STRUCTURE OF SUPERSONIC TURBULENT BOUNDARY-LAYER AFTER EXPANSION REGIONS

The effects of four expansion regions [centered and gradual(R/delta(0) similar or equal to 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 delta(0) downstream of the 7-deg centered expansion. Tbe 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 CO2) made the 14-deg expansion results more difficult to interpret.