Laminar flow through a cube with a hole at bottom

Dear Comsol users,

I am relatively new to comsol, but have been working for quite sometime.

I was just wondering why is it so difficult (not converging even after 100 iterations) to solve a laminar fluid flow in box having a hole (outlet) at the bottom.

Problem description: The 3D box having no slip at all the walls except the inlet wall (face). The exit boundary is "no viscous stress" where the aperture diameter is 60% of the box edge, opposite to the inlet face. The inlet velocity is given at 0.1 m/s. The mesh is physics controlled "normal".

I have attached the comsol for details, if anyone is interested.

Is is something very simple that I am missing.
My final goal is to apply the periodic boundary conditions to the opposite faces in the direction transverse to the primary flow.

Regards,
Sourav