[MdNabil]

I am trying to simulate the fluid(water) flow through a closed loop fluid circuit. (attached image). The red rectangular box is a reservoir from where water needs to be pumped through the full circuit. The circuit(blue) is containing air. I didn't put the pump since it will add complexity. I wanted to adjust the BCs so that they can simulate the scenario.

From the bottom of reservoir comes the tube where I placed my pressure inlet BC. I calculated the static pressure to be 5880Pa and inserted it as the total gauge pressure. The loop ends at the top of the reservoir where I put a 'mass flow outlet' BC. I put 1.667e-3 kg/s.

I used PISO solver. Under relaxation factors are quite low. Pressure 0.3, Momentum 0.4. I am not sure about them though. But the solution never converges. after 100 iterations it shows 'floating point exception'. And the continuity residual curve is fully diverged.

If I don't put any BCs and just let gravity to flow the water, the solution converges. But as soon as I am trying to adjust the BC to simulate a pump, it is not working.

Am I putting the BCs wrong? or the settings? The mesh metrics are quite good. (avg aspect ratioi 1.02, orthogonal quality 0.997). Though around the inlet and outlet regions they are a bit low.

I am trying to find the solution everywhere. But could not guess where the problem is coming from.