Effect of an initial boundary layer thickness on the turbulent flow over a backward-facing step

The effect of an initial boundary layer thickness on the turbulent flow over a backward-facing step is investigated by using large eddy simulations. Three different initial boundary layer thickness, ∕h = 0.26, 0.5, 1.2 and three different Reynolds numbers, Re = 5,100, 24,000 and 45,000 are considered. Expansion ratio is 1.2. The characteristics of the turbulent flow over a backward-facing step (e.g. reattachment length, maximum pressure coefficient, mean streamwise velocity, rms streamwise
velocity fluctuations) significantly depends on both the Reynolds number ( Re ) and the initial boundary layer thickness ( ∕h ) rather than Re or ∕h alone as considered in the literature. In addition at high Re the effect of initial boundary layer thickness shows an asymptotic characteristic in terms of Re to some extent. On the other hand, the instability of the detached
shear layer based on the momentum thickness ( St ) could be reasonably scaled by Re . The frequency corresponding to the shedding model varies depending on both Re and ∕h , which also indicates the fact that the development of the large-scale structures determining the turbulence quantities depends on the two parameters.