Use the Equation of Continuity and the Navier-Stokes Equation to solve this problem. Consider the flow here as fully-developed and it is in its steady state. The gravitational force is in the x direction, so = p − x. A fluid flows in the positive x-direction through a long flat duct of length , width W, and thickness , where ≫ W ≫ . The duct has porous walls at y = 0 and y = , so that a constant cross flow can be maintained, with y = 0, a constant, everywhere. Flows of this type are important in connection with separation processes using the sweep-diffusion effect. By carefully controlling the cross flow, one can concentrate the larger constituents (molecules, dust particles, etc.) near the upper wall.