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Vortical structures in the near wake of tabs with various geometries

Published online by Cambridge University Press:  20 July 2017

A. M. Hamed Open the ORCID record for A. M. Hamed [Opens in a new window] ,
A. Pagan-Vazquez ,
D. Khovalyg ,
Z. Zhang  and
L. P. Chamorro Open the ORCID record for L. P. Chamorro [Opens in a new window]
A. M. Hamed
Affiliation:
Mechanical Science and Engineering Department, University of Illinois, Urbana, IL 61801, USA
A. Pagan-Vazquez
Affiliation:
Mechanical Science and Engineering Department, University of Illinois, Urbana, IL 61801, USA
D. Khovalyg
Affiliation:
Mechanical Science and Engineering Department, University of Illinois, Urbana, IL 61801, USA
Z. Zhang
Affiliation:
Mechanical Science and Engineering Department, University of Illinois, Urbana, IL 61801, USA
L. P. Chamorro*
Affiliation:
Mechanical Science and Engineering Department, University of Illinois, Urbana, IL 61801, USA Civil and Environmental Engineering Department, University of Illinois, Urbana, IL 61801, USA Aerospace Engineering Department, University of Illinois, Urbana, IL, 61801, USA
*
†Email address for correspondence: lpchamo@illinois.edu
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Abstract

The vortical structures and turbulence statistics in the near wake of rectangular, trapezoidal, triangular and ellipsoidal tabs were experimentally studied in a refractive-index-matching channel. The tabs share the same bulk dimensions, including a 17 mm height, a 28 mm base width and a $24.5^{\circ }$ inclination angle. Measurements were performed at two Reynolds numbers based on the tab height, $Re_{h}\simeq 2000$ (laminar incoming flow) and 13 000 (turbulent incoming flow). Three-dimensional, three-component particle image velocimetry (PIV) was used to study the mean flow distribution and dominant large-scale vortices, while complementary high-spatial-resolution planar PIV measurements were used to quantify high-order statistics. Instantaneous three-dimensional fields revealed the coexistence of a coherent counter-rotating vortex pair (CVP) and hairpin structures. The CVP and hairpin vortices (the primary structures) exhibit distinctive characteristics and strength across $Re_{h}$ and tab geometries. The CVP is coherently present in the mean flow field and grows in strength over a significantly longer distance at the low $Re_{h}$ due to the lower turbulence levels and the delayed shedding of the hairpin vortices. These features at the low $Re_{h}$ are associated with the presence of Kelvin–Helmholtz instability that develops over three tab heights downstream of the trailing edge. Moreover, a secondary CVP with an opposite sense of rotation resides below the primary one for the four tabs at the low $Re_{h}$. The interaction between the hairpin structures and the primary CVP is experimentally measured in three dimensions and shows complex coexistence. Although the CVP undergoes deformation and splitting at times, it maintains its presence and leads to significant mean spanwise and wall-normal flows.

JFM classification

Vortex Flows: Vortex Flows Vortex Flows: Vortex interactions Wakes/Jets: Wakes
Type
Papers
Information
Journal of Fluid Mechanics , Volume 825 , 25 August 2017 , pp. 167 - 188
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Copyright
© 2017 Cambridge University Press 

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