Problem of B.C. in Eulerian multiphase model
 

I am simulate fine powders flowing through the nozzle of a hopper. Initially the powder fill the hopper to a position 10 mm below the top. The top of the hopper is open.

I am using FLUENT 6.0 Eulerian multiphase model. I apply outflow B.C. at both the bottom (nozzle) and the top of the hopper.

Now the problem is that the largest velocity of powder exist at the powder surface. I think this result is unreasonable. I guess this problem is due to the B.C. I applied at the top face.

So, what kind of B.C. shohld I apply for the top face of hopper? Thank you very much.