What are the reasons for Divergence due to back flow in TFM simulation?

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  • Last Post 23 October 2018
Taha posted this 23 October 2018

I have simulated TFM simulation and it runs fine but when I add UDF for granular viscosity, I get divergence error due to back flow after certain time steps? What are the possible reasons for this and How to modify UDF to avoid divergence (floating point error)?

Mesh Quality:

Minimum Orthogonal Quality = 7.12847e-01

Maximum Ortho Skew = 2.87153e-01

 

Maximum Aspect Ratio = 3.49884e+00

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abenhadj posted this 23 October 2018

You can try to debug why by using the UDF you end up with divergence. What is in the code included which is not provided by Standard Fluent models for granular viscosity?

Best regards,

Amine

Taha posted this 23 October 2018

I have already used this UDF in another case and its working fine. Also i have used this case without granular viscosity UDF and its working fine. Now problem is that I am not able to run this case with UDF.

so in short combination of UDF and case (in consideration) is not working

abenhadj posted this 23 October 2018

Okay. Perhaps you need to be more Conservative at the beginning of the run (time step size, URF's) and store some UDMI from UDF to see if they are some under/overshoots.

Best regards,

Amine

rwoolhou posted this 23 October 2018

If the model runs for a time then fails, run the model until near the failure point and have a look at the flow field. You're looking for weird results, and excessive velocity/pressure spikes. If you're using a UDF I'd also check what the solver thinks granular viscosity is. 

Taha posted this 23 October 2018

This is the UDF. Kindly help me and let me know what is the the real problem.

https://mega.nz/#!PhggBQYQ!iCdl0U32hHwje4-NL11kB5OMyMXZS1-uDPe1gXMd_Mk

abenhadj posted this 23 October 2018

Please check as rwoolhou suggested what the solver thinks about the granular viscosity. You can post a screenshot of the UDF as I won't hit the link you shared here. 

Best regards,

Amine

Taha posted this 23 October 2018

Granular Viscosity UDF

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