UNSTEADY DYNAMICS OF A LOW ASPECT RATIO MIXING LAYER

This work examines the temporal dynamics in the near-field of a low aspect ratio mixing layer. Using Planar Laser Induced-Fluorescence (PLIF), we investigate mixing dynamics between a rhodamine dye contaminated stream and a clean water uncontaminated stream at different velocity ratios. An in-situ calibration experiment is performed to verify the linearity of the image intensity with rhodamine concentration and to account for spatial variations in laser sheet intensity. The mixing dynamics are analyzed for velocity ratios ranging from one to three, defined as the ratio between the bulk velocity in the small channel to the bulk velocity in the large channel. This range yields mixing layer Reynolds numbers based on the velocity difference, channel width, and kinematic viscosity of water ranging from zero to approximately 1,600. The average and transient properties of the mixing layer are analyzed. The average concentration distribution is presented to quantify the spreading rate of the mixing layer. Of particular interest is the intermittency of the concentration time series at the channel centerline and the nominally clean and contaminated streams, both of which are related to the formation of large scale vortical structures.