Fast-acting boundary-layer suction to control unstarting and unstarted flows

Transient behaviors of unstarting and unstarted flows have been experimentally investigated in a Mach 6 freestream flow with and without boundary-layer suction in a scramjet isolator. The test scramjet model has a rectangular cross section and simulates the full flowpath of a scramjet engine. The suction control is activated by a fast-acting solenoid valve while an unsteady unstarting flow evolves in the model scramjet. A nonreacting jet is injected into the model combustor to mimic the flow choking of unstarting scramjets. The choked flow then initiates an unstart shock wave that reaches the inlet and induces a downstream flow transition from supersonic to subsonic. The temporal evolution of the unstart process has been visualized by high-speed schlieren imaging. The flow visualization shows a strong dependency of the unstarting flow on the local flow conditions, such as static/stagnation pressure gradients and boundary-layer thickness. The suction of 2-3% of the intake mass flow rate delays or pauses the inlet unstart process, and it reduces the restarting process hysteresis. The unstart process is either effectively ceased or marginally delayed, depending on the suction timing with respect to the unstart process.