Roles of sound waves and acoustic feedback in instability and aeroacoustics of supersonic boundary layers
File(s)
Author(s)
Qin, Fufeng
Type
Thesis
Abstract
This thesis investigates the linear and nonlinear evolution of radiating modes in supersonic boundary layers under the influence of impinging sound waves. It is found that the ensuing boundary-layer response is extraordinarily large for a subset of the sound frequency and incident angle, and the resonant over-reflection occurs at a particular pairing of frequency and incident angle. At this point the reflected wave coincides with a locally neutral radiating mode, leading to a fundamental resonance. A composite amplitude equation accounting for both non-parallelism and non-equilibrium effects is constructed to describe the excitation and the nonlinear development of the radiating mode. The analysis is extended to describe the coupling of boundary layers developing on two parallel flat plates through acoustic feedback. Two scenarios are considered: the spontaneous Mach wave of a radiating mode arising from one boundary layer influences the instability of the other, or the spontaneous Mach wave is reflected by the other boundary back to act on the radiating mode. A study is also made of the sound wave with frequency and wavenumber twice those of the radiating mode, in which case the sound wave influences the nonlinear evolution through a subharmonic resonance mechanism. We then investigate a more general case, where the impinging sound and radiating mode have the same phase speed, but the ratio of their frequencies (wavenumbers) is arbitrary. As a result, the mechanism by which the sound affects the evolution of the radiating mode is through a phase-locked interaction taking place in their common critical layer. In all these scenarios, the linear and nonlinear evolution of the radiating mode is significantly affected by the impinging sound wave through the respective mechanisms. Sound waves, whether arriving from the environment or being emitted spontaneously by the instability mode, are thus found to play a crucial role.
Version
Open Access
Date Issued
2022-03-31
Date Awarded
2024-06-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Wu, Xuesong
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Mathematics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)