Technique Of Unsteady Disturbances In Flow Control

Numerous techniques of unsteady disturbance such as unsteady blow/suck, synthetic jet have been researched. It can be seen that active flow control based on unsteady disturbance is promising to change the global structure of the flow by less energy injection and has potential for practical applications.This paper is a preliminary study of a technique for delaying flow separation. Ribbon oscillating in the boundary layer at special positions and frequencies can induced a periodic disturbance downstram which can increasing the Reynolds shear stress, and energize the turbulent boundary layer to delay the flow separation downstream.Smoke visualization experiment of low Reynolds number has been investigated firstly. The ribbon located in the boundary layer and excitated by Lorentz force will vibrate, when the frequency of alternating current on ribbon is equal to its naturally vibrating frequency. It can be seen that the flow separation on the deflected flap can be controlled or delayed when the ribbon upstream is oscillating at several frequencies.The CFD simulations have been conducted. The oscillating ribbon can energize the boundary layer by increasing Reynolds shear stress, and raise the skin friction downstream. The periodic velocity component in the boundary layer is induced by the oscillating ribbon, which decay along the streamwise. The periodic component noted as U~ of different excitation frequencies decays downstream at different speeds. Peak of U~ in the boundary layer move away from the wall along the streamwise. The flow control simulations by CFD method has indicated that oscillting ribbon or springboard like flap can control the separation on the deflect flap and the NACA0012 foil.