A periodically perturbed backward-facing step flow by means of LES, DES and T-RANS: An example of flow separation control

Turbulent flow over a backward-facing step, perturbed periodically by alternative blowing/suction through a thin slit (0.05 H width) situated at the step edge, was studied computationally using (LES) large eddy simulation, (DES) detached eddy simulation, and (T-RANS) transient Reynolds-averaged Navier-Stokes techniques. The flow configuration considered (Re-H= UcH/v=3700) has been investigated experimentally by Yoshioka et al. [1,2]. The periodic blowing/suction with zero net mass flux is governed by a sinusoidal law: v(e)=0.3U(c) sin(2 pi f(e)t), U-c being the centerline velocity in the inlet channel. Perturbation frequencies f(e) corresponding to the Strouhal numbers St = 0.08, 0.19, and 0.30 were investigated (St = f(e)H/U-c). The experimental observation that the perturbation frequency St = 0.19 represents the most effective case, that is the case with the minimum reattachment length, was confirmed by all computational methods. However the closest agreement with experiment (the reattachment length reduction of 28.3% compared to the unperturbed case) was obtained with LES (24.5%) and DES (35%), whereas the T-RANS computations showed a weaker sensitivity to the perturbation: 5.9% when using the Spalart-Allmaras model and 12.9% using the k-omega SST model.