Correlation Measurements Applied to Hypersonic Transitional Boundary Layers

Space-time correlations were measured in the boundary layer of a 7° half-angle, sharp-nosed cone at a free stream Mach number of 8. Measurements were made at nominal local x-Reynolds numbers of 2.3x10⁶, 4.6x10⁶, 5.5x10⁶, 6.8x10⁶, and 9.1x10⁶. These Reynolds numbers spanned the range from laminar, unstable flow, to nearly turbulent flow. Simultaneous measurements with two hot film probes in the boundary layer comprise the primary data set. The mean boundary layer state was characterized with Pitot and total temperature surveys. Correlations were taken with circumferential, streamwise, and vertical probe separations. Preliminary results show that the second mode waves preceding transition occur in wave packets which are relatively limited in their circumferential dimension, typically less than four boundary layer thicknesses, based on a coherence level of 20%. Streamwise phase velocities are between 95% and 100% of the edge velocity, and second mode wavelengths are approximately two boundary layer thicknesses, in agreement with linear theory.