# convergence problem

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June 17, 2011, 04:08
convergence problem
#1
Member

Gregoire Junqua
Join Date: Jun 2011
Location: China
Posts: 58
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Rep Power: 7
Hi there.
I am quite new into modelling but i got a convergence problem as follow DICPCG ask a many iteration. How to make it less?
is improving the grid resolution or lowering the grid resolution could help?
or a better approximation of k and epsilon is the solution?

Quote:
 Courant Number mean: 0.0750431 max: 0.291312 DILUPBiCG: Solving for Ux, Initial residual = 0.00135266, Final residual = 1.80396e-06, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.00114192, Final residual = 1.76496e-06, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.047671, Final residual = 0.00470314, No Iterations 17 time step continuity errors : sum local = 7.64284e-08, global = -2.95465e-10, cumulative = 1.88697e-06 DICPCG: Solving for p, Initial residual = 0.0290166, Final residual = 9.92062e-07, No Iterations 777 time step continuity errors : sum local = 1.58005e-11, global = -3.76457e-14, cumulative = 1.88697e-06 DILUPBiCG: Solving for epsilon, Initial residual = 0.00315981, Final residual = 6.3086e-06, No Iterations 1 DILUPBiCG: Solving for k, Initial residual = 0.0141768, Final residual = 7.62168e-06, No Iterations 1 ExecutionTime = 655.38 s ClockTime = 658 s

 June 17, 2011, 04:34 hello... #2 Senior Member   Tushar Chourushi Join Date: Jul 2009 Location: IIT-Indore, India Posts: 319 Blog Entries: 1 Rep Power: 10 I think the problem is because of the mesh resolution. I just want to know which type of grid have you used? Is it Unstructured? If it's unstructured then it's usual you will observe the error, check out with structured if you get the same. -------- Last edited by Tushar@cfd; June 17, 2011 at 06:27.

 June 18, 2011, 00:02 #3 Member   Alan Russell Join Date: Aug 2009 Location: Boise, Idaho USA Posts: 61 Rep Power: 9 Check the mesh quality with checkMesh - you could have problems with skewed cells, etc. If you get good results from checkMesh, you are ready to start working on the solution. First, instead of refining the mesh, I would do the opposite. A coarse mesh lets you get the 'outline' of the solution quickly, and shows if your setup is valid. If you can get a coarse mesh solution to converge, then start refining your mesh. Good luck, Alan

June 20, 2011, 21:14
#4
Member

Gregoire Junqua
Join Date: Jun 2011
Location: China
Posts: 58
Blog Entries: 1
Rep Power: 7

What do you mean by unstructured?
My grid got a sinus form...
and my
my grid resolution is around 0.01m
and dT = 0.001;

This is the answer of the checkMesh
Quote:
 Checking topology... Boundary definition OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Surface topology movingWall 1500 3002 ok (non-closed singly connected) fixedWalls 1100 2202 ok (non-closed singly connected) outlet 400 802 ok (non-closed singly connected) frontAndBack 880000 883002 ok (non-closed singly connected) Checking geometry... Overall domain bounding box (0 -0.5 0) (11 10 0.1) Mesh (non-empty, non-wedge) directions (1 1 0) Mesh (non-empty) directions (1 1 0) All edges aligned with or perpendicular to non-empty directions. Boundary openness (-6.5805e-20 5.32246e-20 -7.18612e-17) OK. Max cell openness = 1.78614e-16 OK. Max aspect ratio = 4.0807 OK. Minumum face area = 0.000237529. Maximum face area = 0.002625. Face area magnitudes OK. Min volume = 2.37529e-05. Max volume = 2.62471e-05. Total volume = 10.9684. Cell volumes OK. Mesh non-orthogonality Max: 57.1056 average: 28.6341 Non-orthogonality check OK. Face pyramids OK. Max skewness = 1.29563 OK. Mesh OK.
Today i will do some test by improving the grid and the time resolution hope it would work

 June 21, 2011, 00:20 #5 Member   Alan Russell Join Date: Aug 2009 Location: Boise, Idaho USA Posts: 61 Rep Power: 9 In your checkMesh, I see an outlet but no inlet, which will cause problems with the continuity equation (conservation of mass). Most of your setup is like the cavity tutorial, except for the outlet. Can you describe what you are trying to model? You will certainly need some kind of inlet if you have an outlet. Alan

 June 21, 2011, 01:37 #6 Senior Member   Tushar Chourushi Join Date: Jul 2009 Location: IIT-Indore, India Posts: 319 Blog Entries: 1 Rep Power: 10 Can you please upload your case file??

 June 21, 2011, 02:55 #7 Member   Gregoire Junqua Join Date: Jun 2011 Location: China Posts: 58 Blog Entries: 1 Rep Power: 7 I don't specify the inlet, i just want a uniform wind field at the boundaries Moving wall +-------------------------------------------------------------------+ | .................................................. ........................................ | |................................................. .......................................... | Moving Wall .................................................. .....................outlet | .................................................. .........................................| | .................................................. .........................................| _.-._.-._.-._.-._.-._.-._.-._.-._-._.-._.-._.-._.-._.-._.-._.-._.| The ground is a sin function, and i want to view the turbulence around it. [QUOTEFoamFile { version 2.0; format ascii; class volVectorField; object U; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dimensions [0 1 -1 0 0 0 0]; internalField uniform (0 0 0); boundaryField { movingWall { type fixedValue; value uniform (1 0 0); } fixedWalls { type fixedValue; value uniform (0 0 0); } outlet { type zeroGradient; } frontAndBack { type empty; } } // ************************************************** *********************** // ][/QUOTE] If you need more just ask Thank you for your interest anyway

 June 21, 2011, 03:33 #8 Member   Gregoire Junqua Join Date: Jun 2011 Location: China Posts: 58 Blog Entries: 1 Rep Power: 7 I had try a lower grid scale as around 0.05m and there is a good convergence ... How to explain it? my first guess was more the grid scale is small more the convergence is fast ...

 Tags convergence, rans

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