With the second order Euler-Taylor-Galerkin (ETG) finite element method, a large eddy simulation (LES) is used to simulate turbulent flows over a backward-facing step at Re=47 625. The simulation agrees well with Armaly's experiments.The influence of a ripplet on the turbulent flow is calculated. It is shown that large scale eddies and corresponding reattachment lengths change greatly with the ripplet height and its distance to the step. We argue that adding a ripplet could be a simple and effective passive control to the turbulent flow over a backward-facing step.