Transition Experiments on Blunt Cones with Distributed Roughness in Hypersonic Flight

Blunt cones with smooth nosetips and roughened frusta were flown in the NASA Ames Research Center hypersonic ballistic range at a Mach number of 10 through quiescent air environments. Global surface temperature distributions were optically measured and analyzed to determine transition onset and progression over the roughened frusta. Real-gas Navier-Stokes calculations of model flowfields, including laminar boundary-layer development in these flowfields, were conducted to predict values of key dimensionless parameters used to correlate transition on such configurations in hypersonic flow. It was found that nose bluntness has a major influence on roughness-induced transition onset and progression. For small-bluntness cones, pretest computations showed that values of the roughness Reynolds number increased with increasing distance from the beginning of the roughness elements. Transition onset was first observed on the roughened frusta near the cone base, and the transition front progressed forward with increasing freestream pressure at a constant value of the critical roughness Reynolds number. Conversely, for large-bluntness cones, pretest computations showed that values of the roughness Reynolds number decreased with increasing distance from the beginning of the roughness. Transition onset was again observed near the cone base, but forward progression of the transition front with increasing freestream pressure occurred at ever-increasing values of the roughness Reynolds number, a response characteristic of breakdown to turbulence downstream of a "trip ring" or "trip array."