Free shear layer interaction with an expansion-compression wave pair

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 pressure mismatch resulted in the interaction of expansion-compression waves with the free shear layer. Temporally resolved planar images, autocorrelations, 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 with those for the matched condition. The degree of angular fluctuations qualitatively correlated with a nondimensional 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.