Boundary-Layer Transition Experiment During Reentry of HIFiRE-1

The Hypersonic International Flight Research Experimentation program is a hypersonic flight test program. It successfully measured the three-dimensional transition front on a cone at angle of attack in hypersonic flight during its reentry. The test article consisted of a nonablating, 7 deg half-angle, axisymmetric cone with a small bluntness of 2.5mm radius. During transition, angle of attack dropped from 13 to 5 deg and freestream unit Reynolds number increased from 2x10(6) to 8x10(6)/m. Mach number during this time was approximately seven. Earliest transition, determined from fluctuating pressures, occurred on the leeward meridian, at Re-x = 3.1 x 10(6), alpha = 13deg, and M = 7. Attachment line transition occurred at Re-x = 4.7 x 10(6), = 9 deg, and M = 7. The latest transition occurred off the windward meridian at Re-x = 6.0 x 10(6) and phi = 30 and 280 deg. Transition occurred at lower Reynolds numbers than the alpha = 0 deg transition measured during ascent (Re-x = 11 x 10(6) at M = 5.2). The azimuthally averaged transition Reynolds number determined from thermocouples was Re-x = 4.9 x 10(6). Away from the windward meridian, wind-tunnel transition occurred at lower Reynolds numbers than in flight. However, the decrement in transition Reynolds number due to wind-tunnel noise was not as severe as at zero angle of attack. Wind-tunnel windward transition occurred at a higher Reynolds number than in flight. It is posited that the destabilizing effects of wall cooling, which was higher in flight (T-w/T-0 = 0.18) than in ground test (T-w/T-0 = 0.56), outweighed the effect of wind-tunnel noise on the windward transition. High-bandwidth instrumentation recorded periodic pressure fluctuations approximately midway between the windward and leeward meridians before transition. However, they could not be positively identified as a crossflow instability.