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-   -   Problem with simpleFoam (http://www.cfd-online.com/Forums/openfoam-solving/86373-problem-simplefoam.html)

salvoblack March 21, 2011 11:02

Problem with simpleFoam
 
Dear foamers, i have a problem.
Every time I do a calculation with simpleFoam, I have problems related to the type of mesh used. In fact, using an airfoil, if I use a fairly low number of cells as 20,000 I get acceptable results, but if my airfoil is thicker (70000) of the cells i have completely absurd results : the solution diverges. I do not change anything between the various settings, change only the mesh. could you tell me if there is a parameter that should be changed depending on the quality of the mesh??

mirko March 21, 2011 12:22

Quote:

Originally Posted by salvoblack (Post 300355)
Dear foamers, i have a problem.
Every time I do a calculation with simpleFoam, I have problems related to the type of mesh used. In fact, using an airfoil, if I use a fairly low number of cells as 20,000 I get acceptable results, but if my airfoil is thicker (70000) of the cells i have completely absurd results : the solution diverges. I do not change anything between the various settings, change only the mesh. could you tell me if there is a parameter that should be changed depending on the quality of the mesh??

You may have to decrease the time step. Check your courant number.

salvoblack March 22, 2011 06:41

Hello,
I use the cam to calculate the Co courant in openFoam?

maddalena March 22, 2011 07:15

Quote:

Originally Posted by mirko (Post 300373)
You may have to decrease the time step. Check your courant number.

If using simpleFoam the Co is useless, since the simulation is steady state!
Salvo, could you post checkMesh of the old (coarse) mesh and the new (finest) mesh? How do you made the refinement? Increasing the cell number does not mean that you improve mesh quality!!!
What about y+ values?
Have you try to see where the problem may be using simpleFoamResiduals?

mad

salvoblack March 22, 2011 11:23

hi maddalena,sorry for my english.
i put in the terminal Co so i have the Courant number for my mesh.
I have a very very high value of this number:6*10^6. i think that this is the problem for my case.
i know that i can impose Co=1 knowing the distance between cells and the time step. How can I calculate these factors for my mesh???I don't know how calcolate the distance between cells
thanks

maddalena March 22, 2011 18:01

Hi Salvo,
Quote:

Originally Posted by salvoblack (Post 300545)
hi maddalena,sorry for my english.
i put in the terminal Co so i have the Courant number for my mesh.
I have a very very high value of this number:6*10^6. i think that this is the problem for my case.
i know that i can impose Co=1 knowing the distance between cells and the time step. How can I calculate these factors for my mesh???I don't know how calcolate the distance between cells
thanks

your English is not a big issue as far as I can understand your problem. ;)
However... are you really using simpleFoam? If so, than Co has no meaning at all! See equation 2.2 here In your case you should have set deltaT=1, since simpleFoam is a steady state solver, thus deltaT is an iteration counter only! If you want, delete the maxCo line in controlDict file. simpleFoam should no complain about this missing entry, since it does not need it.
In conclusion: the problem lies somewhere else...

mad

salvoblack March 23, 2011 04:54

thank you for yor help, maddalena. i decided to re-make the mesh!!!
just a curiosity, are you italian?? :)

maddalena March 23, 2011 04:56

Quote:

Originally Posted by salvoblack (Post 300662)
thank you for yor help, maddalena. i decided to re-make the mesh!!!
just a curiosity, are you italian?? :)

Yes, I am. That is probably way I can understand your english easily: it is similar to mine! ;)


mad


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