Learning Forum

[Saeed]

Hello,

I am doing a CFD-DEM coupling of a mixing tank and I have a question.

at first the simulation is running ok for a few seconds then particles start to agglomorrate (image attached from before and after) and I recieve the following messages from fluent and Rocky: 

Rocky : solid maximum volume fraction 0.65% did not achieve after 500 iterations 

Fluent: turbulent viscosity limited to viscosity ratio of 1.000000+05 in 29436 cells.

the number of cells will increase over time and then I get the following error and particle disappear : 

DEM-CFD critical explicit time step:
0.183% of the particles exceeded 50% the CFD critical explicit time step.
0.0913% of the particles exceeded 75% the CFD critical explicit time step.
0.0913% of the particles exceeded 100% the CFD critical explicit time step. collected in 11 events of CFD forces.

my particle size is bigger than the mesh size and i thought since Rocky uses iteration for volumetric diffusion there is no issue with it. I want to know if that is the problem which is causing the issue or it is because another reason ?

I have run the simulation with the following time steps and same issue came up. 

CFD: 5e-4s DEM: 4.67e-6 , 5e-7 , 1e-8s 

CFD: 5E-5S DEM: 4.67e-6 , 5e-7

Thanks 

[Jackson Gomes]

Hello Saeed,

If the particle size is larger than the mesh size, it's crucial to ensure that the coupling method being used is appropriate, such as the semi-resolved method. Additionally, depending on the number of particles in your model, the volume fraction needs to be properly calibrated, as it can vary significantly by orders of magnitude.

The issues you are experiencing, such as particle agglomeration and the error messages from Rocky and Fluent, might be related to improper calibration of this volume fraction. Please go to the superior toolbar in Rocky, HELP |Manuals |User Manual and referring to section titled "Rocky 2-Way Coupling Features". This section provides a detailed understanding of the volume fraction and how to adjust it to improve the accuracy of the CFD-DEM coupling.

Best regards,

[Saeed]

Dear Jackson,

Thank you for your reply.

I ran another simulation yesterday with particles being smaller than mesh size and same issue came up and I recieve same error messages except Rocky : solid maximum volume fraction 0.65% did not achieve after 500 iterations.

Can you please help me with that?

Best

[Jackson Gomes]

Dear Saeed.

The semi-resolved method is used in specific cases where the particle is bigger than cell mesh.

Please read carefully the Rocky 2-Way Coupling Features chapter to understand how the volume fraction works. In your case, I think you need to calibrate this parameter.

For further assistance, please refer to HELP |Contact Customer Support.