The aeroacoustics response and shear layer dynamics of confined cavities subject to low Mach number turbulent flow

Abstract

Cavities exposed to low Mach number flow in various engineering applications are often liable for generating flow-excited acoustic oscillations, resulting in large acoustic amplitudes and vibrations. This compromises the safety and reliability of critical equipment due to a phenomenon attributed to interaction between the instability of the shear layer and the acoustic modes of a given system. This thesis experimentally investigates the aeroacoustics response of cylindrical cavities having aspect ratios of h/L = 0.5, 1, and 1.5, where h is the cavity depth and L is the shear layer impingement length, up to flow velocities of Mach 0.4. In view of the cavity confinement, the effects of the admission ratio w/W, where w is the cavity width and W is the duct width, on the aeroacoustics response and shear layer dynamics are also considered. The work extends the investigation to two-dimensional rectangular cavities and square cavities with similar aspect and admission ratios to the cylindrical cavities, as to establish the effect of the cavity shape on the resonance excitation frequencies and hydrodynamic modes of the system. Acoustic pressure measurements present Strouhal periodicities that agree well with values reported in literature. Cylindrical and square cavities with aspect ratio h/L = 0.5, however, exhibit unique behaviour due to the interference of the recirculation region within the cavity, ultimately modifying the symmetry of the shear layer. Particle image velocimetry (PIV) measurements present spatial characteristics of the shear layer dynamics, revealing improved flow modulation with increasing acoustic pressure, and significant asymmetry for shallow aspect ratios. The work presented in this thesis provides novel insight of the shear layer instability in confined cavities, and its effect on the flow-sound interaction mechanism.