The instability of the Mack mode is destabilized by wall-cooling in a high speed boundary layer. The aim of this paper is to study the mechanism of the wall cooling effect on the Mack mode instability by numerical met...The instability of the Mack mode is destabilized by wall-cooling in a high speed boundary layer. The aim of this paper is to study the mechanism of the wall cooling effect on the Mack mode instability by numerical methods. It is shown that the wall-cooling can destabilize the Mack mode instability, similar to the previous conclusions with the exception that the Mack mode instability can be stabilized by wall-cooling if the wall temperature is extremely low. The reversed wall temperature is related to a freestream condition. If the Mach number increases to a large enough value, e.g., about 7, the reversed wall temperature will tend to be zero. It seems that the Mack mode instability is determined by the region between the boundary layer edge and the critical layer. When the wall temperature decreases, this region becomes wider, and the boundary layer becomes more unstable. Additionally, a relative supersonic unstable mode can be observed when the velocity of the critical layer is less than 1 - liMa or is cancelled by the wall-cooling effect. These results provide a deeper understanding on the wall-cooling effect in high speed boundary layers.展开更多
The evolution of Gortler vortices and its interaction with other instabilities are investigated in this paper. Both the Mack mode and the Gortler mode exist in hypersonic boundary-layer flows over concave surfaces, an...The evolution of Gortler vortices and its interaction with other instabilities are investigated in this paper. Both the Mack mode and the Gortler mode exist in hypersonic boundary-layer flows over concave surfaces, and their interactions are crucially important in boundary layer transition. We carry out a direct numerical simulation to explore the interaction between the GOrtler and the oblique Mack mode. The results indicate that the interaction between the forced Gortler mode and the oblique Mack mode promotes the onset of the transition. The forced oblique Mack mode is susceptible to nonlinear interaction. Because of the development of the GOrtler mode, the forced Mack mode and other harmonic modes are excited.展开更多
Properties of wall pressure beneath a transitional hypersonic boundary layer over a 7°half-angle blunt cone at angle of attack 6°are studied by Direct Numerical Simulation.The wall pressure has two distinct ...Properties of wall pressure beneath a transitional hypersonic boundary layer over a 7°half-angle blunt cone at angle of attack 6°are studied by Direct Numerical Simulation.The wall pressure has two distinct frequency peaks.The low-frequency peak with f≈10−50 kHz is very likely the unsteady crossflow mode based on its convection direction,i.e.along the axial direction and towards the windward symmetry ray.Highfrequency peaks are roughly proportional to the local boundary layer thickness.Along the trajectories of stationary crossflow vortices,the location of intense high-frequency wall pressure moves from the bottom of trough where the boundary layer is thin to the bottom of shoulder where the boundary layer is thick.By comparing the pressure field with that inside a high-speed transitional swept-wing boundary layer dominated by the z-type secondary crossflow mode,we found that the high-frequency signal originates from the Mack mode and evolves into the secondary crossflow instability.展开更多
A piecewise acoustic metasurface is designed to suppress the first mode while marginally amplifying the Mack second mode in a Mach 4 flat-plate boundary layer(BL)flow.The results of linear stability theory(LST)and the...A piecewise acoustic metasurface is designed to suppress the first mode while marginally amplifying the Mack second mode in a Mach 4 flat-plate boundary layer(BL)flow.The results of linear stability theory(LST)and the eN method demonstrate the stabilization effect and transition delay performance,respectively.However,the direct numerical simulation(DNS)results indicate that the designed broadband acoustic metasurface actually weakly excites the first mode with a slightly larger fluctuating pressure amplitude at the surface,which is in contrast to the analysis of LST.The discrepancies are found to lie in the‘roughness’effect caused by the recirculation zones inside the microslits and the alternating expansion and compression waves induced at the slit edges,which significantly amplifies the first mode.For further clarification of the competitive mechanism between the acoustic stabilization and‘roughness’destabilization effects of metasurfaces on the first mode,a carefully designed metasurface is installed at the maximum growth rate region,which excites the first mode on the metasurface but inhibits its development downstream.展开更多
基金Project supported by the State Key Program of National Natural Science Foundation of China(No.11332007)the Young Scientists Fund of the National Natural Science Foundation of China(No.11402167)
文摘The instability of the Mack mode is destabilized by wall-cooling in a high speed boundary layer. The aim of this paper is to study the mechanism of the wall cooling effect on the Mack mode instability by numerical methods. It is shown that the wall-cooling can destabilize the Mack mode instability, similar to the previous conclusions with the exception that the Mack mode instability can be stabilized by wall-cooling if the wall temperature is extremely low. The reversed wall temperature is related to a freestream condition. If the Mach number increases to a large enough value, e.g., about 7, the reversed wall temperature will tend to be zero. It seems that the Mack mode instability is determined by the region between the boundary layer edge and the critical layer. When the wall temperature decreases, this region becomes wider, and the boundary layer becomes more unstable. Additionally, a relative supersonic unstable mode can be observed when the velocity of the critical layer is less than 1 - liMa or is cancelled by the wall-cooling effect. These results provide a deeper understanding on the wall-cooling effect in high speed boundary layers.
文摘The evolution of Gortler vortices and its interaction with other instabilities are investigated in this paper. Both the Mack mode and the Gortler mode exist in hypersonic boundary-layer flows over concave surfaces, and their interactions are crucially important in boundary layer transition. We carry out a direct numerical simulation to explore the interaction between the GOrtler and the oblique Mack mode. The results indicate that the interaction between the forced Gortler mode and the oblique Mack mode promotes the onset of the transition. The forced oblique Mack mode is susceptible to nonlinear interaction. Because of the development of the GOrtler mode, the forced Mack mode and other harmonic modes are excited.
基金the National Key Research and Development Program of China 2016YFA0401200 and 2019YFA0405200the National Numerical Wind tunnel(NNW)project,and National Natural Science Foundation of China under contract 11702307.
文摘Properties of wall pressure beneath a transitional hypersonic boundary layer over a 7°half-angle blunt cone at angle of attack 6°are studied by Direct Numerical Simulation.The wall pressure has two distinct frequency peaks.The low-frequency peak with f≈10−50 kHz is very likely the unsteady crossflow mode based on its convection direction,i.e.along the axial direction and towards the windward symmetry ray.Highfrequency peaks are roughly proportional to the local boundary layer thickness.Along the trajectories of stationary crossflow vortices,the location of intense high-frequency wall pressure moves from the bottom of trough where the boundary layer is thin to the bottom of shoulder where the boundary layer is thick.By comparing the pressure field with that inside a high-speed transitional swept-wing boundary layer dominated by the z-type secondary crossflow mode,we found that the high-frequency signal originates from the Mack mode and evolves into the secondary crossflow instability.
文摘A piecewise acoustic metasurface is designed to suppress the first mode while marginally amplifying the Mack second mode in a Mach 4 flat-plate boundary layer(BL)flow.The results of linear stability theory(LST)and the eN method demonstrate the stabilization effect and transition delay performance,respectively.However,the direct numerical simulation(DNS)results indicate that the designed broadband acoustic metasurface actually weakly excites the first mode with a slightly larger fluctuating pressure amplitude at the surface,which is in contrast to the analysis of LST.The discrepancies are found to lie in the‘roughness’effect caused by the recirculation zones inside the microslits and the alternating expansion and compression waves induced at the slit edges,which significantly amplifies the first mode.For further clarification of the competitive mechanism between the acoustic stabilization and‘roughness’destabilization effects of metasurfaces on the first mode,a carefully designed metasurface is installed at the maximum growth rate region,which excites the first mode on the metasurface but inhibits its development downstream.
