Control of Local Mass Transfer in the Separated and Reattaching Flow by a Periodic Forcing

A numerical study based on the Large Eddy Simulation (LES) methodology was made of mass transfer in locally forced turbulent separated and reattaching flow over a backward facing step. The local forcing was given to the flow by a sinusoidally blowing /suction of the fluid into a separated shear layer. The Reynolds number was fixed at 33000 and Schmidt number at 1. The forcing frequency was varied in the range 0 ≤ St ≤ 2, where St is the Strouhal number of forcing. The obtained results revealed the existence of an optimum forcing frequency value, St = 0.25, in terms of the reduced reattachment length. At this frequency the mass transfer is significantly enhanced in the recirculation zone. The influence of the frequency and the amplitude of forcing, in the maximum mass transfer positions and the maximum Sherwood number, are analyzed.