Impact Of Local Cavities On Laminar-turbulent Transition Of Hypersonic Boundary Layers

As the development of the aerospace technology,the hypersonic aircraft design has become a hotspot of the aerodynamics research.Laminar-turbulent transition in boundary-layer flows has significant impact on the friction and heat flux on the surface of the aircrafts,which however is attributed to a variety of factors.This paper studies the impact of rectangular cavities with different depths on the natural transition of a hypersonic boundary layer with oncoming Mach number 6 by direct numerical simulations.The impact mainly includes both the local scattering of the oncoming second-mode disturbance and the local receptivity to the freestream fast and slow acoustic waves as caused by the cavity-induced mean-flow distortion.It is found from the numerical results that,two regimes,depending on the depth of the cavities,emerge in all the three relevant processes,i.e.the mean-flow distortion,the local scattering of the oncoming second mode and the local receptivity to the freestream acoustic waves.For the shallow cavities,two independent separation bubbles appear near the walls on both sides of the cavities,and the mean-flow distortion,such as the pressure and the velocity,becomes stronger as the cavity depth increases;the consequent suppression effect of the cavities on the second-mode disturbance and the local receptivity to the freestream acoustic waves also increase their intensity with the increase of the cavity depth.For deep cavities,a large-scale separation bubble occupies almost the whole cavity,and as the cavity becomes further deeper,the mean-flow distortion is also weakened,which leads to a weaker suppression effect on the second-mode disturbance and a weaker receptivity to the freestream acoustic waves.