Study On Receptivity Of A Hypersonic Boundary Layer Over A Blunt Body Considering Entropy Layer

The boundary-layer transition is an important but unsolved problem in aerocraft design.The understanding of its receptivity is the precondition of studying transition mechanism.The receptivity,which refers to the process that external disturbance penetrate into the boundary layer to trigger unstable wave,provides some key information of the unstable wave in the boundary layer such as its frequency and initial amplitude.For a high-speed vehicle,the free-flow disturbance first interacts with the bow shock,then the transmitted multi-waves behind the shock penetrate into the boundary layer to trigger unstable wave.For a blunt body,there is an entropy layer formed behind the shock.In this thesis,the entropy-layer instability in the supersonic blunt plate and hypersonic blunt cone boundary layer is studied.Taking a plane slow acoustic wave as the free-stream disturbance,the receptivity problem of the blunt-cone boundary layer is studied.Unlike the published studies of high-speed receptivity,the work in this thesis reveals the internal mechanism of the intermediate process that the slow acoustic wave excites the boundary-layer unstable wave,analyses the possible excitation paths in detail,and furthermore identifies the dominant receptivity mechanism.The main work and conclusions are as follows:1.The flow field behind the shock wave can be divided into the outer layer,the entropy layer and the boundary layer.According to the results of direct numerical simulation,two more reasonable methods for identifying the edges of the boundary layer and the entropy layer are presented.The flow in the entropy layer is a rotational flow with large entropy gradient.The entropy-layer edge on the blunt plate is almost parallel to the boundary-layer edge,while the entropy layer on the blunt cone gradually merges with the developing boundary layer in the downstream.2.The unstable entropy-layer mode with low frequency and small growth rate exists in the entropy layer,which is distributed near the nose.For the blunt plate,the downstream evolution of the entropy-layer mode is consistent with that predicted by the linear theory.In this case,there is no unstable two-dimensional T-S wave,which indicates there is no excitation of the unstable wave.For the blunt cone,the entropy-layer mode can excite the unstable first mode in the boundary layer near the neutral point.3.The theory for the interaction between the slow acoustic wave and the shock is applied to the model of a blunt cone.It shows that the disturbance field behind the shock wave can be divided into three regions according to the generation of acoustic wave,which refers to Zone 1 near the nose,Zone 2 in the downstream and Zone 3 in further downstream.In the Zone 1,the disturbances consist of fast acoustic wave,entropy wave and vorticity wave.In the Zone 2,there is no acoustic wave because the angle of the incident wave and the shock exceeds the critical angle,and only entropy wave and vorticity wave are generated.In the Zone 3,the disturbances consist of slow acoustic wave,entropy wave and vorticity wave.The result of the direct numerical simulation shows that the unstable wave in the boundary layer is excited in the downstream.The receptivity coefficient is defined by wall pressure fluctuation at the neutral point.4.According to the theoretical analysis,three possible paths that the disturbance behind the shock excites the boundary-layer unstable waves are further studied.The results shows that the slow acoustic wave in the Zone 3 plays a leading role in the whole receptivity process;the entropy-layer mode in the Zone 2 plays a secondary role;and the excitation efficiency of the fast acoustic wave in the Zone 1 is so small that the fast acoustic wave nearly has no contribution to the excitation of the unstable wave.