Mixed compression inlets offer a potential increase in pressure recovery compared to conventional external compression inlets at Mach numbers above two. However, these inlets suffer from problems with shock boundary layer interactions which cause flow instabilities and severe performance reductions. Previous experiments conducted at the University of Michigan used a wind tunnel with glass side walls and an extensive test section to measure the shock boundary layer interaction associated with a single oblique shock. This work presents an investigation of possible improvements to the current single shock experiment. A 10° oblique shock generator was designed by researchers at the University of Michigan and simulated at the University of Cincinnati. The 10° design did not start in the actual wind tunnel, but was successfully simulated by bypassing the transients from quiescent flow with the help of an initial solution generated from one dimensional inviscid nozzle theory. The residuals from the simulation leveled off at higher levels than expected in some mesh blocks, which indicates unsteadiness. The same case was then simulated in a time accurate manner, and showed very small variations in the solution over time. The magnitude of these variations were large enough to prevent the residuals in the steady simulation from dropping, but small enough that an averaged solution could be used for analysis. A grid dependency study was conducted and found that the 24 million node grid is very close to being grid independent.
A new design was desired that would allow the actual tunnel to start, and this resulted in a 6° oblique shock generator. This geometry allows the tunnel to start, and produces a more benign shock boundary layer interaction than the 10° oblique shock generator. A follow up experiment has been designed where the oblique shock is followed by a normal shock and a subsonic diffuser. This new configuration will provide insights into the effects that combined oblique and normal shock boundary layer interactions have on the health of the boundary layer in the diffuser section of a mixed compression inlet. The extensive glass walls of the wind tunnel will allow direct access for optical measurements of the shock boundary layer interactions and the diffuser section.
Corner separations in a tunnel affect the flow in the center of the tunnel. The oblique shock causes the low momentum flow in the tunnel corners to separate. This in turn creates shocks at the separation bubble leading edge that propagates into the flow-field. Depending on the size of the tunnel and the Reynolds number, these shocks can meet at the tunnel centerline and cause a pressure rise. This pressure rise can help to keep the boundary layer attached by smearing the pressure gradient, or intensify separations by adding an additional region of adverse pressure gradients, depending on where the corner shocks interact relative to the main shock boundary layer interaction.