Propagation of wall pressure perturbations in a large aspect ratio shallow cavity

Wall pressure fluctuations generated by turbulent boundary layers over a shallow cavity are studied experimentally in a low-speed wind tunnel facility. The scope of the present work is to characterize the propagation of the pressure perturbations at the wall by means of pressure cross-correlations and cross-spectra measured through a microphone pair translated along the cavity floor. It is found that the mechanism characterizing the pressure propagation close to the backward facing step and in the middle of the cavity is similar to what is commonly observed in equilibrium boundary layer being the convection velocity smaller than the external mean velocity. On the other hand, in the close vicinity of the forward-step, the hydrodynamic contribution of the pressure fluctuations is accompanied by a relevant acoustic effect characterized by a convection velocity close to the speed of sound. Furthermore, in the regions close to the two steps, the spectral decay of the coherence function, even though of exponential type, is faster than that obtained in the quasi-equilibrium region.