Passive scalar mixing in a planar shear layer with laminar and turbulent inlet conditions

The effect of inlet conditions on the mixing of a passive scalar was investigated in a planar shear layer with inlet boundary layers that were laminar, tripped and naturally turbulent–transitional. Planar laser-induced fluorescence measurements of acetone were used to directly evaluate the shear layer structure, and were processed to determine probability density functions (PDFs) of the mixture fraction. The results agree well with previous studies in aqueous and gaseous systems for laminar inlet conditions. Large-scale structures of a nearly homogeneous composition were found, and the structures spanned the mixing layer width giving rise to a nonmarching style PDF. A high-speed boundary layer that differed from the laminar state (produced by tripping a laminar boundary layer, from the naturally turbulent–transitional state at high flow rates, or by tripping the turbulent–transitional condition) gave rise to a hybrid-style PDF that was markedly different from either the laminar nonmarching case, or the marching shape that has been found by other investigators. The hybrid PDF shape had a marching character on the high-speed side of the mixing layer, and a distributed nature not favoring any specific composition on the low-speed side of the mixing layer. Tripping the low-speed boundary layer produced no change in the hybrid PDF shape, confirming that the difference observed between the high- and low-speed sides was the result of the shear layer development with turbulent inlet conditions, not an inlet condition effect. In addition, with turbulent and transitional inlet conditions all turbulent passive scalar profiles were found to be self-similar and the velocity power spectra displayed a −5/3 slope indicating well-developed turbulent conditions prevailed at a relatively low (104) Reynolds number. Secondary structures were observed in images with turbulent–transitional inlet conditions, but not with tripped inlet conditions.