Active flow control with an oscillating backward facing step in a shallow open channel

An oscillatory, zero-net-mass flux actuator system, Jet and Vortex Actuator (JaVA), is implemented on the step wall of a backward facing step. JaVA can energize the boundary layer by creating jets or vortices thus it may delay flow separation when used properly. The main part of JaVA is a rectangular cavity with a moving actuator plate. The actuator plate is mounted asymmetrically inside the cavity of the JaVA box, such that there are one narrow and one wide gap between the plate and the box. The main governing parameters are the actuator plate's width (b), the amplitude (a) and the operating frequency (f). The main target of the control with active jets on the step wall is to influence directly the main recirculation zone, thus as the actuator plate or the step's vertical wall moves periodically in horizontal direction, a jet emerges into the recirculation zone. Non-dimensional numbers such as the scaled amplitude (S_a = 2πa/b) and the jet Reynolds number (ReJ = 4abf/ν) as well as the cross flow parameter characterize the JaVA-induced flow types and the effects on the recirculation zone. One period consists of one blowing and one suction phase into the recirculation zone. Boundary layer profiles extracted from time-Averaged flow fields of the not actuated (f = 0) and actuated cases at various operating frequencies indicate the effect of active flow control. The interaction between JaVAinduced flow regimes and the boundary layer is investigated numerically in an open channel with a BFS. The computational domain consists of a moving zone along the channel and the motion of the actuator plate is generated by a moving grid imposing appropriate boundary conditions with User-Defined- Functions and the calculations are carried out by a commercial finite-volume-based unsteady, laminar, incompressible Navier- Stokes solver. Numerical simulations and comparisons reveal the JaVA-boundary layer interaction for various governing parameters. Reynolds numbers based on the step height for the shallow open channel flow are Re_h = 225 and 450. The proposed control method based on suction and blowing with an oscillating vertical step seems to be effective in shortening the recirculation zone length and delaying the flow separation downstream of the backward facing step.