Failed to converge in Fluent Multiphase Granular 2nd phase
 

Hi, I am a master student who currently using ANSYS Fluent V19.0 to run a Eulerian Multiphase simulation, the simulation is simple a 2D tube filled with water (primary phase) and 5mm glass bead (2nd phase), after that I set inlet boundary from bottom of the tube with constant velocity of 0.01 m/s and environment pressure outlet at top of the tube, this is a simple fluidized bed setup. The illustration of the initial condition of simulation setup are shown below:

Attachment 86671

The setup detail of the model is shown below:
Simulation type : Eulerian Multiphase with VOF
Analysis type : Transient Implicit
Time Step : 0.001 s
Total Time : 60 second
Glass bead phase : granular 2nd phase with VOF phase localized discretization of 0 (because in future I need to include air bubble into the simulation, so for glass bead 2nd phase, I use VOF phase localized discretization of 0 which is similar to diffused phase setting)
Glass bead density : 2230kg/m3 (denser than water)
Glass bead diameter : 5mm
Drag Model : Wen Yu
Maximum Packing limit : 0.6
Initial volume fraction of glass bead : 0.6 (same as packing limit)

I found out if the inlet velocity is too low (let say 0.01m/s) and cannot achieve minimum fluidized bed (mean the glass bead fludizied bed is stationary, like packed bed and no blow up), the model become unstable, the residual value sudden jump up and failed to converge in the interval of around every 30 time steps (0.03 second), the residual plot are shown below:

Attachment 86672

When I increase the velocity to 0.04m/s and above (over the minimum fluidized velocity), the fluidized bed starts to increase in height, the volume fraction starts to reduce below 0.6, and the simuulation do not have convergence problem anymore.
I suspect this phenomenon is due to the volume fraction of 2nd granular phase is close to 0.6 (maximum packing limit) and the 2nd phase is unstable. (Just my opinion)
In future I will migrate this fluidized bed from this simple tube to a stirred tank. I believe after the stirred tank flow achieved steady state (around 60 second), the velocity in this fluidized bed will over the minimum fluidized velocity and no convergence problem. However, the real case is, just after the stirrer of the tank start to rotate, the velocity in the fluidized bed is still close to zero, it will take time for that region to achieved certain velocity, it will unstable/fail to converge before the tank achieved steady state.
Anyone have idea of how to stabilize the granular 2nd phase (fluidized bed) even with very low inlet velocity?

have you solved the problem?