Transition Analysis for the Ascent Phase of HIFiRE-1 Flight Experiment

The HIFiRE-1 flight experiment provided a valuable database for boundary-layer transition over a 7deg half-angle, circular cone model from supersonic to hypersonic Mach numbers as well as a range of Reynolds numbers and angles of incidence. This paper reports the findings from a computational analysis of the measured in-flight transition behavior during the ascent phase. Given a nearly zero angle of attack, computations indicate that the most likely cause for transition during the flight window of 19 to 22.5s is the amplification of second-mode instabilities in the laminar boundary layer, except in the vicinity of the cone meridian, where a roughness element was placed midway along the length of the cone. The growth of first-mode instabilities is found to be weak at all trajectory points analyzed from the ascent phase. Based on the time histories of temperature and/or heat flux at transducer locations within the aft portion of the cone, the onset of transition across the aforementioned window is found to correlate with an average linear N-factor, based on parabolized stability equations, of approximately 13.3. For times less than approximately 18s into the flight, the peak amplification ratio for second-mode disturbances is too small to cause transition because of the lower Mach numbers at earlier times. Therefore, the observed transition at these times is attributed to an unknown physical mechanism that is potentially related to the step discontinuities in surface height near the changes in surface material.