Modal Analysis of Unstarting Flows Induced by Mass Addition in a Scramjet Combustor

A well-designed Supersonic/Hypersonic air-breathing engine has the intake operating in a started mode to provide a high-performance compression process and generate low drag. Engine unstart, however, leads to reduced air mass capture and low total pressure recovery. Unstart manifests as the disgorging of the shock train system in the engine inlet and isolator, resulting in mass spillage and shock oscillations in the forebody. The downstream flow choking, caused by the pressure rise due to combustion, initiates a boundary layer separation and may onset an unstart. While prior investigations predominantly employed mechanical flaps to understand this evolution and spillage, our study focuses on the characteristics of unstarting flows induced by mass injection. Experiments were conducted at the Indian Institute of Science's Scramjet (Direct connect mode) facility. A sonic nitrogen jet was injected from a 1 mm slot into a Mach 2.2 crossflow transversely from the bottom wall. Reynolds number based on the jet location was 12.32 million. Five jet momentum ratios were tested. Key observations from the pressure and shadowgraph data, and the results from a Proper Orthogonal Decomposition analysis of the snapshots, will be presented at the conference.