help with Laplace equation

giorgk · February 28, 2013, 14:05

Hi,
I started using openFoam few days ago and after I went through alot of documentation and tutorials I decided to try my own very simple example to solve the steady state diffusion equation $\nabla$ ( $\Gamma$ $\nabla$ $\phi$ ) = 0. According to my OpenFoam understanding so far this is equivalent to solving :
fvm::ddt(T) - fvm::laplacian(DT, T) using steadyState as ddtSchemes.

The domain I uses is also a simple one. A box with dirichlet Bcs left and right, fix gradient on the top and zero gradient everywhere else.
The first run looked pretty good as the solution field varied linearly between the dirichlet conditions.
Then as I increased the fixed Gradient on the top the solution field started forming a Hump, which is reasonable again.

Then while keeping the same fixed gradient value that gave me the Hump in my last run I started increasing the diffuse term DT. I would then expect the Hump to start reducing and actually after some large DT value I would expected to get a solution that varied linearly. However the term DT seems to have no impact whatsoever.
Can anyone tell me what I am doing wrong?

Thank you

George

PS: General comment about OpenFoam
I feel I have wasted a lot of time by not find it earlier

giorgk · March 2, 2013, 15:32

I've tried the simplest thing ever and I cannot reproduce in OpenFoam the same solution as in Comsol. (And the one in Comsol is the correct one).
I'm solving laplacian steady state $\nabla(K\nabla h)$ = 0.
My domain is a box with 5km x 5 km and 300 m height.
I assign dirichlet BCs on the left and right 20 m and 40 m respectively and neumann on the top equal to 4.6296e-06 m/s (or 0.4 m/day). The K value is 0.0011574 m/s (100 m/day), and solve it as steady state.
I'm attaching below the results from Comsol and OpenFoam and they are VERY different
This is Comsol solution

and this is OpenFoam solution

So am I using the wrong equation in OpenFoam?

Please someone help

Thank you

prithvi yesudas · November 20, 2013, 10:22

i am trying the same problem with same boundary condition. but getting this error while running blockMesh.
FOAM FATAL ERROR:
Cannot open mesh description file
"/home/guest/OpenFOAM/guest-2.1.1/run/tutorials/cfdproject/constant/polyMesh/blockMeshDict"

From function blockMesh
in file blockMeshApp.C at line 148.
Can u pls upload me your case files since i require to run this problem urgently.

February 28, 2013, 14:05	help with Laplace equation	#1
giorgk New Member Join Date: Feb 2013 Posts: 3 Rep Power: 13	Hi, I started using openFoam few days ago and after I went through alot of documentation and tutorials I decided to try my own very simple example to solve the steady state diffusion equation $\nabla$ ( $\Gamma$ $\nabla$ $\phi$ ) = 0. According to my OpenFoam understanding so far this is equivalent to solving : fvm::ddt(T) - fvm::laplacian(DT, T) using steadyState as ddtSchemes. The domain I uses is also a simple one. A box with dirichlet Bcs left and right, fix gradient on the top and zero gradient everywhere else. The first run looked pretty good as the solution field varied linearly between the dirichlet conditions. Then as I increased the fixed Gradient on the top the solution field started forming a Hump, which is reasonable again. Then while keeping the same fixed gradient value that gave me the Hump in my last run I started increasing the diffuse term DT. I would then expect the Hump to start reducing and actually after some large DT value I would expected to get a solution that varied linearly. However the term DT seems to have no impact whatsoever. Can anyone tell me what I am doing wrong? Thank you George PS: General comment about OpenFoam I feel I have wasted a lot of time by not find it earlier

March 2, 2013, 15:32	What am I doing wrong?? (comsol - openfoam comparison)	#2
giorgk New Member Join Date: Feb 2013 Posts: 3 Rep Power: 13	I've tried the simplest thing ever and I cannot reproduce in OpenFoam the same solution as in Comsol. (And the one in Comsol is the correct one). I'm solving laplacian steady state $\nabla(K\nabla h)$ = 0. My domain is a box with 5km x 5 km and 300 m height. I assign dirichlet BCs on the left and right 20 m and 40 m respectively and neumann on the top equal to 4.6296e-06 m/s (or 0.4 m/day). The K value is 0.0011574 m/s (100 m/day), and solve it as steady state. I'm attaching below the results from Comsol and OpenFoam and they are VERY different This is Comsol solution and this is OpenFoam solution So am I using the wrong equation in OpenFoam? Please someone help Thank you

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November 20, 2013, 10:22		#3
prithvi yesudas New Member prithviyesudas Join Date: Jan 2013 Posts: 6 Rep Power: 13	i am trying the same problem with same boundary condition. but getting this error while running blockMesh. FOAM FATAL ERROR: Cannot open mesh description file "/home/guest/OpenFOAM/guest-2.1.1/run/tutorials/cfdproject/constant/polyMesh/blockMeshDict" From function blockMesh in file blockMeshApp.C at line 148. Can u pls upload me your case files since i require to run this problem urgently.