The Dynamic Characteristics Of Shock Train With Upstream Shock Waves And Downstream Pressure Perturbations

The oblique shock train(OST)is a complex flow phenomenon which involves an interaction between the restricted boundary layer and shock wave in many fluid devices such as hypersonic inlet/isolator,supersonic wind tunnel diffusers.In addition to the complex flow conditions,the shock train also shows the unsteady characteristics.A shock train in the duct tends to oscillate,it shows the unsteady behavior even under steady flow condition.The unsteady shock train and boundary layer interactions(SBLI)are sensiltive to both the upstream shock waves and downstream pressure perturbations.The unsteady phenomenon leads to large-scale of undesirable oscillations,which would cause the structure flutter and unstart of the propulsion system.Therefore,it is of great significance to study the dynamic characteristics of the OST.Firstly,transient pressure measurements,Schlieren system as well as high-speed particle image velocimetry and shear-sensitive liquid crystal technology have been used to describe and rebulid the three-dimensional flow structure of the Ma2.7 ducted flow.The startup process of laval nozzle,the shock reflection hysteresis and the oblique shock train self-excited oscillation are investigated to deepen the cognition of the unsteady characteristics.Secondly,the characteristics of OST forced oscillation are investigated detailed which include the propagation of downstream pressure perturbations,the dynamic characteristics and flow structures of unsteady OST forced oscillation,the impacts of the OST forced oscillation and the modelling analysis of the OST forced oscillation.The downstream periodic pressure perturbations propagate upstream to affect the oblique shock and wall pressure fluctuations through the separation region and subsonic sheer layer.In response to downstream periodic pressure perturbations,the OST is moved upstream and downstream periodically,so that the relative strength of OST matches the imposed pressure variations.The behavior of the unsteady SBLIs structures are very sensitive to the dynamics of shock motion.Changes in relative shock strength that caused by shock velocity and shock position have a dominant effect on the unsteady SBLIs structure.As the back pressure frequency/amplitude have slightly effect on the unsteady SBLIs structure,which indicates that the OST could be treat as the behavior of rigid motion in the duct during the shock motions.Then,to study the interaction of the OST and upstream shocks,ramp/cavity/bump are used to produce different pressure gradient variations of upstream shock waves.The rate of movement of the oblique shock train increases rapidly when it passes through the separation regions,and the deflection direction of the asymmetric separation switches.As the adverse pressure gradient of the ducted flow with upstream shock waves,a hysteresis exists between the OST and upstream shock interactions.A active flow control concept of double ramped ducted flow has been proposed to change the deflection direction of the asymmetric OST by producing faint asymmetric upstream shock waves.Moreover,the response of an OST and different upstream shocks interaction to downstream periodic pressure perturbation have been investigted experimentally.With upstream shock intreaction,the OST was forced to oscillate with the same frequency to downstream pressure perturbation.During the uniform ducted flow,the amplitude of forced oblique shock train oscillation was decreased with the increasing frequency.While in the ducted flow with a ramp,the amplitude of forced shock train oscillation was decreased rapidly and was almost the same with the increasing frequency.At last,the characteristics of the separation shocks forward propagation with upstream shock waves and the switch phenomenon in Ma5 flow have been conducted experimentally in a simple symmetric duct model.With higher and steady backpressure condition,the separation shocks move upstream to the ramp and cause the incident shocks to oscillate.With the overflows of the ramps,a separation shocks deflection switch phenomenon is found to occur randomly during the incident shock oscillation.