Characteristics of the compressible shear layer over a cavity

The compressible shear layer over a 3:1 rectangular cavity was investigated using schlieren photography and planer laser imaging at freestream Mach numbers of 1.8, 2.1, 2.8, and 3.5. The purpose of this investigation was to study the characteristics of the shear layer structures and their convective velocity over this Mach number range. Schlieren images show leading- and trailing-edge shock waves, as well as shock waves emanating from the shear layer, which became less prevalent as the Mach number increased. Streamwise planar laser sheet lighting images indicate the existence of organized roller-type structures at the lower Mach numbers studied (M = 2.1 and 2.8). These structures became less coherent as the Mach number was increased, although the cavity appears to cause the large-scale structures to persist at higher levels of compressibility than found in planar free shear layers. Plan view images indicate that the two dimensionality of the large-scale structures decreased with increasing Mach number. Autocorrelations performed on single-pulse images show that the structure size decreased 63% when the freestream Mach number was increased from 1.8 to 3.5. By double pulsing the laser at delays of 15, 20, and 25 mus, the evolution of the large-scale structures mere investigated and quantified. The correlations were found to decrease by 23% for the same nondimensional time when increasing the Mach number from 2.1 to 3.5. From the shift of the peak correlation, the convective velocity was calculated for each ease and found to vary laterally across the shear layer. At a lateral location most representative of the large-scale structures in the shear layer, the variation in convective velocity with Mach number was best represented by 0.57 times the freestream velocity.