Flow visualization study of role of coherent structures in a tab wake

A simple surface-mounted tapered tab has recently attracted uids research both for its ability to enhance mixing and heat transfer (for which it is known as high-e ciency vortab mixer) and for its generation of coherent structures that are topologically similar to those found in natural turbulent boundary layers. Two types of structures, namely pressure-driven counter-rotating vortex pair (CVP) and hairpin vortices were previously identi ed in the tab wake, but the contribution of individual structures to the mixing enhancement process and how they interact are not known. In the present study, ow visualization using a planar laser-induced uorescence (PLIF) technique is carried out to probe into the ow dynamics in the wake of the mixing tab. By injecting dye at an appropriate location and illuminating the ow in various planes, the structures are visualized clearly. The results show, in contrary to earlier observations, that the two types of structures dominate di erent regions. At the Reynolds number of 700 based on tab height (h), the CVP has more in uence in the region 0¡x=h¡ 1:5. The counter-rotating action of the vortex pair induces a pumping action along the symmetry by which the low-speed uid from the boundary layer is transported to the high-speed outer shear layer. The displaced uid is entrained by the recirculating counter-rotating vortices and is mixed well while convecting downstream. Beyond this region, fully developed hairpin structures contribute more to mixing in a similar way as in a turbulent boundary layer. It is observed that the shedding frequency of hairpin vortices is slightly higher than the pumping frequency of the counter-rotating vortex pair. It is also observed that the hairpin structures loses their identity beyond x=h¿ 15, and there is no large-scale cross-stream mixing visible in this region. c © 2000 The Japan Society of Fluid Mechanics and Elsevier Science B.V. All rights reserved.