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

A simple surface-mounted tapered tab has recently attracted fluids research both for its ability to enhance mixing and heat transfer (for which it is known as high-efficiency vortab mixer) and for its generation of coherent s tructures that are topologically similar to those found in natural turbulen t boundary layers. Two types of structures, namely pressure-driven counter- rotating vortex pair (CVP) and hairpin vortices were previously identified in the tab wake, but the contribution of individual structures to the mixin g enhancement process and how they interact are not known. In the present s tudy, flow visualization using a planar laser-induced fluorescence (PLIF) t echnique is carried out to probe into the flow dynamics in the wake of the mixing tab. By injecting dye at an appropriate location and illuminating th e flow in various planes, the structures are visualized clearly. The result s show, in contrary to earlier observations, that the two types of structur es dominate different regions. At the Reynolds number of 700 based on tab h eight (h), the CVP has more influence in the region 0 < x/h < 1.5. The coun ter-rotating action of the vortex pair induces a pumping action along the s ymmetry by which the low-speed fluid from the boundary layer is transported to the high-speed outer shear layer. The displaced fluid is entrained by t he recirculating counter-rotating vortices and is mixed well while convecti ng downstream. Beyond this region, fully developed hairpin structures contr ibute 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 hig her than the pumping frequency of the counter-rotating vortex pair. It is a lso 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. Al l rights reserved.