Research On Hypersonic Inlet Unstart Multimodes And Mode Transition

Hypersonic inlet is one of the key components of the scramjet.For proper operation,the inlet must operate in a started mode.If an inlet is not started,the performance of the scramjet will be greatly reduced,and it may cause a flight accident.Theoretically,the hypersonic inlet unstart is inevitable.Affected by many factors,there are many modes of hypersonic inlet unstart.It is critical for the control of the hypersonic inlet unstart to master the hypersonic inlet unstart mechanism,and the characteristics of the hypersonic inlet operation mode transition.This paper focuses on the hypersonic inlet unstart,some studies are carried out as follows:For the inviscid flow and viscous flow,the hypersonic inlet unstart is studied.It is found that the boundary layer separation plays an important role in hypersonic inlet unstart for viscous flow.The operation mode of the hypersonic inlet at the overspeed condition is then analyzed.When there is a large scale separation at the cowl,a bow shock is generated before the cowl lip.This new unstart mode is defined as local unstart.Unsteady numerical simulations are performed to study the pulse-starting process of a hypersonic nozzle coupled with a simplified inlet model in a shock tunnel,and the pulse-starting process of a full size hypersonic inlet with a simplified nozzle startup wave system.The local unstart of hypersonic inlet at the overspeed condition is observed.By steady analysis,it is found that the local unstart of inlet essentially results from the transition of the external compression shock reflection from regular reflection to Mach reflection at the cowl.Comparing the inviscid flow and viscous flow,the flow structure at the cowl is changed from shock/expansion wave interaction to shock/boundary layer interaction.Thus the conditions of the local unstart are different for the inviscid flow and viscous flow.By changing the angle of the cowl and the downstream pressure,the hypersonic inlet local unstart/start transition is observed,and a hysteresis exists in the transition.For the low-speed condition and overspeed condition,the processes of the hypersonic inlet unstart induced by fuel injection with low total enthalpy inflow and high total enthalpy inflow,and fuel combustion with high total enthalpy inflow are numerically studied.It is found that the ratio between the fuel mass flow rate and the scramjet air mass flow rate triggering unstart for fuel injection with high total enthalpy inflow is larger than the other cases.And the oscillation during the shock train propagating upstream is more obvious.For the overspeed condition,the separation on the upper wall is more serious.Finally,a bow shock is generated before the cowl lip,the local unstart occurs.When the combustor backpressure oscillates at the low-speed condition and overspeed condition,the shock train oscillates with similar frequency of backpressure.But there is a lag between the shock train oscillation and the dynamic backpressure,especially for high frequency,the phase may be opposite.There is a hysteresis during the shock train oscillation.With the amplitude of the dynamic backpressure increasing,the range of the shock train oscillation increases.With the frequency of the dynamic backpressure increasing,the range of the shock train oscillation increases firstly and decreases subsequently.With the frequency of the dynamic backpressure increasing,the maximum transient backpressure that the hypersonic inlet can resist is increased.For the overspeed condition,the lower wall pressure oscillates more severely,and the hysteresis during the shock train oscillation is always clockwise.However,for the low-speed condition,the hysteresis is counterclockwise when the frequency of the dynamic backpressure is high.Numerical simulations are performed to study the catastrophe characteristics of the hypersonic inlet generic unstart/start transition,local unstart/start transition and local unstart/generic unstart transition.The catastrophe theory,which is an important mathematical method to study the catastrophe and the discontinuous change,is introduced.The catastrophe model of the hypersonic inlet operation mode transition is analyzed.Because the mechanism of the hypersonic inlet flow is complicated,it is challenging to determine the catastrophe model.With the help of numerical simulations,the characteristics of the hypersonic inlet operation mode transition induced by the variation of freestream parameter and backpressure are obtained.The accessible routes for the hypersonic inlet unstart/start transition are studied.The characteristics of the hypersonic inlet geneic unstart and local unstart is analyzed.Then a predictor-corrector method based on the inertial navigation system and forebody surface pressure measurements is proposed to estimate the air mass flow rate of the scramjet,and it is applied to detect the hypersonic inlet unstart.The hypersonic inlet unstart detection methods based on the time domain and frequency domain characteristics of surface pressure near the inlet throat are introduced.Applying the time series analysis,the hypersonic inlet unstart detection methods based on the AIC value,residual-variance of AR model and K-L distance are proposed.For the local unstart,the wall pressure increases more obviously,while the oscillation energy is more dispersed,and there is no obvious dominant frequency.Based on these characteristics,the local unstart and generic unstart of hypersonic inlet can be accurately monitored and identified with the detection methods based on the pressure amplitude and power spectrum.