helo sir, im solving Flat Plate boundary layer the conditions i applied are entrance : u=1;v=0;dp/dx=0; exit : du/dx=0;dv/dx=0;p=0; top : u=1, v=0;p=0; bottom : u=v=0;dp/dy=0;

size of the domain 20:4;

the problem im facing is i have got velocity greater than one. this is due to maintain the conservation of mass. ie at the entrance u=1 it has some mass in this control volume. in the downstream due to boundary layer there is a reduciton in velocity at the bottom, but inorder to maintain the mass conservation it increases the velocity greater than one. so plz let me the correct boundary condition. thanks in advance

Think about the bc at the top, v = 0. This is physically OK 'far' from the plate. Then the question is 'how far is far enough?' With v = 0, you have imposed a duct with a frictionless top surface.

You might try dy/dy = 0 along the top of your domain. From continuity, that should be the case as u -> 0 assymptotically anyway.

Let us know how it comes out.

My response above has (at least) one unfortunate error.

dv/dy -> 0 as u -> 1!

Sorry for the bad job of proofing my own comment!

My recommendation is still to try dv/dy = 0 instead of v = 0.

I'm not a CFD-expert. But, referencing otd's response, the dv/dy=0 condition would still not answer your question of having a velocity larger than 1.

But, that's real physics! The boundary layer turns the channel into a contracting one.

I think, I would worry more if I had the boundary layer and the velocity had got smaller than 1 at or around the exit.

greetings,