Hi, I would like to investigate the flow behaviour within a cavity filled with water. The water inside the cavity is driven by wall jet of air. The characteristic velocity defined Re of the driven cavity is the velocity at the air-water interface. You know that at the interface, Shear stress_air side = Shear stress_water side. Neglecting the surface tension and the motion of the interface, Could you please tell me how can I decide approximately the volumetric flow rate of air stream.

(1). Please describe you boundary conditions and geometry first, before I can answer your question.

Well, it is a rectangular cavity. The no slip boundary condition is applied to all solid faces of the cavity except to the upper air-water interface. The air flows in the chanel is bounded by the lid, where the no slip BC is applied, and the air-water interface. Many thanks,

(1). The air flow rate is related to the velocity profile, which can be used as the inlet boundary condition. You can specify the inlet boundary condition. (uniform or parabolic, or something else)

Dear Chien, I am doing exactely what you said.But how can I know that air flow rate is corresponding the required Re of the cavity which is based on the velocity at the air-water interface before measuring it experimentaly? Thanks.

(1). You don't. (2). Run a few cases, and you can zoom in by interpolation. you can program the logic and let the program do it for you, or you can do it by hand.