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January 3, 2013, 12:02
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Hex and Tet meshes - simplefoam comparison
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#1 |
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Senior Member
Daniele Vicario
Join Date: Mar 2009
Location: Novara, Italy
Posts: 142
Rep Power: 17  |
Hi everybody,
Since I need to use the dynamicTopoFvMesh class I'm trying to understand how my sims (already run with hex meshes) performs using Tet meshes. I started studying the flow rate of a custom valve. Enclosed you can see the same geometry meshed with SHM and with Netgen, togheter with their checkmesh results. Some notes: - Same pressure driven BC for both the cases. - Same turbolence model setup (komegaSST). - Same fvscheme and fvsolution. This is the log file of the flow rate using hex meshes: Code:
# Time inlet outlet
1 -5.700601 5.700579
2 -3.79034 3.790358
3 -4.903445 4.903438
4 -5.377026 5.377034
5 -5.704897 5.704898
6 -5.944335 5.944333
7 -6.119664 6.119663
8 -6.220943 6.220941
9 -6.256514 6.256513
10 -6.250962 6.250963
11 -6.218107 6.218109
12 -6.161133 6.161132
13 -6.088814 6.088814
14 -6.00451 6.004513
15 -5.912531 5.912534
16 -5.815976 5.815976
17 -5.716687 5.716688
18 -5.6167 5.6167
19 -5.517849 5.51785
20 -5.422765 5.422765
21 -5.333158 5.333159
22 -5.250153 5.250153
23 -5.173377 5.173377
24 -5.102728 5.102728
25 -5.037989 5.037989
26 -4.978615 4.978616
27 -4.924211 4.924208
28 -4.874001 4.874
29 -4.827512 4.827511
30 -4.784434 4.784432
31 -4.744479 4.744479
32 -4.708216 4.708216
33 -4.675977 4.675978
34 -4.647641 4.647642
35 -4.622598 4.622598
36 -4.600458 4.600458
37 -4.580474 4.580475
38 -4.562082 4.562083
39 -4.544581 4.54458
40 -4.527438 4.52744
41 -4.510434 4.510434
42 -4.493563 4.493563
43 -4.476761 4.476761
44 -4.460181 4.460181
45 -4.443663 4.443663
46 -4.426965 4.426965
47 -4.410062 4.410062
48 -4.39336 4.39336
49 -4.376991 4.376991
....
2627 -4.27264 4.270662
2628 -4.272626 4.27058
2629 -4.272489 4.270488
2630 -4.272234 4.270317
2631 -4.271874 4.270348
2632 -4.271439 4.270732
2633 -4.27099 4.271581
2634 -4.270574 4.272077
2635 -4.270118 4.271661
Code:
# Time inlet outlet
1 -172.0041 21.71271
2 -29.95186 1.310735
3 -57.05148 23.14831
4 -50.40974 30.45741
5 -42.4858 35.18858
6 -10.63777 21.76366
7 -14.34278 7.928121
8 -19.71502 2.089407
9 -20.83303 9.133087
10 -12.97812 21.45
11 -15.64673 24.9519
12 -18.66453 22.21093
13 -20.08745 5.498206
14 -10.07858 0.3255894
15 -8.387256 3.84717
16 -2.87546 10.25943
17 -4.728935 13.8913
18 -1.959338 13.97463
19 -9.568247 5.668299
20 -7.270501 3.24852
21 -6.705826 5.219731
22 -4.514791 7.281349
23 -6.902184 8.871348
24 -5.602596 7.402167
25 -8.852266 4.557721
26 -6.024786 9.46239
27 -5.604201 6.377496
28 -3.582253 7.762387
29 -4.761439 7.010344
30 -5.44479 5.005502
31 -6.136755 4.06424
32 -4.496904 4.433816
33 -4.910032 5.501521
34 -5.057533 5.617444
35 -5.251011 5.396437
36 -3.316635 4.600008
37 -5.171873 4.411376
38 -5.573408 4.209184
39 -4.3302 4.705713
40 -4.163483 4.676618
41 -2.761881 4.432551
42 -3.619489 3.871926
43 -4.572584 3.528158
44 -3.58269 3.626229
45 -3.723246 3.58942
46 -3.62441 3.603673
47 -3.386411 3.532791
48 -3.105231 3.406442
49 -2.682748 3.179411
50 -2.841984 2.989721
51 -2.799791 2.711039
52 -2.827969 2.447284
53 -2.735649 2.314938
54 -2.527999 2.362736
55 -2.452254 2.342192
56 -2.333465 2.369357
57 -2.174707 2.385321
58 -2.045561 2.35066
59 -2.002889 2.26217
60 -1.992403 2.144051
61 -1.961413 2.003014
62 -1.941028 1.842707
63 -1.969635 1.706767
64 -1.976639 1.629558
65 -1.983343 1.656954
66 -2.024261 1.763623
67 -2.057045 1.869765
68 -2.08375 1.999511
69 -2.208145 2.08836
70 -1.955683 2.163669
71 -2.044087 2.2096
72 -2.084881 2.236304
73 -2.368958 2.315571
74 -2.469138 2.371803
75 -2.61045 2.457574
76 -2.798879 2.556751
77 -3.012586 2.678196
78 -3.194521 2.811004
79 -3.356413 2.947216
80 -3.43074 3.077914
81 -3.443503 3.194778
82 -3.465136 3.298215
83 -3.524945 3.395603
84 -3.6387 3.500317
85 -3.807765 3.615832
....
563 -4.316257 4.31451
564 -4.315974 4.31457
565 -4.316129 4.3145
566 -4.316908 4.31411
567 -4.316524 4.314286
568 -4.316411 4.314671
569 -4.316268 4.314813
570 -4.316568 4.314814
571 -4.316601 4.314811
572 -4.317013 4.314497
Even if the converging result seems good I've got some questions: - The hex mesh case is converging not only faster... but better. Is it possible that mass conservation law is not respected in the first iterations of the tet mesh case ? See differences in the inlet/outlet flow rates... - I used the same fvschemes and fvsolution for both the meshes. I tried other settings (see http://www.cfd-online.com/Forums/ope...tml#post257294) but with more instabilities during the first iterations. Below are my settings: fvSchemes: Code:
/*--------------------------------*- C++ -*----------------------------------*\ | ========= | | | \\ / F ield | OpenFOAM: The Open Source CFD Toolbox | | \\ / O peration | Version: 2.0.0 | | \\ / A nd | Web: www.OpenFOAM.org | | \\/ M anipulation | | \*---------------------------------------------------------------------------*/ FoamFile { version 2.0; format ascii; class dictionary; location "system"; object fvSchemes; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // ddtSchemes { //default Euler; default steadyState; } gradSchemes { default Gauss linear; } divSchemes { default none; div(phi,U) Gauss linearUpwindV grad(U); div(phi,k) Gauss upwind; div(phi,omega) Gauss upwind; div((nuEff*dev(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear corrected; } interpolationSchemes { default linear; } snGradSchemes { default corrected; } fluxRequired { default no; p; } // ************************************************************************* // Code:
/*--------------------------------*- C++ -*----------------------------------*\ | ========= | | | \\ / F ield | OpenFOAM: The Open Source CFD Toolbox | | \\ / O peration | Version: 2.0.0 | | \\ / A nd | Web: www.OpenFOAM.org | | \\/ M anipulation | | \*---------------------------------------------------------------------------*/ FoamFile { version 2.0; format ascii; class dictionary; object fvSolution; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // solvers { p { solver GAMG; tolerance 1e-8; relTol 0.01; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; cacheAgglomeration on; agglomerator faceAreaPair; nCellsInCoarsestLevel 20; mergeLevels 2; } pFinal { solver GAMG; tolerance 1e-8; relTol 0; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; cacheAgglomeration on; agglomerator faceAreaPair; nCellsInCoarsestLevel 10; mergeLevels 1; } U { solver smoothSolver; smoother GaussSeidel; tolerance 1e-8; relTol 0.05; nSweeps 1; } k { solver smoothSolver; smoother GaussSeidel; tolerance 1e-9; relTol 0.01; nSweeps 1; } omega { solver smoothSolver; smoother GaussSeidel; tolerance 1e-9; relTol 0.01; nSweeps 1; } } SIMPLE { nNonOrthogonalCorrectors 1; pRefCell 0; pRefValue 0; residualControl { p 1e-5; U 1e-5; nuTilda 1e-5; } } potentialFlow { nNonOrthogonalCorrectors 10; } relaxationFactors { p 0.5; U 0.7; k 0.6; omega 0.6; } PISO { nCorrectors 2; nNonOrthogonalCorrectors 0; pRefCell 0; pRefValue 0; } cache { grad(U); } // ************************************************************************* // Personally I'm not unsatisfied of the comparison. There is about 1 % difference in the flow rates results and the tet sim is still running. I'm just worrying because I need to use tet meshes with dynamicTopoFvMesh in order to simulate the equivalent of a check valve: an object immersed in a pushing fluid is going to close the outlet... it's one of the worst setup I think... Above all, here http://www.google.it/url?sa=t&rct=j&...55534169,d.bGE it says that "Crushing cell is not supported (Example: complete valve closure for cylinder simulations)" but it was one and half years ago...what about now ? More: Looking in the forum it seems that tet meshes are a little delicate to work with... is there anyone who use dynamicTopoFvMesh on real cases ? I'm sorry for the long thread. Any comment ?
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Daniele Vicario blueCFD2.1 - Windows 7 |
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January 3, 2013, 12:36
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#2 |
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Senior Member
Sandeep Menon
Join Date: Mar 2009
Location: Amherst, MA
Posts: 403
Rep Power: 25 |
Daniele,
I'm not surprised to see that Hex meshes gives better results - they inherently have less non-orthogonality. You'll have to play with the nNonOrthogonalCorrectors with tets and see if that helps. Also, you have a gradient scheme that isn't really the best - try leastSquares. You could also try Hrv's reconCentral schemes from 1.6-ext and see if that changes things. |
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January 3, 2013, 13:01
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#3 |
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Senior Member
Daniele Vicario
Join Date: Mar 2009
Location: Novara, Italy
Posts: 142
Rep Power: 17 |
Thanks for the suggestions. Unfortunately I cannot use the ext version, but I'll try others schemes. Any news regarding the crushing cells limitation ?
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Daniele Vicario blueCFD2.1 - Windows 7 |
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January 3, 2013, 13:08
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#4 |
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Senior Member
Sandeep Menon
Join Date: Mar 2009
Location: Amherst, MA
Posts: 403
Rep Power: 25 |
Well, I suppose you could compile the sources for reconCentral in 2.1.x - I don't think there's anything in there specific to 1.6-ext.
The closest thing I can see in OpenFOAM to complete valve closure is either by changing a GGI / AMI patch at run-time to wall - which probably has adverse stability issues where you'll have to either under-relax or reduce the time-step. Or possibly by switching cells off by marking cells in a specific zone and deactivating it. You could try playing around with porous zones too... |
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January 3, 2013, 13:10
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#5 |
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Senior Member
Philippose Rajan
Join Date: Mar 2009
Location: Germany
Posts: 552
Rep Power: 25 |
Hello,
I use Tetrahedral meshes quite often (well... almost always), and as you have seen, the convergence of a simulation when using Tet meshes is in most cases not as good as for the same system with a hex dominant mesh. However, I would suggest that you try the following in addition to the suggestions by Sandeep: 1. As Sandeep mentioned.... for Tet meshes, non-orthogonal correctors are almost always a requirement. I use 2 non-orthogonal correctors, and it works quite well. 2. I noticed that your relaxation factors are quite high.... You could try with lower values to start of with (say for the first 250 - 300 iterations). Again... a suggested value is: 0.3 for all the variables. The results from Checkmesh seem to be quite ok for the Tet mesh, and I dont think you can get too much better than that.... as far as non-orthogonality goes. You could try one more thing..... In netgen, in the meshing options, you can choose the number of optimisation steps for the surface and volume mesh generation...... Set the Surface Mesh optimisation to around 15 steps, and the volume mesh optimisation to around 12..... The mesh generation might take a little longer, but the quality of the resulting meshes should become a little better. A great evening ahead! Philippose |
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January 3, 2013, 15:32
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#6 |
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Senior Member
Daniele Vicario
Join Date: Mar 2009
Location: Novara, Italy
Posts: 142
Rep Power: 17 |
@Sandeep:
I'll try to use reconCentral. I don't think I'm going to find particular problems compiling the source. Actually I'm more worried about your next suggestion...I just want to be sure: is it possible with the current release of dynamictopofvmesh class to manage crushing cells (i.e. the closing valve example) ? If not, it seems to me that the piston engine video shows this functionality... am I so wrong ? @philippose: Many thanks ! I'm going to play a little with those parameters and see what happens...
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Daniele Vicario blueCFD2.1 - Windows 7 |
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January 3, 2013, 15:36
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#7 |
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Senior Member
Sandeep Menon
Join Date: Mar 2009
Location: Amherst, MA
Posts: 403
Rep Power: 25 |
You're right. Valve closure is not supported at the moment.
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January 3, 2013, 16:18
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#8 |
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Senior Member
Daniele Vicario
Join Date: Mar 2009
Location: Novara, Italy
Posts: 142
Rep Power: 17 |
Well, that's....interesting. I'm not enought prepare to fully understand why it can't work but I'm still however interested in testing my cases.
Even because your class is the most automatic system you can have in OF that can manage dynamic meshes... is it ? I read a lot regarding GGI and AMI (but never try none of them) but, as you said, they seem too...delicate for my application. However easy question: is there any plan to support crushing cells in the near future ?
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Daniele Vicario blueCFD2.1 - Windows 7 |
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January 3, 2013, 16:25
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#9 |
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Senior Member
Sandeep Menon
Join Date: Mar 2009
Location: Amherst, MA
Posts: 403
Rep Power: 25 |
Valve closure: Can't guarantee it. Mainly because I haven't found a need to do it till now. I could look into it if there's significant interest, but that largely depends on whether I have the time on my hands.
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January 3, 2013, 16:58
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#10 |
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Senior Member
Daniele Vicario
Join Date: Mar 2009
Location: Novara, Italy
Posts: 142
Rep Power: 17 |
Ok. Thanks anyway for your work.
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Daniele Vicario blueCFD2.1 - Windows 7 |
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January 4, 2013, 02:18
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#11 |
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Senior Member
Daniele Vicario
Join Date: Mar 2009
Location: Novara, Italy
Posts: 142
Rep Power: 17 |
Just a small note:
In fvSolution, using relTol=0 brings the inlet flow rate to be equal to the outlet one since the very first step. This however increase each iteration time. Still don't know which value is better (from the speed point of view). Probably it depends on the case.
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Daniele Vicario blueCFD2.1 - Windows 7 |
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