Boundary layer suction timing and location effects on unstarting flows in a model scramjet

Abstract

The effects of the suction timing and location on inlet unstart were investigated to determine the necessary response time and appropriate location of a flow control device. Inlet unstart is one of the causes for scramjet-powered engine failure, and boundary layer suction could suppress or delay the phenomenon. A model scramjet having a rectangular cross-section was tested in a Mach 4.5 freestream via nitrogen-jet-injection-induced inlet unstart. High-speed schlieren imaging and pressure measurements were performed to study the characteristic behaviors of the unstarting and unstarted flows. Suction extracted approximately 1–2% of the freestream flow depending on the test conditions. Boundary-layer suction successfully prevented inlet unstart with the most upstream suction location for the jet momentum ratio near the unstart threshold condition but lost the control authority as the suction delay time increased. Suction was not able to prevent unstart but was delayed with downstream suction locations, and a similar deterioration in the suction control performance was observed an increasing suction delay time. For a jet momentum 10% higher than the threshold, suction could not prevent unstart but suppressed the propagation speed of the unstart shockwave. Fluctuations in the wall pressure were significantly reduced with suction. For all tested cases, better control performance was achieved with a faster suction trigger timing. The results from the flow visualization and pressure measurements indicated that suction must be applied before the unstart shockwave passes through the suction location to achieve effective flow control.