Effect of Tripping on Hypersonic Turbulent Boundary-Layer Statistics

The effect of varying three-dimensional, cylindrical post-type trip size on the mean and turbulent velocity profiles of a Mach 7.6 turbulent boundary layer is examined using particle image velocimetry. It is shown that the effect of under and overtripping is to amplify the wake component of the mean velocity profile and outer-layer turbulence intensity, confirming trends from incompressible flow. Such results indicate that overly aggressive tripping introduces artificial large-scale turbulence that requires longer downstream distances to decay. For the current experiment, adequate tripping was obtained for trip sizes between 1.7 and 2.3 times the laminar boundary-layer displacement thickness at the trip, delta*(tr), with the optimum size approximately 2.3 delta*(tr). The wake strength for the optimally tripped cases followed the correlation of Fernholz and Finley (AGARDograph 253, Neuilly sur Seine, France, 1980) at the same Reynolds number, providing a good indicator for under- or overtripping. These results confirm that compressible boundary layers mimic incompressible trends but require larger trip sizes and have increased sensitivity, making a boundary layer free from initial conditions harder to achieve.