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-   -   different results between serial solver and parallel solver (http://www.cfd-online.com/Forums/fluent/87839-different-results-between-serial-solver-parallel-solver.html)

wlt_1985 April 30, 2011 05:09

different results between serial solver and parallel solver
 
I run my model with serial solver and parallel solver respectively. I found that the results especially the velocity does have large difference in power of magnitude. For example, serial solver 2m/s but parallel solver 20k m/s. What is wrong with the parallel solver? In addition, turbulent viscosity limited to viscosity ration does appear in parallel solver but none in serial solver. Pls help.

Thanks a lot

zhenglun.wei May 1, 2011 17:14

I also found some difference between the serial and parallel version, but not that severe. Hope you'll find the solution asap.

wlt_1985 May 2, 2011 16:46

Quote:

Originally Posted by zhenglun.wei (Post 305815)
I also found some difference between the serial and parallel version, but not that severe. Hope you'll find the solution asap.

Thansk for care. :)

cfd_newbie May 2, 2011 21:18

Quote:

Originally Posted by wlt_1985 (Post 305684)
I run my model with serial solver and parallel solver respectively. I found that the results especially the velocity does have large difference in power of magnitude. For example, serial solver 2m/s but parallel solver 20k m/s. What is wrong with the parallel solver? In addition, turbulent viscosity limited to viscosity ration does appear in parallel solver but none in serial solver. Pls help.

Thanks a lot

Hi Raymond,
Are you sure you have inputed the exact same initial and boundary conditions in both the cases ? If the results are still different then you should report the problem to Fluent. They have a dedicated set of people who look into this issue and they might give you a patch to fix this problem.
Raashid

wlt_1985 May 4, 2011 08:50

Quote:

Originally Posted by cfd_newbie (Post 306002)
Hi Raymond,
Are you sure you have inputed the exact same initial and boundary conditions in both the cases ? If the results are still different then you should report the problem to Fluent. They have a dedicated set of people who look into this issue and they might give you a patch to fix this problem.
Raashid


Dear Raashid,

I have found the causes of the problem. There were no much differences in FINAL RESULTS for both cases. Since the calculation for parallel solver was much faster than the serial solver, hence the WRONG RESULTS due to the model setting have been shown earlier than the serial soler. These errors were due to the TURBULENT VISCOSITY RATIO LIMITATION during iteration. Initially, the limit of turbulent viscosity ratio is set to 1e+05 and there was no such warning of limitation after the initiation has been activated. However, after hundred iterations, this warning of limitation has been shown and the velocity has suddenly been increased after that particualr moment. I have gone through few posts in this forum. These errors were due to poor mesh and they could be solved by having grid adaption. I tried to have grid adaption. However, the residuals of continuity was keeping increasing and going to diverge. I have no idea on solving my problem. Can please help me for my problem? Thanks for help.

Regards
Raymond

cfd_newbie May 4, 2011 21:01

Quote:

Originally Posted by wlt_1985 (Post 306230)
Dear Raashid,

I have found the causes of the problem. There were no much differences in FINAL RESULTS for both cases. Since the calculation for parallel solver was much faster than the serial solver, hence the WRONG RESULTS due to the model setting have been shown earlier than the serial soler. These errors were due to the TURBULENT VISCOSITY RATIO LIMITATION during iteration. Initially, the limit of turbulent viscosity ratio is set to 1e+05 and there was no such warning of limitation after the initiation has been activated. However, after hundred iterations, this warning of limitation has been shown and the velocity has suddenly been increased after that particualr moment. I have gone through few posts in this forum. These errors were due to poor mesh and they could be solved by having grid adaption. I tried to have grid adaption. However, the residuals of continuity was keeping increasing and going to diverge. I have no idea on solving my problem. Can please help me for my problem? Thanks for help.

Regards
Raymond

Hi Raymond,
The gird adaptation technique may or may not work depending on problem to problem. According to my experience (more experienced Fluent users can correct me), mesh adaptation can work well for heat transfer kind of problems but for external aerodynamics problems you should have a good grid to start your problem from the beginning.
Raashid

litterfree May 9, 2011 08:06

the unit for the two cases are same?

wlt_1985 May 9, 2011 11:21

Quote:

Originally Posted by cfd_newbie (Post 306299)
Hi Raymond,
The gird adaptation technique may or may not work depending on problem to problem. According to my experience (more experienced Fluent users can correct me), mesh adaptation can work well for heat transfer kind of problems but for external aerodynamics problems you should have a good grid to start your problem from the beginning.
Raashid

Dear Raashid,

Thanks a lot. I am trying to do it with better grid from the beginning.

Best regards

wlt_1985 May 9, 2011 11:24

Quote:

Originally Posted by litterfree (Post 306794)
the unit for the two cases are same?

Dear Litterfree,

Yes. Both cases are same units.

Best regards

meangreen May 12, 2011 16:36

Make sure your choice of partitions is not affecting your solutions.

yochule April 11, 2012 15:25

dear raymond, i hope you are still in the forum.
i'm havin some problems with local parallel and dynamic gradient adaption. my problem it's that when i set up the gradient adaption scheme, the parallel computing stop working and the next iterations became in serial... so the solutions take so much time. can you tell me if you did found the same issue?
i'm runing in gnu/linux ubuntu amd64, whith the command: ./fluent 2ddp -t4 -pnet
you can check my post here: http://www.cfd-online.com/Forums/flu...-adaption.html

hope you can help me.
thanks a lot (and excuse me to get into your post)


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