Transition Experiments on Blunt Bodies with Isolated Roughness Elements in Hypersonic Flight

摘要

Smooth titanium hemispheres with isolated three-dimensional surface-roughness elements were flown in the NASA Ames Research Center hypersonic ballistic range through quiescent CO_2 and air environments. Global surface intensity (temperature) distributions were optically measured and thermal wakes behind individual roughness elements were analyzed to define tripping effectiveness. Real-gas Navier-Stokes calculations of model flowfields, including laminar boundary-layer development in these flowfields, were conducted to predict key dimensionless parameters used to correlate transition on blunt bodies in hypersonic flow. For isolated roughness elements totally immersed within the laminar boundary layer, critical roughness Reynolds numbers for flights in air were found to be higher than those measured for flights in CO_2; i.e., it was easier to trip the CO, boundary layer to turbulence. Tripping effectiveness was found to be dependent on trip location within the subsonic region of the blunt-body flow-field, with effective tripping being most difficult to achieve for elements positioned closest to the stagnation point. Direct comparisons of critical roughness Reynolds numbers for three-dimensional isolated versus three-dimensional distributed roughness elements for flights in air showed that distributed roughness patterns were significantly more effective at tripping the blunt-body laminar boundary layer to turbulence.