Identification and Assessment of Scramjet Isolator Unstart and Operability Metrics

Computational fluid dynamics (CFD) simulations play a strong role in the design and development of aerospace and defense vehicles, including high-speed applications where testing under the correct operational conditions is not yet viable. In this study, metrics for the onset of isolator unstart are identified. An assessment of the variance of operating variables and their impact on metrics for the onset of isolator unstart and operability metrics was performed, utilizing a nozzle–isolator assembly from NASA Langley Research Center as a demonstration case. The effects of increasing backpressure ratio and decreasing inflow Mach number on these metrics and the underlying contributions of shock physics were investigated in detail. A major conclusion from this study is that both inflow Mach number and backpressure ratio can strongly impact pseudo shock train and shock–boundary layer interactions inside the isolator, but inflow Mach number has a stronger impact than the backpressure ratio. The research presented in this paper demonstrates that the isolator performance can shift from start to unstarted and operable to inoperable with a small variance in operating conditions. Another important insight presented in this research is that the length of the pseudo shock train and the Mach stem height change discontinuously with both the backpressure ratio and the inflow Mach number. Therefore, the length of the pseudo shock train and height of the Mach stem are strong indicators of the onset of unstart, which is an important consequence for instrumentation and closed-loop adaptive feedback control system design for scramjet flight operations.