Experimental Analysis of the Turbulent Shear Stresses for Distorted Supersonic Boundary Layers
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An experimental analysis of the turbulent shear stresses for a supersonic boundary layer distorted by streamline curvature-induced pressure gradients was performed using laser Doppler velocimetry. Four pressure-gradient flows were examined: a nominally zero-pressure-gradient case (M = 2.8, Re = 1.1 104, = 0.02); a favorable-pressure gradient (M = 2.9, Re = 1.5 104, = -0.5); an adverse-pressure gradient (M = 2.7, Re = 1.2 104, = 0.9); and a successive-pressure gradient (M = 2.5, Re = 1.2 104, = -1.0, following a region of = 0.9). For the favorable-pressure gradient, the turbulent shear-stress levels across the boundary layer decreased by 70-100%, as compared to the zero-pressure-gradient boundary layer. For the adverse-pressure gradient, a 70-100% increase was observed. For the combined-pressure gradient, the shear stresses returned to values similar to the zero-pressure-gradient flow. A new pressure gradient parameter was found to correlate well with the peak shear-stress amplification. It was also postulated that the shear-stress amplifications were in part the result of the nonuniform bulk dilatation/compression and streamline divergence/convergence, implying a forcing phenomena that influenced the statistical uv correlation. The combined-pressure-gradient flow demonstrated that the turbulent structure adjusts relatively rapidly to the distortion. Numerical simulations of the mean velocity obtained with a k- turbulence model were found to agree very well with the present data. With the exception of the zero-pressure-gradient flow, the magnitudes of the turbulent shear stresses were not accurately reproduced; however, correct trends were predicted.