Behaviors Of Shock Train In Isolator With Background Shocks And Its Control

Hypersonic inlet and isolator are the key compression components of scramjet engine,the internal flow of them is crucial for the performance and stable operation of scramjet engine.When these two components work cooperatively,the shock wave and expansion waves inside the inlet,termed as background shocks,will inevitably affect the shock train in the isolator.Therefore,the current paper mainly focuses on the behaviors of shock train under the interference of background shocks.Firstly,the generation mechanisms of the background shocks are analyzed and the characteristics of background shocks under two typical inflow conditions of isolator with the incoming Mach number M₀ of 4.92 are carried out.When the inflow is under small separation state,double shocks reflect alternately in the isolator,and the flow in the isolator behaves symmetrically.When the inflow is under large separation state,there’s only single shock which reflects in the isolator,turning the entire duct flow upward and downward alternately.For both flow states,the background shocks attenuate gradually along the flow direction.Meanwhile,due to the hybrid interference between the cowl shocks and the boundary layers near the side and bottom walls,supersonic corner vortex is generated.In general,the evolution histories of the corner vortex under the influence of the background shock waves can be classified into two types,namely,from the adverse pressure gradient region to the favorable pressure gradient region and the reversed one.The Hopf bifurcation occurs and a limit cycle appears at the zero-pressure gradient point.It’s found that the limit cycle is related to the nonlinear high order terms of the velocity expression.In addition,three conservation laws inside the limit cycle are derived.Then,the behaviors of shock train under three entrance Mach numbers,i.e.M_in=2.54,2.98 and3.46,in the hypersonic inlet/isolator model are experimentally investigated.The results demonstrate that the shock train in the hypersonic inlet/isolator departs from that with simple inflow condition:When the entrance Mach number M_in is 2.54,the shock train appears to be asymmetrical owing to the asymmetrical effect of the bottom and top wall boundary layer;whereas the shock train experiences four stable and three unstable interactions due to the stronger background shocks under M_in=2.98 and3.46.These stable/unstable interactions arise alternatively.Besides,the background shocks can shorten the shock train and enhance the compression capability of the isolator.Moreover,the curvature effect on the internal flow of inlet/isolator with the incoming Mach number of 4.92 is experimentally,numerically and theoretically discussed.As the duct curvature increases,the background shocks get stronger and the left-running expansion waves become the dominated expansion waves in the isolator.It’s found that the flow patterns show apparent differences when the shock train interacts with the background shocks:In small curvature isolators,the massive separation of shock train situates near the top or bottom wall,respectively,whereas the large separation always locates near the bottom wall in large curvature isolators.As for the unstable shock train/background shock interaction,violent flow oscillations may appear in small curvature isolators,whereas rapid forward movement occurs in large curvature isolators.In addition,the radial pressure gradient direction changes from being centripetal to being centrifugal alternatively in small curvature isolators,whereas the direction keeps centripetal in large curvature isolators.Meanwhile,the amplitude of radial pressure gradient increases with the duct curvature.However,the streamwise adverse pressure gradient in the shock train region decreases with the duct curvature,and the maximum sustainable backpressure ratio decreases with a percentage as high as 17%of that of straight isolator.In general,large duct curvature has detrimental effects on the shock train,but adequate duct curvature(e.g.the curvature radius between 20⁵0H_iso)may be beneficial since it can suppress the fluctuation level to some extent.Based on the experimental results,it’s discovered for the first time that the shock train exhibits energy-level-transition-like phenomenon under the influence of background shocks.The energy level corresponds to the stable background shock/shock train interaction.A prediction method of shock train patterns under different energy levels are introduced,which is based on the pressure difference induced by the background shock as well as the resistance difference of the top and bottom boundary layer.When the shock train is under one energy level,it has a stable margin to some extent.When the shock train migrates from one level to another,three transition processes exist,i.e.oscillation,rapid forward movement,and relatively smooth motion.The external force of these motions is considered to be the local boundary layer near the reflection point of background shock.The specific motion depends on the intensity of the leading shock of shock train as well as the features of the local boundary layer.At last,a fluidic ramp flow control concept has been proposed to suppress the violent low frequency oscillation during unstable background shock/shock train interaction.The experimental results reveal that the background shocks are rebuilt adjustably and get weakened.Besides,the shock train moves upstream smoothly with less unstable interaction.A slight oscillation with the dominant frequency of 655Hz occurs only when the shock train locates very upstream.