Measurements of Natural Turbulence During the BOLT II Flight Experiment

A Mach 6.0 flight experiment was performed to characterize the turbulent skin friction and heat flux associated with natural transition for vehicle-length Reynolds numbers up to 45 million. This boundary-layer turbulence 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. The test geometry used concave curvature and swept leading edges to introduce a boundary layer with stationary laminar vortex streaks, competing transition mechanisms, and complex early turbulence. The analyses also showed that the spatial evolution of turbulence varied with respect to the location of the vortex heating streaks. Prominent overshoots were observed in the early turbulence within the streak. Turbulence data was collected for Reynolds numbers ReL up to 45×106. A common Rex=12×106 was identified as the start of equilibrium turbulence for the data presented. Conjugate heat transfer simulations, both laminar and turbulent, agreed well with the experimental data, including the laminar leading edge. The Reynolds analogy ratios based on the curve fits to the data, including compressibility, were generally between 0.9 and 1.0. The observed variations were likely the result of the spatial separation of the sensors and different definitions of Stanton number normalization between flight and theory.