Predicted Cd is dependent on time step (pimpleFoam)

sharonyue · March 1, 2024, 10:41

In this test case, a Re = 1.5e4 single phase flow around a cylinder was simulated. It was found the predicted Cd is dependent on deltaT.
When deltaT=1e-4, time averaged Cd equals 1.19, which match the experiments (Cd=1.2).
When deltaT=1e-5, time averaged Cd equals 1.6, which is much higher.
Please see the attached picture.

Run the test case by OpenFOAM-10. See the value of Cd
change deltaT=1e-5, see the value of Cd
In a laptop, one can run the test case in serial for 300s-400s to see the variation of Cd.

This is also reported here: https://bugs.openfoam.org/view.php?id=4059

Test case link:

https://drive.google.com/file/d/1LaS...usp=drive_link

sharonyue · March 4, 2024, 02:22

The bug report is closed by OpenFOAM foundation.
Please also see the last pages of this: https://www.tfd.chalmers.se/~hani/ku...es/ddtCorr.pdf

It looks this problem comes from ddtcorr implementation.

geth03 · March 4, 2024, 03:36

as Henry Weller already mentioned, the Courant Number is too large obviously for your simulation and Courant Number independency is achieved for Co<<1.

sharonyue · March 5, 2024, 10:15

1. Co<<1 does not provide the correct results. I tried 0.01, it predicts high Cd value.
2. Providing Co < 1, results should not be very time dependent. A little bit difference should be acceptable.
3. It does not make sense using a smaller time step, but the results are worse. At least it cannot be justified by theory (sometims it happens though), especially for such a easy single phase test case.
4. https://www.tfd.chalmers.se/~hani/ku...es/ddtCorr.pdf In this slide, they also found with ddtcorr, it predicts higher Cd. Exactly the same problem in my test case.

These days I use the pimpleFoam provided by ESI OpenFOAM-v2306, whereas the ddtcorr is turned off. It does not have this problem.

sylvester · March 6, 2024, 08:16

Quote:

Originally Posted by sharonyue

These days I use the pimpleFoam provided by ESI OpenFOAM-v2306, whereas the ddtcorr is turned off. It does not have this problem.

ddtCorr may be switched off, but default on:
https://www.openfoam.com/documentati...10b5389e09c0b4

sharonyue · March 6, 2024, 10:52

Alright, so, I have no idea what happens. It requires lots effort to debug. I think it comes from the algorithm instead of coding errors.

March 4, 2024, 02:22		#2
sharonyue Senior Member Dongyue Li Join Date: Jun 2012 Location: Beijing, China Posts: 838 Rep Power: 17	The bug report is closed by OpenFOAM foundation. Please also see the last pages of this: https://www.tfd.chalmers.se/~hani/ku...es/ddtCorr.pdf It looks this problem comes from ddtcorr implementation. __________________ My OpenFOAM algorithm website: http://dyfluid.com By far the largest Chinese CFD-based forum: http://www.cfd-china.com/category/6/openfoam We provide lots of clusters to Chinese customers, and we are considering to do business overseas: http://dyfluid.com/DMCmodel.html Last edited by sharonyue; March 5, 2024 at 10:28.

March 5, 2024, 10:15		#4
sharonyue Senior Member Dongyue Li Join Date: Jun 2012 Location: Beijing, China Posts: 838 Rep Power: 17	1. Co<<1 does not provide the correct results. I tried 0.01, it predicts high Cd value. 2. Providing Co < 1, results should not be very time dependent. A little bit difference should be acceptable. 3. It does not make sense using a smaller time step, but the results are worse. At least it cannot be justified by theory (sometims it happens though), especially for such a easy single phase test case. 4. https://www.tfd.chalmers.se/~hani/ku...es/ddtCorr.pdf In this slide, they also found with ddtcorr, it predicts higher Cd. Exactly the same problem in my test case. These days I use the pimpleFoam provided by ESI OpenFOAM-v2306, whereas the ddtcorr is turned off. It does not have this problem. __________________ My OpenFOAM algorithm website: http://dyfluid.com By far the largest Chinese CFD-based forum: http://www.cfd-china.com/category/6/openfoam We provide lots of clusters to Chinese customers, and we are considering to do business overseas: http://dyfluid.com/DMCmodel.html

March 6, 2024, 10:52		#6
sharonyue Senior Member Dongyue Li Join Date: Jun 2012 Location: Beijing, China Posts: 838 Rep Power: 17	Alright, so, I have no idea what happens. It requires lots effort to debug. I think it comes from the algorithm instead of coding errors. __________________ My OpenFOAM algorithm website: http://dyfluid.com By far the largest Chinese CFD-based forum: http://www.cfd-china.com/category/6/openfoam We provide lots of clusters to Chinese customers, and we are considering to do business overseas: http://dyfluid.com/DMCmodel.html

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March 4, 2024, 03:36		#3
geth03 Senior Member Join Date: Dec 2019 Location: Cologne, Germany Posts: 355 Rep Power: 8	as Henry Weller already mentioned, the Courant Number is too large obviously for your simulation and Courant Number independency is achieved for Co<<1.