Effect of an expansion-compression wave pair on free shear layer dynamics

Experiments were performed using high-speed cinematography to spatially and temporally resolve compressible planar mixing layer structures in pressure matched and unmatched conditions. The unmatched conditions (overexpanded and underexpanded) were achieved by enforcing a static pressure difference between the supersonic planar jet exit (M~ 1.65) and the ambient quiescent flow below. The gas dynamics resulted in the interaction of expansion-compression waves with the free shear layer. An unsteady component of this interaction caused by the convection of the sonic line undulations, is postulated to excite the shear layer. From the temporally resolved planar images, auto- and temporal- correlations were obtained for a portion of the flowfield just downstream of the expansion-compression interaction. The results showed increased fluctuations in shear layer structure angles for the unmatched pressure conditions as compared to those for the matched condition. The degree of fluctuation correlated with a non-dimensional pressure gradient parameter and the time scale was consistent with the eddy passage frequency. The passive scalar convection speeds for the unmatched conditions were found to be significantly lower than for the matched cases. The deviations of the passive calar convection speeds from the isentropic speeds for matched and unmatched eases arc discussed.