Structure of Supersonic Turbulent Boundary Layer After Expansion Regions

The effects of four expansion regions [centered and gradual (R/6o ~ 50) expansions of both 7 and 14 deg] on a fully developed Mach 3 turbulent boundary layer were investigated. Instantaneous visualizations were made possible by the presence of scalar water condensation in the freestream and its absence in the higher temperature boundary layer. The elongated longitudinal structures previously found in the flat plate boundary layer are present downstream of the expansions. Large-scale structures increase in scale across the expansions. Structure angles also initially increase but are found to return to the flat plate value 10#o downstream of the 7-deg centered expansion. The rapid quenching of small-scale turbulence by the expansions results in a more intermittent boundary layer visually dominated by large-scale structures. Convection velocities derived from double-pulse correlations are reasonable in the flat plate and 7-deg centered expansion boundary layers. Excess condensation downstream of the 14-deg expansions (probably CC>2) made the 14-deg expansion results more difficult to interpret. Nomenclature n = normal distance above the surface R = radius of curvature for the gradual expansions, correlation coefficient Ree = Reynolds number based on boundary layer momentum thickness s = streamwise distance along the surface measured from the start of the convex curvature U = mean velocity vector U = mean streamwise velocity UT = friction velocity V = mean normal velocity jc = horizontal distance measured from (s, n) = (0, 0) y = vertical distance measured from (s, n) = (0, 0) Ap = pressure difference across the expansion region SQ = boundary layer thickness at s = 0 mm <$vis = boundary layer thickness defined by 99% of the freestream intensity SRMS = normal distance above the boundary where the peak in the rms profile occurs 9 = boundary layer momentum thickness v = kinematic viscosity TO = surface shear stress ahead of the expansion region