Oscillation Characteristics Of Shock Train And Its Suppression In An Isolator With Incident Shock

As one of the important parts of scramjet,the isolator plays a vital role in engine working.On the one hand,the isolator further compresses the airflow captured by the inlet to provide a stable airflow suitable for combustion.On the other hand,it prevents the high pressure and disturbance generated by the combustion chamber from being transmitted to the inlet to avoid the occurrence of inlet unstart.The function of the isolator is mainly realized by shock train,so the oscillation characteristics of the shock train directly affect the safe and stable operation of the isolator.The shock waves generated by the upstream inlet will enter the isolator,forming incident shock and causing non-uniform flow in the isolator,thereby affecting the oscillation characteristics of shock train.So far,the oscillation characteristics of shock train in the isolator with incident shock are still not clear.Moreover,the change of incoming flow during the actual flight of engine will inevitably lead to the continuous variation of incident shock.However,due to the difficult of changing incoming flow during experiments,no public reports on the experimental study of isolators with changing incident shock have been seen.In this dissertation,a direct-connected variable Mach number experimental system has been built to realize the variation of incident shock.Relying on this experimental system,the oscillation characteristics of shock train in an isolator with incident shock have been studied,and the fluidic control method has been used to suppress the shock train oscillation.The following works have been conducted:Firstly,the oscillation of shock train in isolators with fixed incident shock and backpressure is studied.According to the asymmetric structure change of shock train during the oscillation,the oscillation can be divided into three modes.The pressure gradient on the isolator wall caused by the incident shock affects the oscillation intensity of shock wave train.Through the correlation analysis and phase analysis of the wall pressure,it is found that the cause of shock train oscillation is the instability of the separation zone induced by the leading shock.The pressure at the position of shock train leading edge on the top and bottom walls are different,resulting in the asymmetric structure of shock train.The oscillation mode of shock train can be judged by calculating the pressure difference at the shock train leading edge on the top and bottom walls during the entire oscillation process.Secondly,the oscillation of shock train in isolators with fixed incident shock and oscillating backpressure is studied.The oscillation can be divided into two types.The oscillation amplitude of shock train caused by type Ⅱ oscillation is much higher than that of type Ⅰ oscillation,therefore type Ⅱ oscillation has a greater impact on the safe operating boundary of the engine.The shock train oscillation is driven by the backpressure change.However,Type Ⅰ oscillation is mainly affected by backpressure and limited affected by the upstream conditions of shock train,while Type Ⅱ oscillation is generated when the shock train passes through the negative pressure gradient region near the upstream shock reflection point during the shock train oscillation.Both types of oscillations are accompanied by oscillations caused by inherent unsteadiness of the shock train.The change of backpressure amplitude affects the occurrence of two types of oscillations,and has different effects on the amplitudes of two types of oscillations.Thirdly,the study on the motion characteristics and oscillation characteristics of shock train in the isolator with varying incident shock and backpressure is carried out.By changing the incoming flow Mach number,the incident shock varies continuously and causes the movement of the background waves in the isolator,thus causing the movement of shock train.Hysteresis occurs in shock train position during the variation of incident shock.The reason for the hysteresis is that the negative pressure gradient region causes a double solution of flow field in the isolator.When the incident shock and backpressure change simultaneously,the shock train exhibits complex motion.Three factors affect the movement trend of shock train,and the influence of each factor change with the change of incoming flow Mach number and backpressure.In an isolator with varying incident shock and linearly changing backpressure,the shock train oscillation is still caused by the instability of the separation zone after the leading shock,and the oscillation intensity increases with the increasement of Mach number.In an isolator with varying incident shock and oscillating backpressure,the shock train oscillation is dominated by the backpressure change.There is no obvious relationship between the change of shock train oscillation amplitude and the change of Mach number.Finally,research on fluidic control methods to suppress shock train oscillations is carried out.Through the comparison of various fluidic control methods,considering the oscillation mechanism of the shock train in the isolator with incident shock,the distributed suction and jet method is selected for research.The distributed suction and jet method can achieve the suppression of negative pressure gradient of the isolator wall through two effects,and thus effectively suppress the oscillation intensity of shock train.When the fluidic control is applied,the downstream disturbance can affect the upstream area of shock train through the jet,but the pressure oscillation caused by jet is weaker than the pressure oscillation caused by shock train oscillation.By suppressing the oscillation of the shock train,the safety margin of the scramjet can also be increased.