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

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 (Re_h) 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 (D_x and D_y) and width to height ratio (D_x/D_y) all obey to the lognormal distribution. With increase of Re_h from 4400 to 9000, D_x decreases and D_y increases, but the mean diameter (D=0.5 × (D_x + D_y)) keeps around (0.28~0.29) h. As the increase of Re_h, the vortical shape change toward a uniform condition, which may be contributed by enhancement of the shear intensity.