Effects of Expansions on a Supersonic Boundary Layer: Surface Pressure Measurements

Multipoint wall pressure measurements are used to investigate the response of a Mach 3, fully developed, compressible, turbulent boundary layer (ReQ - 25,000) to centered and gradual (/?/8 - 50) expansions, both of 7- and 14-deg deflection. Although rms fluctuation levels decrease across the expansions, the rms normalized by the local static pressure remains nominally constant. Just downstream of the expansions, normalized power spectra are more concentrated at low frequencies (/< 10-15 kHz) than upstream, suggesting small-scale turbulence is quenched. This spectra alteration is more prominent for centered expansions and larger deflections. The spectra evolve very quickly after the centered expansions and very slowly after the gradual expansions. Downstream of the expansions, space-time correlations do not lend themselves to the derivation of convection velocities, signifying a severe distortion of the boundary layers. Measurements immediately after the gradual expansions compare well with those further downstream of the centered expansions of the same deflection, suggesting the distance from the beginning of the expansions is the appropriate length scale for characterizing the boundary-layer evolution. After the expansions, a band of elevated spanwise coherence (around 15-30 kHz) and elevated spanwise correlation levels emerge. Increases in streamwise coherence and correlation are less pronounced. At the last measurement stations, the boundary layers remain far from equilibrium.