Hi, Here are the details in as concise a manner as possible: 1. I'm using In-compressible Navier Stokes Module in laminar flow, time dependent solution. 2.I have a rectangular region in which I have some fluid. 3.there are cilia inside the fluid, which move with a time and space-varying velocity V=ycos(5t+6x)\ 4. However, the movement of the cilia is oscillatory, so the net velocity of the fluid is some other value, say U 5. there is a bulk/volume force acting on the fluid, which is a function of both U and V...say |F|=(U-V)... 6. Obviously, U is also time varying, and determined by V....so how do I define it in COMSOL?