Cinematic particle image velocimetry of high-reynolds-number turbulent free shear layer

The objective of this research was to study the time-evolving velocity field in a two-stream, turbulent, planar free shear layer using a cinematic particle image velocimetry technique. The water shear layer had a velocity ratio of 0.23 and a Reynolds number of 2.62×10⁴ based on velocity thickness and velocity difference. The cinematic particle image velocimetry system employed an argon-ion laser, a scanning mirror, and a 35-mm movie camera. Experimental data obtained by this technique yielded a combined spatial and temporal evolution of the two-dimensional velocity and spanwise vorticity fields. (The resulting set of 400 velocity vector fields is available by contacting the second author.) The detailed velocity field structure of the shear layer was significantly different from previous lower Reynolds number flow visualizations in that the classical well-defined eddies and braids were replaced with complex three-dimensional agglomerated vortices of both signs. The velocity field evolution was also notably different from that of the passive scalar field, where the former exhibited stronger temporal variations and reduced spatial coherency. Temporal and spatial correlations yielded transverse distributions of convection velocities based on both streamwise velocity perturbations and vorticity. Additionally, the spatial autocorrelation was performed to show the average eddy shape, and a Lagrangian tracking correlation method was used to estimate eddy lifetime.