Measurements of Natural Transition During the BOLT II Flight Experiment

The smooth-wall transition process was characterized during flight at Mach 6.0 on a research geometry with concave curvature and swept leading edges, which introduced a boundary layer with stationary laminar vortex streaks, competing transition mechanisms, and complex early turbulence. This flight, termed BOLT II, was the second in a series coordinated by the Air Force Office of Scientific Research. Surface heat flux, skin friction, and pressure fluctuation spectra were acquired to characterize the transition process. Transition was first observed in a mixed second mode and crossflow mode region on [Formula: see text] side of the vehicle at [Formula: see text]. The mixed-mode transition appeared to occur in a narrow streak between 0.10 and 0.12 m off the centerline. In the [Formula: see text]-side mixed-mode region, transition took place at higher Reynolds numbers, [Formula: see text]. The analyses also showed that the spatial evolution to turbulence varied with respect to the location of the vortex heating streaks. Near the centerline, the flow experienced transition at [Formula: see text]. This transition front bifurcated as it moved upstream, which indicated correlation to near-centerline counter-rotating roll-up vortices. High-frequency pressure data captured major transition modes on the [Formula: see text] half of the vehicle, including the near-centerline vortex (100–200 kHz), mixed mode (300–500 kHz), and a higher-frequency mode (650–750 kHz) further outboard.