Receptivity Of Hypersonic Boundary Layers Due To Acoustic Waves Over Blunt Wedges

Boundary-layer transition is one of key issues that must be considered in the design of hypersonic aircraft.As the initial stage of natural transition,receptivity provides initial information such as the amplitude,frequency and phase of disturbances for the evolution of unstable waves in the boundary layer.For a blunt body,the upstream entropy gradient is significant after the bow shock,forming an entropy layer.The disturbance in the entropy layer have the potential to excite unstable waves in the boundary layer during downstream propagation.In this paper,using the theoretical method,the linear interaction between the acoustic wave and the bow shock is analyzed,the acoustic disturbance component and its distribution after the shock are obtained.After that,through direct numerical simulation,the acoustic receptivity process of the second mode of the blunt wedge boundary layer is studied,and multiple possible receptivity routes are analyzed in detail,The critical receptivity route that excites the second mode is identified.The main conclusions obtained are:1.When the slow acoustic wave is incident,the disturbance in the boundary layer has gone through three stages,namely,the entropy layer mode,the fast mode and the nonmodal disturbance,before the second mode is generated.The entropy-layer mode generates the second mode through the fast mode.The fast mode generated at the leading edge in synchronization with the fast acoustic wave can also directly excite the second mode through intermodal conversion downstream,but the efficiency of the second mode generated through intermodal conversion are quite low.The dominant receptivity route is accomplished by the extracted disturbance in the entropy layer.In addition,when the slow acoustic wave is incident,with the increasing of bluntness,the amplitude of the disturbance generated at the front edge becomes larger,and there emerges large-amplitude disturbance in the entropy layer downstream.2.When the fast acoustic wave is incident,the fast acoustic waves are generated after the bow shock.The fast sound wave synchronizes with the fast mode at the leading edge,the latter evolves in the boundary layer and eventually excite the second mode through intermodal conversion downstream.Compared with the case of the slow acoustic wave,it is found that the acoustic wave amplitude generated after the shock is larger when the fast acoustic wave is incident,and the synchronized fast mode undergoes a process of forced growth near the leading edge.As a result,the amplitude of the second mode excited by the fast acoustic wave is one order of magnitude larger than that of the slow acoustic wave.