Aeroacoustics and shear layer characteristics of confined cavities subject to low Mach number flow

Abstract

Numerous engineering applications involve low Mach number flow in cavity aperture geometries, which can generate flow-excited acoustic oscillations and hence high amplitudes of noise and vibration. This is caused by the instability of the shear layer interacting with the acoustic modes of the system. In this study, we experimentally investigate ducted cylindrical cavities with aspect ratios (Depth/Diameter) of 0.5, 1, and 1.5 up to flow velocities of Mach 0.4. We examine the influence of the cavity confinement (Cavity Width/Duct Width) on the aeroacoustics response, as well as the downstream impingement behaviour of the shear layer. Rectangular and square cavities bearing similar geometric parameters are also investigated to compare the effect of the cavity shape on the shear layer dynamics and the aeroacoustics response. The results confirm resonance behaviour at frequencies well predicted by the theory, as well as Strouhal periodicities that agree with reported literature. However, cylindrical and square cavities with aspect ratio of 0.5 exhibit stronger dependence on the aspect ratio due to the interference of the recirculation region by the cavity floor, ultimately modifying the symmetry of the shear layer. This asymmetric flow pattern affects the velocity-dependent tones, generating much lower Strouhal numbers than the estimated values, and must be accounted for in equipment design. Moreover, Particle Image Velocimetry (PIV) measurements further illustrate the spatial characteristics of the shear layer dynamics for the various shaped cavities. The visualizations demonstrate enhanced flow modulation at higher acoustic pressure amplitudes, as well as reveal the flow asymmetry and the three-dimensional shear layer topology in cylindrical and square cavities.