Time Resolved Investigations of Streamtraced Inlet Restart Dynamics

Obtaining reliable restart of hypersonic inlets in the event of accidental unstart remains a key performance metric that challenge the operational boundary of powered hypersonic vehicles. The present work investigates the restart dynamics in a self-starting axisymmetric hypersonic Buseman inlet, which have a strong practical relevance in future platforms. The restart process was triggered by rapidly decreasing the aerodynamic blockage from a high bandwidth counter injected jet, which allows the examination of the restart dynamics without being masked by the back pressure transience. Two dimensional time resolved pressure fields over inlet and isolator surfaces and time resolved external shock field during the restart process are obtained at 10 kHz repetition rate. Both these measurements revealed a complex unstart shock motions during restart that include small amplitude relatively broadband oscillations prior to restart, a large amplitude periodic oscillation at the early restart phase, and subsequently followed by a dominant downstream shock motion. A complex flow structure within the external contraction portion of the inlet was discernable in the schlieren imagery with shear layer eddies being spilled out of the inlet. The pressure fields further evidenced a strong variation in the streamwise and azimuthal direction reinforcing the occurrence of flow spillage along both directions. Overall, the restart duration was determined to be approximately 11 ms, which is substantially greater than the unstart time scale of 8 ms obtained in the same inlet using the same back pressuring system. The critical process that extends the restart time is the periodic shock oscillations, which is termed as intermediate buzz in prior investigations, that pervade the first half of the restart process. The power spectral density of the unstart shock motions showed pointed to a shift in the mechanisms that drive the unstart shock motions during restart.