Investigation of a Busemann Intake at Negative Angle of Attack

A high-speed, shape-transitioned, inward-turning intake was tested in Purdue’s Boeing/AFOSR Mach 6 Quiet Tunnel. The inlet model, called the Indiana Inlet (INlet), had a total contraction ratio of 4.68:1 and a design point of Mach 6 at 0° angle of attack. The model was outfitted with a suite of high-frequency pressure transducers, and the external flowfield was imaged with high-speed schlieren photography. The INlet was tested under low freestream disturbance levels for a variety of freestream unit Reynolds numbers and at-4° angle of attack. An unsteady shockwave near the leading edge of the inlet forebody, indicative of boundary layer separation, was detected from high-speed schlieren images in the-4° angle of attack cases. The oscillation frequency of the unsteady shockwave matched low frequency waves detected by surface-mounted pressure transducers. Downstream of the unsteady shockwave, a 20 kHz boundary layer instability was detected by pressure transducers. The strength and rate of growth of this instability was linked to freestream Reynolds number. The highest tested Reynolds numbers led to spectral broadening indicative of a transitional boundary layer within the mixed contraction region of the model. The existence and behavior of the unsteady shockwave suggests a coupling phenomenon between a hypothesized separation bubble and the 20 kHz boundary layer instability. Additionally, there is likely a critical negative angle of attack when this hypothetical separation bubble becomes unavoidable for this geometry.