| The receptivity mechanisms of supersonic and hypersonic boundary layers to freestream disturbances are studied by using both linear stability theory (LST) and direct numerical simulations (DNS). Beside of perfect flow, a computational code is developed for numerical simulation of transient hypersonic nonequilibrium flow by combining a fifth-order shock-fitting scheme with additive semi-implicit Runge-Kutta (SIRK) methods. Meanwhile, a LST code based on multi-domain spectral method is developed to study the boundary-layer stability characteristics relevant to the receptivity study. Receptivities of supersonc and hypersonic boundary-layers to freestream disturbances are studied under three different flow conditions, i.e., a Mach 4.5 flow over a flat plate, a Mach 8.0 flow over a sharp wedge with half-angle 5.3°, and a Mach 10.0 oxygen flow over a flat plate. For each flow condition, linear stability characteristics of the boundary-layer wave modes and their mutual resonant interactions are studied by both LST and DNS. Then, four types of freestream disturbances, i.e, fast acoustic waves, slow acoustic waves, entropy waves and vorticity waves, are introduced in front of the oblique shock to investigate how freestream disturbances interact with the shock, penetrate the boundary layer and ultimately induce boundary-layer disturbances. The effects of incident wave angles, forcing wave frequencies, and wall temperature perturbation conditions on the receptivity are studied. For Mach 10.0 oxygen flow, receptivities are studied in both perfect gas and thermochemically nonequilibrium regime to investigate the real gas effects on the receptivity and stability of hypersonic boundary-layer flow. It is found that a family of stable wave modes play a very important role in the receptivity process of excitation of the unstable Mack modes. These stable modes are termed mode I, mode II, etc, which can interact with both fast acoustic waves and the Mack-mode waves. In receptivity to freestream slow acoustic waves, Mack mode waves are generated near the leading edge. The receptivity to freestream entropy waves or vorticity waves is mainly through the interaction between viscous boundary layer and fast acoustic waves. There is significant real gas destabilizing effect on the second-mode waves in Mach 10.0 oxygen flow over a flat plate. |
