Effects of External Forcing on Entrainment Ratio and Fluid Composition in Turbulent Shear Layers

Abstract : The objective of this research was to determine how the imposition of two-dimensional external disturbances affects the mixing field in plane two- stream shear layers. The study relates to the effects of forcing on mixing of species, the kind of mixing that is necessary for chemical reaction and combustion. The effect of forcing on mixing was documented by measuring the probability density function (pdf) of the concentration field in a nonreacting liquid shear layer forced by 2-D disturbances. The concentration field was measured using the laser induced fluorescence (LIF) technique in the passive scalar mode. Results show that composition distribution of mixed fluid in forced shear layers is essentially uniform across the width of the shear layer similar to previous results in natural layers. Forcing changes both the amount and the composition of mixed fluid. For the cases studied, forcing results in an increase of the total amount of mixed fluid (integrated across the layer width) and a shift of the predominant mixed-fluid concentration to larger values. The increase in the amount of mixing in the forced layer appears to be due mostly to the increase in the layer width and not so much the result of improved small- scale mixing. Details of the forcing waveform shape have a significant influence on the structure of the flow and the mixing field.