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January 24, 2013, 08:45 |
dambreak tutorial's weird velocity field.
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#1 |
Senior Member
Dongyue Li
Join Date: Jun 2012
Location: Beijing, China
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Hi guys:
In the dambreak tutorial,water is set to the right side of the box. after I set the alpha is like this:you can see the mesh is fine. but what happend to the velocity field? I just alter the alpha field. and it should be stationary.so the velocity should be zero rite? |
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January 24, 2013, 17:28 |
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#2 |
Retired Super Moderator
Bruno Santos
Join Date: Mar 2009
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Greetings Forrest,
Without knowing the exact steps you've taken, it's a bit complicated to replicate your images... so... what exact steps did you take? Best regards, Bruno
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January 24, 2013, 21:02 |
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#3 | |
Senior Member
Dongyue Li
Join Date: Jun 2012
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Quote:
The time step is absolutely the same with the original dambreak tutorial. What I change is ONLY the alpha field. Code:
defaultFieldValues ( volScalarFieldValue alpha1 0 ); regions ( boxToCell { box (0 0 -1) (1 0.292 1); fieldValues ( volScalarFieldValue alpha1 1 ); } ); contralDict is attached. Code:
application interFoam; startFrom startTime; startTime 0; stopAt endTime; endTime 1; deltaT 0.001; writeControl adjustableRunTime; writeInterval 0.05; purgeWrite 0; writeFormat ascii; writePrecision 6; writeCompression uncompressed; timeFormat general; timePrecision 6; runTimeModifiable yes; adjustTimeStep yes; maxCo 0.5; maxAlphaCo 0.5; maxDeltaT 1; |
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January 26, 2013, 15:48 |
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#4 |
New Member
Stephen Lucchesi
Join Date: Jul 2011
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It's likely to be to do with the mesh. There is a different mesh size used above the obstacle, the boundary between these meshes is likely causing the issue.
i'm suprised there is not also an effect in the areas lower than the top of the obstacle as there is also a mesh boundary there. |
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January 26, 2013, 19:47 |
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#5 |
Retired Super Moderator
Bruno Santos
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Greetings to all!
Stephen is right about the mesh. Since the cells aren't evenly shaped, they induce the concentration of those vortices. After doing some tests, since I was very curious about this and I finally had the opportunity to play with it, here's what I found out:
Now that I come to think of it, I very vaguely remember some presentation on this subject... Anyway, there you go, the result is explained Best regards, Bruno
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January 26, 2013, 22:54 |
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#6 | |
Senior Member
Dongyue Li
Join Date: Jun 2012
Location: Beijing, China
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Hi Bruno,
Thanks very much. just as you said ,Its caused by the mesh. but I cant catch whats this mean. Quote:
And I tried some other cases. 1) alpha is uniform 0; namely its all air. and in 1 sec, the max is 9e-30,looks like its normal.the image is attached. 2) alpha is uniform 1, its totally water. and I set the BC is this: Code:
atmosphere { type inletOutlet; inletValue uniform 1; value uniform 1; } 3) alpha is uniform 1, its totally water too. and I set Code:
atmosphere { type inletOutlet; inletValue uniform 0; value uniform 0; } What confused me is, in reality, if there is a bottle filled with water(for example 3 in which air can flow), There should not be like air can drop in the water,even like this there should be much more bubbles. Why I am testing this is because when I am doing my 3D case I find the same problem about the velocity.so I turn to the dambreak tutorial and make a comparision.then problem arises. Well, You absolutely handled much problems,if you can explain it more ,I would be very grateful.Thanks in advance. |
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January 27, 2013, 06:49 |
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#7 |
Retired Super Moderator
Bruno Santos
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I didn't show any images before because it seemed simple enough. First about the set-up in ParaView:
Now, comparing snapshots (notice the names of the images! They are not sorted in time):
In conclusion:
Bruno
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January 27, 2013, 09:48 |
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#8 | |
Senior Member
Dongyue Li
Join Date: Jun 2012
Location: Beijing, China
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Quote:
This is a huge information for me. Iam a beginner.HOO.But its veru useful. I will check it out later and try some other cases to testify. Thanks very much for you consistant assistance Bruno.now it seems more clear. Last edited by sharonyue; January 27, 2013 at 21:14. |
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October 18, 2016, 15:57 |
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#9 | ||
New Member
Islem Megdiche
Join Date: Feb 2016
Location: Liverpool
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Quote:
Thx for the explanation !! Quote:
I am currently modeling a dam Break case, I used same settings as the damBreak tutorial, when the mesh is coarse the simulation works fine whether turbulence is included or not, but when I refine the mesh to satisfy the y+ condition imposed by the turbulence model, the simulation crash due to spurious current i.e velocity go crazy thus deltaT decreases. The smallest element I am using is 3e-04 m. If I try to run the problem as a laminar case using the same mesh, the simulation doesn't crash but It takes a long time. I will have to refine the mesh further, beacuse I will run KW sst model which requires y+=1 near the wall to solve the boundary layer thus a finer mesh that the one required by k-epsilon, The first cell height should 9.6e-06m near the wall which will lead to more spurious current as the latter increases when the mesh decreases. I would say that my mesh (that I used to run k-epsilon model leading the simulation to crash) is structured mesh except that I have sudden change in the mesh size in some zones of the computational domain and some non-orthogonal faces in some area, I just wanted to reduce the number of cells to gain in computation time but as I said it is causing problem when turulence modelling is included. Any advice in how the mesh should be in relation to spurious current and turbulence modelling ?? I appreciate you help in advance Regards, islem |
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