I'm trying to solve a fluid flow in a porous material problem in Abaqus/CFD using steady-state solver. I was able to solve simple flow around a cylinder problem with steady-state solver. But can't obtain solution on a more complex fluid domain.

When I'm trying to solve it with Abaqus/CFD steady-state solver the CFD Steady State Incompressible Flow Nonlinear Residuals initially decrease and then start oscillating, i.e. v_x and v_z will reduce, while v_z will increase and vice versa. After a number of such oscillations v_z abruptly becomes zero, then v_x and v_y become zero and then -nan. I've attached a part (beginning and end) of sta file.I was able to obtain solution for a simple flow around a cylinder problem with steady-state solver, but can't get solution for my complex domain.

I tried to reduce under-relaxation  factors to 0.1, but it didn't help.

Can any one suggest me how can I obtain the solution?