Investigation Of The Shock Train Instability And Control Method In Scramjet

The scramjet compresses the incoming flow by the forebody and inlet,resulting in the existence of shock-wave/boundary-layer interaction and flow distortion caused by many reflected shock waves in the inlet/isolator.These factors make the flow in the engine have nonlinear characteristics,which leads to some abrupt changes and bifurcations.However,most of the researches on the inlet/isolator are carried out in an ideal uniform flow field,ignoring the complex flow field in the real scramjet.In this situation,the potential issues caused by the complex flow field cannot be exposed.Therefore,this topic focuses on the study of the shock train instability in the hypersonic inlet/isolator and its impact on the scramjet.Based on the influences of the flow field,the rules are summarized and the characteristic parameters of shock train motion are analyzed.Then the dynamic characteristics of the shock train,the dynamic mechanism and the criterion of the unsTablemotion are analyzed.Finally,the margin control method considering the impact of incident shocks for scramjet is proposed.To provide a solution to the influence of the unsTableshock train motion on the engine,the following works are carried out in this subject:Firstly,the study on the influences of the unsTableshock train motion on the dual-mode scramjet is carried out.On the one hand,the state of the intake system affects the safety of the engine.In addition,it affects the downstream combustion,that is,indirectly affects the engine performance.When the incident shocks exists,the mass flow at the exit of the isolator fluctuates violently during the acceleration period.However,from the ground combustion tests it can be found that the existence of the incident shocks do not affect the thrust of the engine.The shock-wave/boundary-layer interaction regions locate near at the entrance of the isolator when the angle of attack is large and the flight Mach number is low.When the shock train locates in this region,it moves forwards quickl y.The formation of large-scale separation at the entrance leads to the inlet unstart.Then based on the development process of ramjet engine control and the impact of shock train motion on the engine,the necessity of the shock train control in scramjet engine is demonstrated.Secondly,considering the different effects of the unsTableshock train motion on the scramjet,the influence of the flow field evolution in the isolator on the shock train motion is studied.In the ascent process,the varying flow conditions will lead to different distributions of the shock-wave/boundary-layer interactions in the isolator.At the entrance of the isolator,if the shock-wave/boundary-layer interaction region exists simultaneously at the upper and lower walls,the shock train oscillates greatly.By the analysis of the influence of backpressure on the shock train motion,it can be seen that the rising rate of the backpressure has an important influence.When the dominant frequency of the backpressure is higher than that of the disturbance caused by the shock-wave/boundary-layer interaction,the movement of the shock train will be dominated by backpressure.When the incident shocks disturbs the shock train motion,the fluctuation of the downstream backpressure is less obvious than that of the wall-pressure upstream.In addition,when the oscillating backpressure is considered,the disturbance of the incident shocks does not have a serious impact on the pressure change in the combustor,which further explains why the incident shocks does not affect the thrust of the engine.According to the analysis of characteristic parameters,the unstableshock train motion is related to the flow parameters in the boundary layer.Thirdly,the mechanism of the unsTableshock train motion is studied deeply.The influence of the wall-pressure gradient along the tunnel is added to the Billig correlation.The modified model can qualitatively predict the shock train trajectory with the influence of the incident shocks.The mechanism modeling method for the shock train is proposed,and further the modeling work is extended to the flow field with incident shocks.The dynamic model can qualitatively characterize the unsTableshock train motion with the assistance of the flow parameters obtained by numerical simulations.The linear stability analysis shows that with uniform flow field there is a sTablereal pole and a pair of sTableconjugate poles in the shock train system.However,when the incident shocks exists,the maximum pressure that the boundary layer can sustain is highly nonlinear.The change in the sign of the pressure gradient causes the system structure to vary from sTableto unstable,and then an unsTablepole appears in the shock train system and dominates the shock train movement.Finally,according to the controllability analysis,the sudden movement is uncontrollable,while the oscillation part is still controllable but the fast response of the control system is required.According to the observability of the shock train,margin monitoring relies heavily on the flow parameters along the tunnel.The influence of the unsTableshock train motion on the control system is analyzed.It is found that the unsTablemotion would seriously affect the anti-interference ability of the control system.Combined with the instability mechanism and instability criterion,a margin control method for scramjet is proposed.The control method is validated on a direct-connected wind tunnel,and it is preliminarily proved that the method could effectively realize the margin control and ensure the safe and efficient operation of the scramjet engine.