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

Abstract

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 identiÿed 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.

Numerical simulation of vortex structures and heat transfer behind a hill in a laminar boundary layer

“…A symmetrical vortex pair is generated by this flow behind the hill. This vortex is similar to the CVP (counter-rotating vortex pair) observed by Elavarasan and Meng in visualization experiments for the flow around a tab [7]. The flow around the hill in this study is also similar to the flow observed in earlier studies [1,3].…”

“…Acarlar and Smith [5] observed the behavior of hairpin vortices downstream of a hemispherical protuberance in a laminar boundary layer, and examined the boundary layer characteristics in the redeveloping region. For the flow past an inclined trapezoidal tab, Gretta and Smith [6], Elavarasan and Meng [7], and Yang et al [8] experimentally elucidated the dynamics of a hairpin vortex in a tab © 2008 Wiley Periodicals, Inc.…”

Tackling turbulence with Holographic Particle Image Velocimetry (HPIV)

“…From this 3D velocity field, instantaneous vorticity field is computed to identify the hairpin vortex structures found in this flow using PIV and flow visualization. 26,28 Shown in Fig. 10 is an iso-surface of the vorticity field, where three hairpin vortices are identified in the 3D volume.…”

Four-dimensional dynamic flow measurement by holographic particle image velocimetry

“…9 are consistent with findings from previous flow visualization, PIV, and direct numerical simulations on the tab wake. [35][36][37] The choice of vorticity isosurface threshold ͑ T ͒ must ensure that the errors in the vorticity structures be sufficiently small while major vortex structures are properly revealed. Although it is difficult to give a direct measure on the vorticity error, a comparison between the statistics of residual divergence and vorticity data provides a good estimate.…”