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

Abstract

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, $\delta /h$ = 0.26, 0.5, 1.2 and three different Reynolds numbers, ${\text{Re}} =$ 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 (${\text{Re}}$) and the initial boundary layer thickness ($\delta /h$) rather than ${\text{Re}}_{\theta }$ or $\delta /h$ alone as considered in the literature. In addition at high ${\text{Re}}_{\theta }$ the effect of initial boundary layer thickness shows an asymptotic characteristic in terms of ${\text{Re}}_{\theta }$ to some extent. On the other hand, the instability of the detached shear layer based on the momentum thickness ($St_{\theta }$) could be reasonably scaled by ${\text{Re}}_{\theta }$. The frequency corresponding to the shedding model varies depending on both ${\text{Re}}$ and $\delta /h$, which also indicates the fact that the development of the large-scale structures determining the turbulence quantities depends on the two parameters.