| Author: | Chen, Yifeng |
| Title: | Instabilities and control strategies in hypersonic boundary-layer flows |
| Advisors: | Wen, Chihyung (AAE) Guo, Peixu (AAE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Subject: | Boundary layer control Fluid dynamics Aerodynamics, Hypersonic Hypersonic planes Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Aeronautical and Aviation Engineering |
| Pages: | xxiv, 167 pages : color illustrations |
| Language: | English |
| Abstract: | Boundary-layer instability and transition remain significant and unresolved challenges for hypersonic vehicles. Transition prediction and control require a comprehensive understanding of the character, energy source, and interaction of instabilities. These instabilities include both modal and nonmodal disturbances in linear and nonlinear stages that lead to transition. However, the aforementioned fundamental aspects remain incompletely understood and warrant further systematic investigation. The primary objective of this investigation is to elucidate the fundamental energy mechanisms underlying various boundary-layer instabilities and to evaluate potential transition control approaches, especially for a passive control method- acoustic metasurface. By employing three established energy-based methodologies- the momentum potential theory, inviscid Lagrangian energy analysis, and relative phase analysis- a comprehensive unified framework for the source terms governing the exponential evolution of the second mode is developed. For the Mach 6 flow condition under investigation, all three methodologies converge on identical local energy amplification pathways driven by two dominant source mechanisms: the dilatation term in the near-wall region and either the Reynolds thermal stress term or heat exchange term in the outer layer region, contingent upon the chosen energy norm. By extending relative phase analysis to multiple instability modes and influencing factors, the analysis reveals that obliqueness results in a strengthened first mode while a weakened second mode via Reynolds shear stress. The wall cooling stabilizes the first mode via thermoacoustic phase opposition but destabilizes the second mode through a newly generated in-phase region of wall-normal internal energy transfer. Acoustic metasurfaces mimic wall heating to suppress the second mode while amplifying oblique first modes through in-phase shear production. Furthermore, the wall-normal transport of internal energy at the critical layer demonstrates increased phase deviation for the substantially weaker supersonic mode, explaining its reduced contribution to energy growth. Resolvent analysis of hypersonic flow over blunt wedges identifies two distinct growth regimes in the entropy layer (pattern A) and boundary layer (pattern B) of travelling waves, distinguished by their energy gain characteristics between optimal and sub-optimal disturbances. Both regimes exhibit energy growth dominated by nonmodal processes, which may subsequently facilitate first-mode instability development under adiabatic wall conditions. Wall cooling suppresses Pattern B's modal/nonmodal growth but minimally affects Pattern A. Through the combined analysis of slow acoustic wave receptivity and resolvent response, a plausible mechanistic explanation was proposed for the experimentally observed "transition reversal" phenomenon. Furthermore, the analysis identifies concurrent Orr and lift-up mechanisms governing nonmodal growth. Finally, transition triggered by multiple dominant instabilities is examined, integrating linear and nonlinear, modal and nonmodal growth stages. The acoustic metasurface effect is modelled using a rigorously validated time-domain impedance boundary condition (TDIBC). The resolvent analysis identifies two distinct optimal disturbance peaks corresponding to the oblique first mode and planar Mack second mode. The critical finding reveals that acoustic metasurface placement profoundly influences transition delay and skin friction reduction. Positioning within the second mode's linearly unstable region delays the transition by suppressing streaks in the oblique breakdown scenario. However, during late transition stages, the metasurface increases skin friction overshoot attributable to intensified shear-induced dissipation from reinforced detuned modes associated with combination resonance. This adverse effect can be mitigated by restricting metasurface placement upstream of the overshoot region. This study provides fundamental insights into instability mechanisms and transition processes in hypersonic boundary-layer flows while advancing effective control strategies. The findings offer significant potential to accelerate the development of next-generation hypersonic vehicles through improved transition prediction and control capability. |
| Rights: | All rights reserved |
| Access: | open access |
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