Transition and Separation over a Hollow Cylinder-Flare at Mach 5

Hypersonic laminar and transitional Shock Boundary Layer Interaction (SBLI) over a θ = 8◦ hollow cylinder-flare model is characterized in a Mach 4.8 flow at unit Reynolds numbers ranging from 4.6 to 13.7 million per meter. While incoming flow is laminar at all unit Reynold’s numbers tested, the state of the flow at reattachment varies from laminar to turbulent. A separation length scaling from schlieren imaging shows good agreement with previous studies on transitional separation bubbles. Additionally, since a completely laminar state was achieved at the lowest unit Reynolds number, a change in the scaling trend is shown for the laminar case. Kulite pressure sensors over the flare are used to resolve the unsteady pressure fluctuations both inside the separated region and downstream of reattachment. The transition to turbulence is characterized by a broad rise in frequency spectra with two distinct frequency bands showing elevated spectral content. Comparison of the frequency spectra with the attached boundary layer shows separation amplifies the low frequency band in the first mode range. The second elevated band of frequency content is found not to originate in the boundary or shear layer but through a possible entropy layer above the hollow cylinder surface. Both observations from pressure measurements are supported by high-speed schlieren which shows the spatial structures associated with these frequency bands.