Thank you for your input, Sir.

So for this case, I should keep running on the coupled solver with pseudo-transient? Using the 0.1 timescale factor, right?

And for the keeping drag force at first, until how many iterations approximately that I could start to turn on the rest of all forces?

As your reference, I have tried with increasing the mesh and try to run with pseudo-transient but I put all the forces on since the beginning of the simulation. I use 0.1 timescale factor for both liquid and solid time scale. On the 10.000 iterations, the simulation is okay. But when I try to change the timescale factor from 0.1 to 1, it suddenly got a floating point exception in the simulation.

Looking forward to your kind response. 

Best regards,