Effect Of Local Indentations On Subcritical Transition In Hypersonic Boundary Layers

Laminar-turbulent transition in hypersonic boundary layers is of fundamental importance in aerospace engineering applications.Using numerical simulations and theoretical analysis,the effect of local indentations on the subcritical transition in hypersonic boundary layers on a blunt plate with an oncoming Mach number of 6 and an angle of attack of-4~0 is studied.1.Based on the Harmonic linearized Navier-Stokes(HLNS)system,a numerical approach to calculate the optimal disturbance for elliptic problems is developed.The effect of local indentations on the linear evolution of the streaky structures is quantified.Simultaneously,the nonlinear evolution of streaks is studied by direct numerical simulations(DNS),together with the impact of local indentations.The numerical results indicate that the initial amplitude of the optimal disturbance determines the amplitudes and locations of the streaks when reaching the nonlinear saturation state;the mean-flow distortion induced by the local indentation always produces weak destabilizing effects on the streak amplitude.2.The impact of local indentations on the evolution of the secondary instability modes is studied by DNS method.It is found that for the most unstable sinuous mode,a moderate-sized indentation wound promote its amplitude,and the maximum destabilizing effect occurs when the streamwise length scale of the local indentation is comparable with the secondary-instability wavelength.