacoustics synchronizing PIV and microphone measurements

The goals of the present study are the detection, visualization and analysis of the sound propagation process from a strong tonal sound source to the acoustic far-field produced by flow around a square rod. Furthermore, the flow structures related to the aeroacoustic source mechanism are investigated. Velocity measurements were conducted using the particle image velocimetry technique (PIV) in a wind tunnel experiment in anechoic conditions. Simultaneously, the acoustic pressure fluctuations were recorded by microphones in the acoustic far-field. The PIV observation windows were shifted stepwise from the source region to the vicinity of the microphones. This enables to trace the acoustic propagation. The PIV measurement has been performed with a high dynamic range of more than 72 dB to reproduce the acoustic fluctuations through the flow field in observation areas outside the source region. The calculation of the cross-correlation function between the velocity and the pressure fluctuations has been applied which acts like a filter of the velocity fluctuations yielding a proxy variable for the acoustic particle velocity. The temporal evolution of this quantity indicates the propagation of the acoustic perturbations. It can be shown that wave propagation emanates from coherent flow structures in the near-field through the shear-layer of the open jet to the far-field.