Linear stability analysis of the onset dynamics of scramjet unstart

Recently, interest has increased in using scramjet engines as a means of long-range high-speed flights and economical access to outer space. One of the most perilous causes of scramjet malfunctions is the unstart event that is initiated by excessive heat release from combustion. When unstart occurs, a strong moving-shock structure is first formed in the engine, and the shock structure propagates upstream and finally spills out of the engine inlet. The unstart event is detrimental to the engine because (1) the moving-shock structure imposes high pressure and thermal loads on the inner walls of the engine during the unstart process, and the walls can be ruptured due to the loads; and (2) the airflow into the engine is greatly diminished when the shock structure is disgorged by the engine, leading to loss-of-thrust and engine stall. Because the probability of unstart grows with increasing heat release from combustion, the danger of unstart is an important limiting factor in the performance of scramjets. Therefore, the onset mechanisms of the unstart event need to be understood to prevent or delay the unstart process. However, the detailed dynamics has not been fully understood yet, even though many studies have examined unstart onset mechanisms. For instance, Korkegi (1975) suggested empirical correlation functions for estimating the critical pressure rise above which unstart occurs, based on the assumption that shockinduced flow separation of turbulent boundary layers on the engine walls causes the unstart process. In ground tests of the HyShot II scramjet model (Frost et al. 2009), the critical pressure rise in the model agreed with the Korkegi limit, and therefore the authors presumed that unstart was initiated by flow separation of the boundary layers. In a later ground experiment of HyShot II, however, Laurence et al. (2013) could not find large-scale boundary-layer separations, and they concluded that flow separation was not the main cause of unstart. Instead, they proposed thermal choking as the responsible unstart mechanism. However, further conclusions regarding the onset mechanism could not be drawn because of the limited diagnostics in the experiments. The primary objective in this study is to find the onset dynamics of the unstart event based on linearized system. Section 2 describes the linearized system dynamics that will be discussed throughout this study. In section 3, the detailed methodology and the scramjet configuration are presented. The linearized dynamics at the unstart onset point is then discussed in section 4. Finally, section 5 summarized the findings and make suggestions for future work.