Momentum and mass transfer in developing liquid shear mixing layers

Abstract

Flow structure in developing mixing layers is experimentally investigated with an emphasis on the second moment terms. Instantaneous streamwise and vertical velocities and concentration are simultaneously measured using a combination of a laser-Doppler velocimeter and a laser-induced fluorescence technique. The results show that the turbulent momentum and mass transfer in developing mixing layers can be negative (counter-gradient flux) at middle and small eddy scales. As a result, the Reynolds shear stress in the developing mixing layer becomes smaller than that in the forcibly developed mixing layer, whereas the momentum production term in the developing mixing layer is larger than that in the developed mixing layer. As the mixing layer develops, the flow gradually becomes turbulent and turbulent momentum and mass transfer becomes positive at small scales. Consequently, the counter-gradient diffusion appears only at the middle scales before it fully develops. With respect to the streamwise mass transfer in the developing mixing layer, the fluid mass at large scales can also be transferred negatively in the off-central region of the mixing layer.