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Interpretation of pressure icoFoam

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Old   May 5, 2007, 23:36
Default Hi, I was running icoFoam w
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Billy
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Hi,

I was running icoFoam with lid-drive cvavity and I noticed that pressure changed in same degree, as I refined the mesh. Doubling the cells in xy direction, the pressure doubles also. Documentation says:

"In a closed incompressible system such as the cavity, pressure is relative: it is the pressure range that matters not the absolute values"

But why does the pressure range change? With 10x10 grid pressure range is -20-20, with 20x20 grid it is -40-40, with 40x40 it is -80-80. Shouldn't it converge to a certain range?

Thanks in advance,

Billy.
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Old   May 6, 2007, 10:46
Default Increasing even more, using a
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Increasing even more, using a 60x60 grid gives very high pressure range like -1E+4-1E+4.

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Old   May 6, 2007, 19:15
Default I also tried a grid with one a
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I also tried a grid with one and two layers, the pressure is increases in the same degree that the layers increase.




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Old   May 6, 2007, 19:15
Default I also tried a grid with one a
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I also tried a grid with one and two layers, the pressure is increases in the same degree that the layers increase.




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Old   May 7, 2007, 11:16
Default The figure shows pressure cont
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The figure shows pressure contours around the inlet. The outlet is located on other side.
Both domains have same thickness.
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Old   May 7, 2007, 15:41
Default Basically the pressure seems t
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Basically the pressure seems to increase at the same rate the volume of the cell decreases.
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Old   May 8, 2007, 17:45
Default Does anyone know why this happ
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Does anyone know why this happens? Don't want this topic to die.
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Old   May 8, 2007, 17:51
Default Please, post a case. I'll try
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Please, post a case. I'll try to run it.

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Old   May 8, 2007, 18:00
Default I would think about two things
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I would think about two things:
1. where is the reference pressure cell and
what is the value you are setting in your case.
2. Dont you think that it would be better to
have more dense mesh in the region of big
gradients(4 cells is not enough, remember you
calculate everything on discrete values).

hope this helps
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Old   May 8, 2007, 18:38
Default Thanks. Two test cases are inc
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Thanks. Two test cases are included: one and two layer hex meshes.

hexlayer1.tgz
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Old   May 8, 2007, 18:54
Default I wasn't able to include secon
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I wasn't able to include second case because it is bigger than 50k. But if you have Gmsh, the number of layer can be changed edting line 3:

l = 2.

Then running gmshToFoam.
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Old   May 8, 2007, 19:12
Default You may have to set inlet velo
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You may have to set inlet velocity like

0.1, 0, 0.
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Old   May 8, 2007, 19:15
Default Patch 0 is inlet, Patch 1 is
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Patch 0 is inlet,
Patch 1 is outlet,
Patch 2 is wall,
Patch 3 is wall.

Rafael,

I don't think it has to do with mesh density. The figures were done with interFoam and two fluids but this happens also with the icoFoam case I posted and the pressure variation is smoother.
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Old   May 8, 2007, 19:20
Default I post two more figures with i
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I post two more figures with icoFoam.




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Old   May 8, 2007, 19:30
Default I believe the reference pressu
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I believe the reference pressure is only important in closed cavities. This case the outlet is set to zero.
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Old   May 9, 2007, 17:42
Default The gradient seems ok. The pro
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The gradient seems ok. The problem is the value. Anyone have suggestions why this happens? It almost looks like the pressure is per layer.
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Old   May 9, 2007, 18:06
Default Hi Billy I should state tha
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Joakim Möller
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Hi Billy

I should state that I do not know the details of your simulation, but I must ask what your are trying to simulate:

1) From your pictures, it looks to me that your are trying to run a 2D simulation. Why use more than one layer?

2) Have you reached grid-independence? You have walls in your simulation, but I can't see that you have resolved any boundarylayers. When
you change the mesh-density, how does the velocities change?

Best regards

/Joakim
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Old   May 9, 2007, 19:44
Default 1) I am trying to simulate the
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1) I am trying to simulate the flow inside a thin plate. I want to use several layers because I might have a temperature variation across thickness that I want to capture. This way I would solve equations for flow as well as temperature. However, I want to know also how much pressure is needed, so pressure shouldn't vary so much with the amount of layers I have.

2) This is for laminar flow not turbulent flow. I don't think this difference is due to grid indpendence, there is no cross thickness flow and I don't think pressure should be 1x, 2x, 3x higher as I refine the mesh.

With 3 layers, pressure is 3 times higher. I think this has to do with the pressure correction scheme.


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Old   May 9, 2007, 23:41
Default Hi Billy Isn't your problem
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Hi Billy

Isn't your problem a 3D problem?

I would assume that you get a parabolic flow profile in the xy-plane between the plates. Your strange results is probobly due to your b.c. I assume that you are currently using empty as b.c. at bottom and top since you started with one layer. Shouldn't you use wall- or atleat slipwall conditions instead and something like 10 cells in the z-direction?

Regards

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Old   May 10, 2007, 10:35
Default Actually I tried with 10 layer
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Actually I tried with 10 layers and it blows up. Courant just increases to infinity. There is an inlet, outlet and all other surfaces are walls. I will try another geometry to see what happens.

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