Flow Mechanism And Control Methods Of The Hypersonic Inlets With Two Or Three Dimensional Compression

Unstarted oscillation phenomena,with strong unsteady and destructive characteristics,comesfrom the abnormal operating states of the supersonic inlets and hypersonic inlets.Using the methods of theoretical analysis and wind tunnel test,this dissertation deals with the flow characteristics and dynamic mechanism of the unstart phenomena of the hypersonic inlets with two or three dimensional compression,of which new methods for buzz frequency prediction and oscillation controlling are carried out on the basis.At first,to enrich the understanding of hypersonic inlet unstart,a rectangular hypersonic inlet is tested at the low-speed mode(Ma 5),the design mode(Ma 6),and the over-speed mode(Ma 7).Results indicate that the unstart processes are basically the same for the low-speed mode and the design mode.An unstart phenomena dominated by the ramp-side flow separation,which exhibits violent fluctuations along the entire duct with a low frequency,is observed at these two operating modes.While operating at the over-speed mode,due to the existence of the detached cowl shock,the oscillation phenomena exhibits relatively gentle fluctuations and the characteristic frequencies are mainly determined by the shock train in front of the throat section.And then,a hypersonic inlet with side compression,which involved with three-dimensional compression,has been tested at a freestreem Mach number of 6.0 to enrich the understandings of hypersonic inlet unstart.The inlet operates in a big buzz mode and exhibits a series of unsteady flow patterns similar to those of rectangular hypersonic inlets in a buzz cycle.Once the separation bubble induced oblique shock is expelled over the cowl lip,a supersonic reverse flow is observed in the contracting part of the inlet which acts as an inverted supersonic nozzle.When the external unstart shock system moves back toward the cowl lip,two kinds of secondary oscillations with high dominant frquencies are observed in turn by the time-accurate pressure system.The two mentioned secondary oscillations are both acoustic resonance modes,and one is formed in the duct between the isolator exit and the leading edge of high density air column in the combustor whereas the other one is formed in the duct between the cowl lip and the inlet throat.Furthermore,a three-dimensional inward turning inlet has been tested at a freestreem Mach number of 5.0.During the unstart process with a relative high attack angle of 6 deg,the separation bubble located in the inlet entrance experience two totally different evolution processes at random,which makes a complex unsteady flow pattern combined by the relatively gentle fluctuation and the relative violent fluctuation.However,either with the increasing of the throttling ratio or the reducing of attack angle,such combination of two different fluctuations will be replaced by the kind of relative violent fluctuation with a unique base frequency gradually.Based on the previous study of the oscillation mechanism of the hypersonic inlets with two and three dimensional compression,a frequency prediction method is developed to predict the frequency of the hypersonic inlets buzz.It divides the single buzz cycle into two stages,which are the mass filling-up stage and the external shock motion stage.As to the mass filling-up stage,the analysis of the upstream moving process of the terminal shock system is replaced by the quantitative analysis of the stored air in the duct.And the stagnation speed of sound of the freestream is regarded as the maximum speed of the moving external shock system.Besides,the numerical method has to be calibrated by the previous and current experience data.It can be seen that,the prediction method can propose a rapid and accurate frequency of the unstarted hypersonic inlets.At last,a variable hypersonic inlet is designed to validate the oscillation flow control methods,which are mechanical control method based on a movable slot-plate and fluidic control method based on the high pressure air injection.With the help of the spillage offered by the slot-plate,the mechanical method can afford the discrepancy of the capture ability and the exhaust capability of airflow during this oscillation cycle,result in controlling the oscillation caused by the unstart.As to fluidic control method,the injected high pressure air can form a fluidic baffle-board,which turns the main flow out of the entrance.For that reason,the choking downstream is released and the oscillation flow can be controlled.In general,the two control methods with high practicality can offer efficient,general and fast responding control effects.