Aeroacoustics of deep cavity flows

Abstract

Flow-acoustic resonances occurring within deep cavities have been observed in various engineering applications. These resonances occur when specific operating conditions cause airflows over deep cavities to excite self-sustained oscillations that couple with least-damped acoustic modes to generate intense aerodynamic noises. Consequently, these flow-acoustic resonances can lead to extreme noise, violent unsteady structural loads and threaten the mechanical integrity of the system. Hence, it is important to understand the underlying physical mechanisms of the aerodynamic noise generation in deep cavity flows. This thesis uses high-resolution large-eddy simulations to investigate the flow-acoustic resonance in turbulent flows passing over deep cavities at low-subsonic flow speeds across three distinct inclination angles. Several theoretical methods, including Doak's momentum potential theory, modal and non-modal analyses, are employed to gain insights into the intricate noise generation and amplification mechanisms within orthogonal and inclined deep cavity flows. Accordingly, the work presented in this thesis is structured into two main parts. The first part of this thesis investigates the flow-acoustic resonance in orthogonal deep cavity flow at three different flow speeds. The subsequent analysis reveals strong evidence of efficient fluid-acoustic coupling between the shear layer oscillations and the nearby depthwise acoustic modes in all cases, and the optimum Mach number at which the pronounced acoustic response occurred is identified. Consequently, an improved frequency prediction model tailored explicitly for orthogonal deep cavity flows at low subsonic flow as a function of inflow boundary-layer property is proposed. The second part of this work investigates the flow-acoustic resonance in inclined and deep cavity flow across three distinct inclination angles. Notably, a marked contrast in aeroacoustic behaviour between inclined and orthogonal cavities is observed at the elevated flow speed, which do not follow the existing flow-acoustic resonance theories. Specifically, inclined cavities display a markedly enhanced resonance with the peak frequency being significantly lower in comparison to the orthogonal cavity configuration. It is postulated that the amplified flow-acoustic resonances in inclined cavities are linked to a low-frequency extension of the first hydrodynamic mode through enhanced shear layer undulations when the acoustic particle displacement is comparable to the momentum thickness. Experimental evidence supports this hypothesis. The elucidation of this hypothesis holds potential in advancing our understanding and mitigation of flow-acoustic resonance in inclined and deep cavity flows.

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ORCID for Jae Wook Kim: orcid.org/0000-0003-0476-2574

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