3D Backward Facing Step
 

We are trying to solve a 3D backward facing step problem. Our solution is not converging.

1.We have given constant temperature along the bottom walls, and rest of the walls are adiabatic.
2. Expansion ratio:2, Aspect Ratio:8,
3. We have used simple scheme, second order upwind for momentum and energy.
4. We are trying to solve for Mixed Convection (Ri=1).
5. We tried solving by taking both outflow and pressure-outlet at outlet, however solution was not converging in both the cases.
6. We have tried upto 12000 iterations.
7. At first we solved only for pure forced convection (in which we hadn't taken boussinesq approx.), and it converged.