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Open AccessArticle

Experimental Investigation of Coherent Vortex Structures in a Backward-Facing Step Flow

1
State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing Hydraulic Research Institute, Nanjing 210029, China
2
Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA
*
Authors to whom correspondence should be addressed.
Water 2019, 11(12), 2629; https://doi.org/10.3390/w11122629
Received: 14 October 2019 / Revised: 23 November 2019 / Accepted: 10 December 2019 / Published: 13 December 2019
(This article belongs to the Special Issue Advances in Hydraulics and Hydroinformatics)
Coherent vortex structures (CVS) are discovered for more than half a century, and they are believed to play a significant role in turbulence especially for separated flows. An experimental study is conducted for a pressured backward-facing step flow with Reynolds number (Reh) being 4400 and 9000. A synchronized particle image velocimetry (PIV) system is developed for measurement of a wider range of velocity fields with high resolution. The CVS are proved to exist in the separation-reattachment process. For their temporal evolution, a life cycle is proposed that vortices form in the free shear layer, develop with pairings and divisions and finally shed at the reattachment zone, and sometimes new vortical structures are restructured with recovery of flow pattern. The CVS favor the free shear layer with frequent pairings and divisions particularly at the developing stage around x/h = 2~5 (x: distance from the step in flow direction, h: step height), which may contribute to the high turbulent intensity and shear stress there. A critical distance is believed to exist among CVS, which affects their amalgamation (pairing) and division events. Statistics show that the CVS are well organized in spatial distribution and show specific local features with the flow structures distinguished. The streamwise and vertical diameters (Dx and Dy) and width to height ratio (Dx/Dy) all obey to the lognormal distribution. With increase of Reh from 4400 to 9000, Dx decreases and Dy increases, but the mean diameter (D=0.5 × (Dx + Dy)) keeps around (0.28~0.29) h. As the increase of Reh, the vortical shape change toward a uniform condition, which may be contributed by enhancement of the shear intensity. View Full-Text
Keywords: coherent vortex structure; backward-facing step; synchronized PIV; separation and reattachment; free shear layer; vortical evolution coherent vortex structure; backward-facing step; synchronized PIV; separation and reattachment; free shear layer; vortical evolution
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MDPI and ACS Style

Wang, F.; Gao, A.; Wu, S.; Zhu, S.; Dai, J.; Liao, Q. Experimental Investigation of Coherent Vortex Structures in a Backward-Facing Step Flow. Water 2019, 11, 2629.

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