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Roughness surfaces likely present on high-speed flight vehicles due to ablation can greatly impact the laminar-turbulent transition process. In this work, effects of a randomly distributed roughness patch with roughness Reynolds number R e k k = 474 on the stability and transition in a Mach 6.5 boundary-layer flow over a flat plate have been investigated via stability analyses and direct numerical simulation (DNS). The roughness patch induces several streamwise streaks downstream. The streaks slightly stabilize the Mack mode instability, yet sustain strongly unstable shear-layer modes, achieving a significantly larger integrated growth rate than the smooth case. The most amplified shear-layer mode extracts energy primarily through the spanwise velocity gradient and develops nonlinearly into hairpin vortices residing on the strongest low-speed streak. The hairpin vortices eventually contaminate the whole flowfield, leading to a fully turbulent state. We further assess the influences of wall-temperature ratio and the roughness geometry on the flowfield and the pertaining instability characteristics. The results reveal that the high wall-temperature ratio weakens the streak amplitudes and shear-layer instabilities; while randomly distributed roughness tends to induce larger-amplitude streaks than the regular counterpart with the same R e k k , the flowfield of the former one can even be more stable than the latter. We find that the spanwise gradient of streamwise velocity should also be considered along with the streak amplitude in determining the strength of shear-layer instabilities. [ABSTRACT FROM AUTHOR]

Nonmodal linear stability analysis results are presented for hypersonic flow over a cone at 6° angle of attack complementing earlier modal stability analysis. Based on the parallel flow assumption, singular value decomposition is applied to obtain the optimal linear combination of global crossflow modes. The optimal disturbance exhibits significant transient growth in the initial short distance and progressively follows the path of the most unstable mode downstream. The largest transient energy gain is observed for disturbances at around 40 kHz close to the most amplified modal frequency and tends to increase with the Reynolds number. The optimal disturbance initially exhibits two amplitude peaks in the azimuthal direction, one lying in the leeward region where the unstable crossflow modes reside and the other in the windward region where the adjoint modes exist. As the optimal disturbance travels downstream, the second amplitude peak rapidly shifts toward the leeward side and reaches the optimal energy gain when it eventually merges with the first amplitude peak. The evolution process of the optimal disturbance indicates that the optimal disturbance might have exploited the locally crossflow instability through traveling from the windward side to the leeward side. [ABSTRACT FROM AUTHOR]