Causality Correlation Analysis for Aeroacoustics using PIV in the Near-Field

Abstract

The investigation of the cause and effect relationship between near-field quantities in a turbulent flow and associated pressure fluctuations in the far-field is of great interest in aeroacoustic research. Common approaches for the prediction of the acoustic far-field from the near-field data are mostly based on an acoustic analogy, where the source strength is determined by experiments or numerical simulations. In this way it is possible to estimate how much noise is generated in a certain flow field. But typically the source strengths in the near-field largely cancel themselves out and only a relatively small portion radiates to the far-field. To predict this destructive interference correctly, the sources have to be determined with high accuracy in the complete source domain. Additionally a far-field prediction does not necessarily give a deep insight into the source process itself. Often, even the simple question of where the sound comes from cannot be unequivocally answered. The attempt to answer this question is to measure simultaneously the acoustic pressure in the far-field together with some other near-field quantity. In this manner the far-field pressure can be correlated with the near-field quantity to identify flow structures which are subject to the same physical phenomenon as the aeroacoustic sources in a statistical manner. In the short review presented here we are describing its implementation by means of cross correlating the far-field radiated sound pressure of different flow configurations with velocity fluctuations in the near-field measured via PIV.