A two-element high-lift configuration at stall condition is investigated by a numerical simulation based on the Reynolds-averaged Navier-Stokes equations and eddy-viscosity turbulence models. Flow separation can be delayed by periodic vertical suction and blowing through a slot close to the leading edge of the flap. By simulating different excitation frequencies and intensities an optimum flow control can be identified and in cooperation with experimental investigations the mechanism of separation control is studied. Special attention is drawn to the turbulent time-scales to check the applicability of statiscal turbulence models for unsteady flows.