Mixing enhancement behind a backward-facing step using tabs

Mixing enhancement behind a backward-facing step is experimentally investigated using multiple tabs located at the edge of the backward-facing step. The Reynolds number is 24000 based on the free-stream velocity and step height with a turbulent boundary layer flow at the inlet. A parametric study is performed to find the optimal tab configuration of minimizing the reattachment length. The parameters of the tab are its height and spanwise width, and the spanwise spacing between two adjacent tabs. For each tab configuration, we measure the reattachment length and wall pressure, and for the optimal configuration the streamwise velocity behind the step. For most tab configurations considered, the reattachment length decreases and turbulence intensity increases at almost all spanwise locations. The reattachment length and wall pressure show significant variations along the spanwise direction. It is found that the tabs introduce strong three-dimensional disturbances to the separating shear layer by producing a pair of counter-rotating streamwise vortices and entrain high momentum fluids into the recirculation region, which greatly enhances mixing behind the backward-facing step. (C) 2007 American Institute of Physics.