[Kossivi]

Hello All,

I'm running pimpleDyMFoam on an industrial case fan system. But the time step is very low (10e-7) at the beginning but it increases to 10e-6 later. The problem is that for almost 3 weeks now the case doesn't converge after many revolutions. I tried to put higher the maxco to increase the time step but no real effect observed. The mesh is about 6 millions cells.
Knowing that start the pimpleDyMFoam running using the steady state (simpleFoam-MRF) to have a jump-start.
My question am I doing something wrong ?
And How can I get a good options for reasonable time computation?

Attached the controDict, fvSolution and fvScheme.


Thanks in advance.
Kossivi
controlDict.txt

fvSchemes.txt

fvSolution.txt

[kkk]

Hi

Could you show me solver log?
How many times does p iteration become?

Regards,
kkk

[Kossivi]

Hello kenyoh,
Thanks for reply.
p iterations become 3 or 2 iterations.
See attached a part of the log file (too heavy to upload).

Thanks,
Kossivi
log1.txt

[kkk]

Hi Kossivi

It seems to be very stable caluclation.
I guess there is no way except finding mesh where Co is high and re-meshing there.

Or,
increasing maxCo,
increasing parallel numbers.

Sorry I couldn't help you.
kkk

[Kossivi]

Thanks very much for the help.
I increase the maxCo and it speeds up the timeStep.
At which extent the maxCo can be increased generally for real case?

Thanks,

Kossivi

[kkk]

Hi Kossivi

Actually, I have not set maxCo more than 10.
Simulation usually become in fail when I set maxCo more than 10.
But I've heard around 100 was OK from someone.

Generally simulation is not constrained by Courant number in implicit method.
You would be able to set maxCo as much as higher possible.
But it means you extremely decrease the accuracy of the time-discrete comparing with one of space-discrete.
I guess it's better to decrease the accuracy of the space-discrete and set smaller maxCo.

Regards,
kkk

[Kossivi]

Hi kenyoh,
Thank you for your answer. It really helps.
I have another question. It is regarding the convergence of the solution. I know for transient the convergence is not straight as is steady state case. But what I do for this case is to monitor the moment and the pressure rise (p_outlet - p_inlet) and compare them to the steady state results with the (simpleFoam-MRF). If the results after let's say 2 revolutions are similar to the one of the steady state so I conclude for convergence. of course I look at the pressure convergence too.
Is that way of doing correct ?

regards,

Kossivi

[kkk]

Hi Kossivi

Yeah, I think so.
I guess you might have to monitor more long time for making sure it really become steady state.
If transient simulation become steady state, the monitoring value will become constant after that.

If the monitoring value does not change after 2 revolutions, Then you can stop the simulation.

Regards,
kkk

[Kossivi]

Hi kenyoh,
Thanks very much for your reply.
It really helps.

regards,
Kossivi.

[Kossivi]

Hi kenyoh,
I have a question regarding the comparison of a steady turbomachinery (MRFSimpleFoam for example) and the unsteady turbomachinery calculation.
How can I compare the results a the same angle ?
Is simple for know the angle of the unsteady solution. But how to know it for the steady one ?
Any idea on it will be appreciated.

Thanks,

Kossivi

[kkk]

Hi Kossivi

Actually I have never used MRFSimpleFoam, so this is my guess.
I guess the rotation force might be added to N-S equation as a inertial force in MRFSimpleFoam.
Just imagine that you were seeing the result of pimpleDyMFoam on the rotating inertial system.
The results of both might be the same.
Then you could evaluate it at any angle you like.

Regards,
kkk

[Kossivi]

Originally Posted by kkk

Hi Kossivi

Actually I have never used MRFSimpleFoam, so this is my guess.
I guess the rotation force might be added to N-S equation as a inertial force in MRFSimpleFoam.
Just imagine that you were seeing the result of pimpleDyMFoam on the rotating inertial system.
The results of both might be the same.
Then you could evaluate it at any angle you like.

Regards,
kkk

Sorry for the late reply. It clear. And I have been able to speed up my case.
Thanks.