Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Aeronautical and Aviation Engineering | en_US |
| dc.contributor.advisor | Wen, Chih-yung (AAE) | en_US |
| dc.creator | Lu, Jiachen | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/14331 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Linear instability and control methods of first Mack mode and crossflow mode | en_US |
| dcterms.abstract | First Mack modes and crossflow instabilities play critical roles in supersonic boundary layer transitions, whereas they have not received sufficient attention. Several critical issues require further investigation: First Mack modes lack a physics-based comprehension of their behavior across varying spanwise wavenumbers. Their distinct responses to control techniques at different spanwise wavenumbers have not been systematically evaluated. The identification of crossflow instabilities and their independence from first Mack modes in hypersonic regime remain contested. The stabilization mechanisms of crossflow modes have not been clearly elucidated. This thesis addresses these theoretical and engineering challenges through direct numerical simulation, linear stability theory, and momentum potential theory. | en_US |
| dcterms.abstract | The first part focuses on the linear instability and control strategies of first Mack modes at varying spanwise wavenumbers in a Mach 4.5 flat-plate boundary layer. As the spanwise wavenumber increases, the streamwise wavenumber initially rises before declining, with its maximum marking the transition of the first mode from an acoustic to a vortical nature. These distinct physical characteristics result in divergent responses to laminar control techniques and differing stabilization mechanisms: Wall cooling slightly stabilizes planar first modes and significantly stabilizes oblique first modes by suppressing the thermal components. Similarly, wall suction exhibits a weaker stabilization effect on planar first modes compared to their oblique counterparts, with thermal and vortical components in the oblique mode damped under steady suction. For porous coatings, slightly oblique first modes are modulated by scattering effects and wall admittance, whereas highly oblique first modes are destabilized by mean-flow distortions induced by the coatings. Grooves, despite their macroscale geometry relative to porous coatings, govern planar first modes through wall admittance mechanisms, where acoustic components are modulated by wall admittance. For highly oblique first modes, mean-flow distortions are responsible for the destabilizing effects. | en_US |
| dcterms.abstract | The second part provides new evidence for the consistency of first Mack and crossflow modes. Boundary-layer receptivity to three-dimensional slow acoustic and vorticity waves over a Mach 5.9 sharp wing is systematically investigated under varying sweep angles. Uniform behaviors between these modes highlight their consistency: Linear stability theory categorizes both instabilities as S modes. The vortical component dominates both modes, emphasizing their inherent vortical nature. These modes share identical receptivity pathways: For slow acoustic waves, strong acoustic components are generated at the leading edge through the synchronization mechanism and subsequently diminish. The vortical components grow steadily throughout the receptivity process, resulting in a high-growth-rate non-modal growth phase of perturbations. After adjustments of acoustic and thermal components, the linear growth occurs. While for vorticity waves, leading-edge disturbances primarily consist of vortical components, arising from interactions between vorticity waves, shock waves, and boundary layers. Acoustic and vortical components go through adjustment in their shape, and thermal components exhibit initial decay followed by recovery, resulting in a lower-growth-rate non-modal growth phase. Ultimately, the linear growth phase commences. | en_US |
| dcterms.abstract | The final part investigates the stabilization mechanisms of crossflow modes in a Mach 6 blunt-wing boundary layer. Wall cooling can effectively stabilize the crossflow mode by predominantly suppressing the thermal component. Wall suction exhibits limited control effects and directly influences the vortical component. Porous coatings and macro-slit grooves exhibit moderate stabilization effects on the crossflow mode. Mean-flow distortions are identified as the primary driver of their stabilization effects. Stabilization mechanisms of crossflow instabilities exhibit significant parallels with those of first Mack modes, further highlighting their physical consistency. | en_US |
| dcterms.extent | xii, 110 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2025 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.accessRights | open access | en_US |
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