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 an alternating blowing/suction through a thin slit situated at the step edge was studied computationally using the LES (Large Eddy Simulation), DES (Dettached Eddy Simulation) and T-RANS (Transient Reynolds-Averaged Navier-Stokes) techniques. The flow configuration considered (Re_H = U_cH/ν = 3700) has been investigated experimentally by Yoshioka et al. (2001). The periodical blowing/suction with zero mass flux is governed by a sinusoidal law: v_e = 0.3U_csin(2πf_et), 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_eH/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 applied. However, the closest agreement with experiment (the reattachment length reduction of 28.3% compared to the unperturbed case) was obtained with the LES (24.5%) and DES (35%) methods whereas the T-RANS computations show a weak sensitivity to the perturbation: 5.9% when using the Spalart-Allmaras model and 12.9% using the k–ω SST model.