Transition Experiments on Blunt Bodies with Distributed Roughness in Hypersonic Free Flight

Models of hemispherical and blunt large-angle conical geometries were flown in the NASA Ames hypersonic ballistic range to investigate the influence of distributed surface roughness on transition to turbulence. Transition-zone fronts were defined from optically measured global surface temperature distributions. Measurements of preconditioned surface roughness, surface temperature, average transition-front location, and freestream environment were combined with real-gas Navier-Stokes calculations of model flowfields, including laminar boundary-layer development in these flowfields, to transform all data into dimensionless parameters used to correlate transition onset/location on rough blunt bodies in hypersonic flow. A combination of present and previously published results led to the formulation of a transition correlation applicable over all three regimes potentially encountered by an ablating blunt-body heat shield undergoing atmospheric entry: the smooth-wall asymptote, the critical-roughness Reynolds number regime, and the large-roughness/low-Reynolds-number asymptote.