OpenFOAM v.2.x Courant Number 4 time higher then in previous versions

makaveli_lcf · February 16, 2012, 05:57

Hi Foamers!

I got and issue using OpenFOAM v2.x:

Courant Number jumps 4 times if compare to OpenFOAM v1.6 and 1.7.x.
I tried cases for different geometries ofcouse with the same setting between OF versions and result is always the same!
Actually this issue results in a calculation time increasing 4 times. Unappropriated!

Please, comment on that issue, what is your experience with v2.x usage?

Cheer,
Alex

makaveli_lcf · February 16, 2012, 08:54

Reported at Mantis:

http://www.openfoam.com/mantisbt/view.php?id=424

makaveli_lcf · February 16, 2012, 10:05

Issue closed (http://www.openfoam.com/mantisbt/view.php?id=424#c1039):

Quote:

The method for calculating the Courant number has been changed since version 1.7.x to be better represent the stability requirement of collocated meshes. In general the change hase resulted in slightly lower values.

A change in the method to calculate the Courant number does not in itself change the stability of the algorithm or lead to divergence as it is not used in the algorithm; only to set the time-step. If you have a case which behaved differently for the same fixed time-step between different versions of OpenFOAM the issue does not relate to a change in the way in which the Courant number is calculated.

So then it means that OF 2.x is less stable with the same fixed time step...
Will do more checks and report regarding actual calculation time.

alberto · February 17, 2012, 12:02

Quote:

Originally Posted by makaveli_lcf

Issue closed (http://www.openfoam.com/mantisbt/view.php?id=424#c1039):

So then it means that OF 2.x is less stable with the same fixed time step...
Will do more checks and report regarding actual calculation time.

What solver? I was aware of the change in the definition of Co, which was made to remove inconsistency at boundaries, but I haven't experienced major issues with 2.1.x.

makaveli_lcf · February 17, 2012, 16:20

Hi Alberto!

pimpleFoam. According to my test to keep the same Co number (with which solution does not diverge) actual calculation time increased 2 times (I remind with the same constant dt I've got Co_new_OF = 4*Co_old_OF).

makaveli_lcf · February 17, 2012, 16:22

Slow down, to my opinion, is not 4 times because now turbulence is solved at last PIMPLE iteration. But of course I tried with turbulence updated each iteration: the same result, solution diverges with the dt I used before with old OF versions..

alberto · February 17, 2012, 22:14

Quote:

Originally Posted by makaveli_lcf

Slow down, to my opinion, is not 4 times because now turbulence is solved at last PIMPLE iteration. But of course I tried with turbulence updated each iteration: the same result, solution diverges with the dt I used before with old OF versions..

OK, let's try to do some troubleshooting, if you like

Do you use any particular boundary condition? For example mapped, or time-varying, or custom BC? Some of these were updated in 2.x.

makaveli_lcf · February 18, 2012, 07:55

Nope, standard k-epsilon (also tried with realizable which I mostly use) with standard wall functions for k, epsilon, nut, fixed values for U at inlet and zero gradient at outlet, pressure 0 at outlet and zero gradient at the inlet. I think it would be better if I upload here some test case. Will do it on Monday.

alberto · February 18, 2012, 22:28

Yes, that's a good idea. The BC's are surely not the reason.

February 16, 2012, 05:57	OpenFOAM v.2.x Courant Number 4 time higher then in previous versions	#1
makaveli_lcf Senior Member Dr. Alexander Vakhrushev Join Date: Mar 2009 Posts: 256 Blog Entries: 1 Rep Power: 19	Hi Foamers! I got and issue using OpenFOAM v2.x: Courant Number jumps 4 times if compare to OpenFOAM v1.6 and 1.7.x. I tried cases for different geometries ofcouse with the same setting between OF versions and result is always the same! Actually this issue results in a calculation time increasing 4 times. Unappropriated! Please, comment on that issue, what is your experience with v2.x usage? Cheer, Alex __________________ Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at

February 16, 2012, 08:54		#2
makaveli_lcf Senior Member Dr. Alexander Vakhrushev Join Date: Mar 2009 Posts: 256 Blog Entries: 1 Rep Power: 19	Reported at Mantis: http://www.openfoam.com/mantisbt/view.php?id=424 __________________ Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at

February 17, 2012, 16:20		#5
makaveli_lcf Senior Member Dr. Alexander Vakhrushev Join Date: Mar 2009 Posts: 256 Blog Entries: 1 Rep Power: 19	Hi Alberto! pimpleFoam. According to my test to keep the same Co number (with which solution does not diverge) actual calculation time increased 2 times (I remind with the same constant dt I've got Co_new_OF = 4*Co_old_OF). __________________ Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at

February 17, 2012, 16:22		#6
makaveli_lcf Senior Member Dr. Alexander Vakhrushev Join Date: Mar 2009 Posts: 256 Blog Entries: 1 Rep Power: 19	Slow down, to my opinion, is not 4 times because now turbulence is solved at last PIMPLE iteration. But of course I tried with turbulence updated each iteration: the same result, solution diverges with the dt I used before with old OF versions.. anmartin likes this. __________________ Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at

February 18, 2012, 07:55		#8
makaveli_lcf Senior Member Dr. Alexander Vakhrushev Join Date: Mar 2009 Posts: 256 Blog Entries: 1 Rep Power: 19	Nope, standard k-epsilon (also tried with realizable which I mostly use) with standard wall functions for k, epsilon, nut, fixed values for U at inlet and zero gradient at outlet, pressure 0 at outlet and zero gradient at the inlet. I think it would be better if I upload here some test case. Will do it on Monday. __________________ Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at

February 18, 2012, 22:28		#9
alberto Senior Member Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,912 Rep Power: 36	Yes, that's a good idea. The BC's are surely not the reason. __________________ Alberto Passalacqua GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) OpenQBMM - An open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Very high courant number, time step continuity errors and bounding epsilon	erncyc	OpenFOAM Running, Solving & CFD	1	March 24, 2011 12:00
Error while running rhoPisoFoam..	nileshjrane	OpenFOAM Running, Solving & CFD	8	August 26, 2010 13:50
calculation diverge after continue to run	zhajingjing	OpenFOAM	0	April 28, 2010 05:35
Differences between serial and parallel runs	carsten	OpenFOAM Bugs	11	September 12, 2008 12:16
Could anybody help me see this error and give help	liugx212	OpenFOAM Running, Solving & CFD	3	January 4, 2006 19:07