EXPERIMENTAL INVESTIGATION OF THE UNSTEADY STRUCTURE OF A TRANSITIONAL PLANE WALL-JET

A laminar wall jet undergoing transition is investigated using the par ticle image velocimetry (PIV) technique. The plane wall jet is issued from a rectangular channel, with the jet-exit velocity profile being p arabolic. The Reynolds number, based on the exit mean velocity and the channel width, is 1450. To aid the understanding of the global flow f eatures, laser-sheet/smoke flow visualizations are performed along str eamwise, spanwise, and cross-stream directions. Surface pressure measu rements are made to correlate the instantaneous vorticity distribution with the surface pressure fluctuations. The instantaneous velocity an d vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent interactions between the outer-region (free-shear-layer region) and inner-region (b oundary-layer region) vortical structures. Results show that under the influence of the free-shear-layer vortex, the local boundary layer be comes detached from the surface and inviscidly unstable, and a vortex is formed in the inner region. Once this vortex has formed, the free-s hear-layer vortex and the inner-region vortex form a vortex couple and convect downstream. The mutual interactions between these inner- and outer-region vortical structures dominate the transition process. Fart her downstream, the emergence of the three-dimensional structure in th e free shear layer initiates complete breakdown of the flow.