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Strange Result for Versteeg Testcase (2D, convection only, steady state) |
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#1 |
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Chris
Join Date: Jun 2011
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Hi everybody,
I'm new to OpenFOAM and tried to implement a testcase from the Versteeg and Malalasekera book[1]. GoogleBooks To solve the problem I used the scalarTransportFoam solver and set DT = 0 as it is a convection only problem. I used the boundary condition from the book except for the two outlet patches, where I used zeroGradient for phi. The result I get are quite normal as long as I don't reach a certain number of cells. For example if my domain is 1 x 1 m and I use more than 21 cells in the x_1 and the x_2 direction the result is completely instable and has nothing to do with the analytic solution. I solved the div(phi,T) term with Gauss linear. But the solution doesn't converge either if I use QUICK or vanLeer to compute the gradient. Only upwind always give a reasonable but not really accurate solution. Does anybody have an idea what problem is? Thanks in advance. caramelo |
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#2 |
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Santiago Marquez Damian
Join Date: Aug 2009
Location: Santa Fe, Santa Fe, Argentina
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Can you post an sketch of the problem and BC's used?
Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC) - CONICET/UNL Tel: 54-342-4511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe - Argentina. http://www.cimec.org.ar |
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#3 |
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Chris
Join Date: Jun 2011
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You can see a sketch in the GoogleBooks links i posted. I just added the whole case as an attachment. With this setting you will get a reasonable result. If you now increase the numbers of cells in the blockMeshDict, no reasonable result will be received any more.
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#4 |
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Laurence R. McGlashan
Join Date: Mar 2009
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When you get finer grid cells, I think there's an issue with the Courant number for non-upwind schemes. Try changing ddtSchemes to Euler and make the endTime in controlDict something like 2; I get 'better' results with that.
I note that: "At the first and last nodes where the diagonal intersects the boundary a value of 50 is assigned to \phi." Might that be an issue?
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Laurence R. McGlashan :: Website Last edited by l_r_mcglashan; August 16, 2011 at 17:56. |
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#5 |
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Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
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It is simply a question of conditioning of the linear problem you are trying to solve. You would need under-relaxation (have to modified the code adding T.relax()) to use the code as steady state.
Alternatively, simply perform a pseudo-transient simulation, as suggested by Laurence, since the effect is the same as under-relaxing. P.S. It is not a problem in the schemes ;-) Best,
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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. ![]() |
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