Research On The Separation Phenomena And Mechanism Of Over-expanded Single Expansion Ramp Nozzle For RBCC Engine

This paper studies the separation patterns and separation pattern transitions of single expansion ramp nozzle(SERN)under the over-expanded condition with numerical simulations,theoretic analysis and wind tunnul experiments.Firstly,the separation patterns in straight wall nozzles have been summarized.It is different from the traditional rocket nozzle that the over-expanded flowfield in SERN presents high asymmetry.The free shock separation(FSS)is the most common separation pattern in rocket nozzle,and restricted shock separation(RSS)just appears in the special configurations.However,in SERN,RSS(ramp)is the most common separation pattern.In the numerical simulation results,two special separation patterns,FSS and RSS(flap),have also been discovered.In the flowfield of over-expanded rocket nozzle,there is a large normal shock,so the separation shock on the expansion wall is small and the separation regions are restricted.For SERN,the separation shocks,oblique shocks,are the most important shocks in the flowfield and the normal shock is restricted.Because of lacking the effects of normal shock,there are much more differences between the several separation patterns in SERN.Then,the processes of separation pattern transitions in straight wall nozzles have been researched.The separation pattern transitions in rocket nozzle will cause the side load and the side load maybe damages the structure of nozzle.The side load appears in separation pattern transition in SERN as well,but the side load is lift for the SERN.The side load will cause the challenge of flight control and the nozzle structure.In the processes of startup,shutdown,accelerating,decelerating,the development process of the flowfield are different,because the mechanisms of the interaction with the separation zones and the environment gas are different.The startup and shutdown processes are not the simple inverse processes.Because of the existence of shock structures,there will be a hysteresis loop and special processes of separation pattern transitions.And then,the results in straight wall nozzle have been generalized into the large area ratio SERN.The major effects on the flow separation patterns in the over-expanded SERN have also been discussed.In the results of truncated ideal contour nozzle and cubic curve contour,the separation pattern of RSS(flap),RSS(ramp)and FSS are also been observed.The external Mach number makes more influence on the separation pattern transitions.The initial expansion angle and the length of flap will affect the type of the separation pattern.The length of nozzle will affect the transition NPR or transition Mach number.Finally,the nozzle designed under Ma6 condition is given.The process of external Mach number varying linearly has been simulated by numerical method.The process of the vehicle flying with the real trajectory has also been simulated by numerical method.From the results,it can be found that the transformation law of the external Mach number will not affect the type of separation pattern transition,but it will affect the transition Mach number and the performance before and after the transition.