Aeroacoustic, acoustic and fluid dynamic characterization of rectangular partial enclosures

Wall pressure fluctuations generated by interaction of turbulent jets with the wall of rectangular partial enclosures (RPEs) are studied experimentally over a broad range of parameters. The scope of the present work is to characterize the propagation of the pressure perturbations in the RPEs by means of wall pressure auto-spectra, cross-spectra and cross-correlations measured through microphones located along the wall. In order to interpret the pressure measurement, the acoustic and fluid dynamic behaviours are investigated analytically, numerically and experimentally for several cavityneck section ratios. The flow structures as the vortex formed in the cavity and the recirculation zone in the neck are studied in details. The acoustic behaviour of this geometry is investigated with particular emphasis on the first dominant mode analysis. It is pointed out that the first mode frequency scales as an Helmholtz resonator frequency. Taking into account this scaling a reduced form of the Strouhal number, that leads the spectra to collapse, is proposed. Furthermore it is found that the mechanism characterizing the pressure propagation at high frequency close to the bottom wall is strongly affected by the adverse pressure gradient that modifies the jet-wall interaction. The fluid dynamic contribution of the pressure fluctuations at high frequency is accompanied by a relevant acoustic effect characterized by a convection velocity close to the speed of sound at low frequency. The dynamic of the recirculation zone, characterized using the Proper Orthogonal Decomposition technique, plays an important role from the aeroacoustic viewpoint that produces in some conditions an increase of the wall pressure fluctuations at low frequency.