Learning Forum

[Aras karimi]

Hi everyone,

I use the device with the following specifications for simulation in Fluent:

(FLUENT 2022-R1, Windows OS, Intel i9-13900k, 32GB RAM). My processor has 24 physical cores.

First, I opened the mesh file with 22 cores in Fluent and tried to solve that the residuals do not converge well and errors appear in the console:

 

Once again, I opened the mesh file with 16 cores in Fluent and tried to solve that, compared to the previous state, the resiguals converge better, but still messages appear in the console:

 

And for the third time, I opened the mesh file with 4 cores in Fluent and tried to solve that no message appeared in the console this time:

 

It's very strange and very disappointing. Does that mean I can't use the number of cores of my device for simulation?

Why do very strange things happen in the convergence process with the increase in the number of cores?

The number of my mesh is very large and using 4 cores is very boring and unbearable.

Please help me what to do?

Thankyou

Regards

I should also say that when I open the mesh in Fluent, at the very first moment, a message with the following theme appears in the console:

 

Is this normal?

I hope to solve my problem.

Regards

[Federico]

Are you using any UDFs for your case?

[Aras karimi]

No, I do not use UDF.

My problem is 3D airfoil simulation.

[Rob]

Which solver are you using, and how many cells in the model? 

[Aras karimi]

- Fluent

- About 1 million hexahedral cells produced with ICEM.

[Rob]

Pressure based coupled solver?  

[Aras karimi]

Yes Pressure based.

coupled solver for velocity and pressure equations.

[Rob]

OK, I suspect it's the partitions. With a very high level of partitioning on a relatively small mesh you can see instabilities in the solver.  Using fewer cores, many more cells or SIMPLE should resolve this. 

[Aras karimi]

My processor has 24 physical cores. which includes 16 efficient cores and 8 performance cores.

When I select the mesh file in Fluent with 16 cores, as soon as Fluent opens, I receive a message in the console as follows:

 

Is everything normal? Do I need to adjust anything?

Should I use 8 cores or 16 cores?

[Rob]

For 1M cells I'd mesh on one core and probably solve on between 4 and 10 (hardware & licence permitting). With 8 performance cores available I'd probably not use more than 8: if you use the less efficient cpu it may slow everything down. 

[Aras karimi]

My approximate mesh is 1 million. It is very likely that it will be 4 or 5 million after studying on the mesh.

I even select 4 cores for Fluent, but I still get the following message in the console:

 

The problem is from somewhere else and I think that some adjustments must be made.

Dear Rob, Is it possible to ask the experts of Fluent Group about this?

Using processor cores to reduce the solution time, but on the contrary to increase the solution time, which means really crazy)

Thanks.

[Rob]

The warning is just that; it means Fluent hasn't assigned cpu and has told the OS to sort it out. Typically it happens when Fluent can't identify free cores due to hardware load. We see it on our laptops, and rarely on the cluster if running two jobs on the came box. 

Re the speed. For every partition data must be transferred between the compute nodes. That has a time overhead. As the number of cores increases the time spent on each core calculating the equations decreases (everything gets faster), however the time spent passing data increases (slowing things down). At some point the sheer number of partitions and data passing exceeds the benefit of adding more cores.