I want to simulate the flow of water in a tank. Suppose the one side of the tank is open at upstream end and the water is coming in from this side. The same quantity of water is going out from the correspoding downstream end keeping the water level constant. Of course, the bottom boundary will be non slip wall boundary conditions (BC) where all the velocity componants are zero. Near side wall and far side wall BCs will also be same as the bottom. The BCs on top of the water surface will be slip wall type. What about the boundary condition on upstream side wall and downstream side wall? Thank you in adavance... Sas..

If I am understanding you correctly, the flow is for a tank with four wall and two inlets. The four walls all should be modelled with no-slip (zero velocity) conditions.

not fully understood. however, inlet conditions if and when known - Dirichlet type. outlet condtion when not known - Neumann type 1st deriv (or second deriv even better from physics point).read patankar.

The following answer is assuming that you are considering an incompressible assumtion ---

In the upstream side a Dirichlet condition has to be specified. In you case it would be velocity of water entering the tank.(u=U_inf, v=0,w=0) and no condition on pressure

On the Downstream side a Neumann condition has to be specified. An efficient condition is to set the variation of normal stress to zero.