First and Fifth Hypersonic International Flight Research Experimentation's Flight and Ground Tests

The Hypersonic International Flight Research Experimentation (HIFiRE) program is a hypersonic flight-test program executed by the U.S. Air Force Research Laboratory and the Australian Defence Science and Technology Group, including flight, ground test, and computation. HIFiRE flights one and five (HIFiRE-1 and HIFiRE-5, respectively) were devoted to boundary-layer transition. The body of knowledge from the research campaigns showed a complex transition behavior. Axisymmetric flows and crossflow transition were strongly affected by wind-tunnel noise, with transition occurring at lower Reynolds numbers in the wind tunnel. Similarly, three-dimensional flows with inflected velocity profiles, such as the leeward side of HIFiRE-1 or the HIFiRE-5 centerline, exhibited much lower transition Reynolds numbers in ground tests as compared to flight. The disparity between flight and wind-tunnel transition Reynolds numbers was less pronounced for attachment-line transition. The supposition was that this transition mechanism was either less affected by wind-tunnel noise or was more susceptible to disparities in wall cooling between the wind tunnel and flight.