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About UEqn in icoFoam.

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Old   June 4, 2013, 09:56
Default About UEqn in icoFoam.
  #1
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Dongyue Li
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Hi All,

This is UEqn code in icoFoam:
Code:
fvVectorMatrix UEqn
        (
            fvm::ddt(U)
          + fvm::div(phi, U)
          - fvm::laplacian(nu, U)
        );
AFAIK,momentum equation can be devided into three components,this three components u v w should be calculated in three algebraic equations,see equation 6.8 and 6.10 in H K Versteeg's book.so why is there only UEqn's matrix?where is VEqn and WEqn?



Thanks in advance.
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Old   June 4, 2013, 11:37
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ngj
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Niels Gjoel Jacobsen
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Hi Forrest,

The key point is the type of the matrix, namely

Code:
fvVectorMatrix
This tells you that you are solving for a matrix variable, which in this case is the velocity vector U, which has a component for each of the three Cartesian directions.

Kind regards

Niels
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Old   June 4, 2013, 19:40
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Quote:
Originally Posted by ngj View Post
Hi Forrest,

The key point is the type of the matrix, namely

Code:
fvVectorMatrix
This tells you that you are solving for a matrix variable, which in this case is the velocity vector U, which has a component for each of the three Cartesian directions.

Kind regards

Niels
Thanks Niels,

Can you tell me in this situations below,what is the fvvectormatrix?
If there were just three nodes,after dixcretisation of the governing equation.I have:
Code:
1 2 3   u_1     2
4 5 6   u_2  = 3
7 8 9   u_3     4

11 22 33   v_1     22
44 55 66   v_2  = 33
77 88 99   v_3     44

111 222 333   w_1     222
444 555 666   w_2  = 333
777 888 999   w_3     444
I think UEqn should not be this:
Code:
1     2    3  
4     5     6   
7     8     9   
11   22   33   
44   55   66   
77   88   99   
111 222 333   
444 555 666   
777 888 999
this looks good which only can be solved for v_1 v_2 v_3:
Code:
11   22   33   
44   55   66   
77   88   99
Where am I wrong?Thanks alot!
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Old   June 6, 2013, 00:02
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Any ideas?
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Old   June 6, 2013, 22:44
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I try to print the UEqn:

Code:
fvVectorMatrix UEqn
        (
            fvm::ddt(U)
          + fvm::div(phi, U)
    
        );
    
    Info<< UEqn << endl;
I got this in a 2*2 mesh:

Code:
Lower triangle = 4(-0.05 -0 -0 -0.05)

diagonal = 4(0.05 0 0.05 0)

Upper triangle = 4{0}


[0 4 -2 0 0 0 0]
4{(0 0 0)}

3
(
2{(0 0 0)}
6{(0 0 0)}
0()
)


3
(
2{(0 0 0)}
6((0.05 0 0) (0.05 0 0) (-0.05 0 0) (-0.05 0 0) (0 0 0) (0 0 0))
0()
)
3*2 mesh:
Code:
Lower triangle = 7(-0.05 -0 -0.05 -0 -0 -0.05 -0.05)

diagonal = 6(0.05 0.05 0 0.05 0.05 0)

Upper triangle = 7{0}


[0 4 -2 0 0 0 0]
6{(0 0 0)}

3
(
3{(0 0 0)}
7{(0 0 0)}
0()
)


3
(
3{(0 0 0)}
7((0.05 0 0) (0.05 0 0) (-0.05 0 0) (-0.05 0 0) (0 0 0) (0 0 0) (0 0 0))
0()
)
But I cannot see anything from this..
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Old   June 12, 2013, 19:37
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no one is here?
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Old   June 20, 2013, 04:00
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[LaTeX Error: Syntax error]

In this formula, I think the left equation in Jasak's thesis page.145 eqn(3.136) have three componets, so there are three equations, but why is there only one matrix:Ueqn?where is Veqn and Weqn?
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Last edited by sharonyue; June 27, 2013 at 22:07.
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Old   June 27, 2013, 22:06
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Still does not get anywhere now.
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Old   June 30, 2013, 21:15
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Code:
Lower triangle = 7(-0.05 -0 -0.05 -0 -0 -0.05 -0.05)

diagonal = 6(0.05 0.05 0 0.05 0.05 0)
especially this: In a matrix, if the diagonal coefficients are 6, is the upper and lower triangle coefficients should be (5+4+3+2+1)=15?
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Old   June 30, 2013, 22:48
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This is a piece in Fabian Peng Karrholm's Ph.D thesis, :see the image.

OpenFOAM use A.22 to represent this three equations: A.18 A.19 A.20.
But in this thesis there are no more explanations regarding this.
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Old   July 2, 2013, 23:27
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Hi, the matrix coefficients for the momentum equation are the same, so you have only one matrix. The source term has different values in each direction. Check lines 143-208 in http://foam.sourceforge.net/docs/cpp/a04765_source.html

Regards.
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Old   July 2, 2013, 23:44
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Quote:
Originally Posted by santiagomarquezd View Post
Hi, the matrix coefficients for the momentum equation are the same, so you have only one matrix. The source term has different values in each direction. Check lines 143-208 in http://foam.sourceforge.net/docs/cpp/a04765_source.html

Regards.
Hi,

After a long struggle, I just made a discretisation of momentum equation manully and found you are right, so I know why there was only one matrix.but I still have many problems so I didnot update this.

While do you know why Ueqn.A() do not equal to the diag.term of Ueqn?Ueqn.A() extract the diag. term from the Ueqn??

And why is this Ueqn looks so mass. Its not like a matrix.

BTW, I read your note regarding to interFoam solve. Its totally wonderful and help me alot.

Thanks for your answer about the momentum predictor. I need time to look it deeply, and I tried to download your thesis about several weeks ago but failed, can you send me one copy? Email:chivasbeijing@qq.com

Regards!
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