Measurements of an axisymmetric hypersonic shear-layer instability on a cone-cylinder-flare in quiet flow

Instabilities generated in Mach-6 quiet flow about an axisymmetric cone-cylinder-flare geometry with a 5 degrees half-angle sharp cone were studied in the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT). A small, 3.5 degrees flare angle was initially tested to provide a baseline with a model that introduces a compression to the flow but does not induce a separation. A 10 degrees flare was then used to make measurements with an axisymmetric separation bubble. Surface heat flux and pressure fluctuation measurements were made with IR thermography and PCB and Kulite pressure sensors, respectively. Off-the-surface density gradient fluctuations were observed using focused laser differential interferometry for the separated flow. Quiet-flow measurements resulted in laminar boundary layers through the downstream edge of both models. The shear layer was found to be mostly steady in space under quiet flow, as opposed to large spatial fluctuations seen in imagery from conventional noise tunnels. In addition to the second mode, a traveling instability was observed which, based on computational analysis, corresponds to oblique waves that are amplified by the shear layer above the separation bubble. This instability was found to phase lock with the second mode as it propagates downstream. Artificial perturbations were then introduced to the 10 degrees flare model upstream of the separation to study the resulting wave packets as they moved through the shear and boundary layers. This paper provides measurements that may be comparable to hypersonic flight due to the low-disturbance environment of the test facility.