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Governing equation for simpleFoam

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Old   April 24, 2006, 04:40
Default Hi, Im doing my thesis work an
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newbee
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Hi, Im doing my thesis work and need to know which the governing equations are for the simpleFoam application.

If I understand the code correctly it should be:

div(phi, U) + turbulence->divR(U)= -grad(p));

where:
-phi is the potential of U (but it has the dimension m³/s?)

what does turbulence->divR(U) represent?

what does the following lines do?
volScalarField AU = UEqn().A();
U = UEqn().H()/AU;
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Old   April 24, 2006, 04:50
Default Hi! For the first part of y
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Andreas Hauffe
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Hi!

For the first part of your question I can help you. In the Programmer's Guide on page 54 you can see this equation. R is just nueff*grad(U) and nueff is nu+nut and is calculated by the turbulence model. Phi should be U in this case. And the p is pressure/density.

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Old   April 24, 2006, 05:06
Default Hi! Ups, I think your right
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Hi!

Ups, I think your right with phi. It is the potential of U. But part of the discretised form is multiplyed with a surface value because of the surface integrals, right?

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Old   April 24, 2006, 05:11
Default phi is in general the flux (rh
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phi is in general the flux (rho*u)

>what does the following lines do?
>volScalarField AU = UEqn().A();
>U = UEqn().H()/AU;

This lines are need to precondition (jacobian) the U matrix.

hth
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Old   April 24, 2006, 05:22
Default > what does turbulence->divR(U
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Gavin Tabor
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> what does turbulence->divR(U) represent?

This is the turbulence modelling. When you average the NSE you get back a very similar set of equations - the Reynolds Averaged Navier-Stokes equations - which have one additional term, which represents the effect of the fluctuations around the average (the turbulence) on the mean flow. We usually write this as div(R) where R is a 2nd rank tensor (the Reynolds Stress). Since we don't know this term (the closure problem) we need to model this; which is usually done by assuming that the turbulence has a dissipative effect and creating a 'turbulent viscosity' (which is usually so large it swamps the physical viscosity) and write R=-nu grad(u).

In FOAM this all gets swallowed into the turbulence model; turbulence->divR is getting this out of the model in a convenient (and mathematically stylish) way.

Gavin
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Old   April 24, 2006, 05:42
Default Hi! As far as I understood
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Andreas Hauffe
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Hi!

As far as I understood these things in this case div(R) is not only the Reynolds stress tensor of the RANS equations. But it is only a question of definition I think.
div(R) = laplacian(nu,U) + Reynolds stress tensor

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Old   April 24, 2006, 05:52
Default Hi! Next mistake. It should
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Andreas Hauffe
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Hi!

Next mistake. It should be
div(R) = laplacian(nu,U) + div(Reynolds stress tensor)
There is a mistake in Hrv's PhD. Thesis on page 71.

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Old   April 24, 2006, 05:58
Default Thanks a lot! This was big
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Thanks a lot!

This was big help. I will follow my papers on Reynolds equation on average flow values.

-a thought-
isnt div(UU)=2U*div(U)

which would give
1/2*div(phi, U) + turbulence->divR(U)= -grad(p));
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Old   April 24, 2006, 06:12
Default No, there is no mistake in Hrv
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Hrvoje Jasak
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No, there is no mistake in Hrv's Thesis on page 71.

Hrv
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Old   April 24, 2006, 06:25
Default Hi! In this case I want to
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Andreas Hauffe
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Hi!

In this case I want to apologise to you for my mistake. But for my understanding I have to ask why in equation (2.30) the Reynold stress tensor is just added? So this equation consits of 5 vectors and a tensor.
Where is my mistake?

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Old   April 24, 2006, 06:32
Default Hmm, my mistake in fact: it sh
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Hmm, my mistake in fact: it should say div{(overline{Uprime Uprime})} right? and I am missing the div.

Will fix it asap.

Hrv
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Old   April 24, 2006, 06:40
Default Hi! I think div{(overline{U
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Andreas Hauffe
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Hi!

I think div{(overline{Uprime Uprime})} is right.
But your PhD thesis is just a great work to understand what OpenFOAM does. And in comparison to stupid things I'm telling sometimes it is without mistakes!

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Old   April 24, 2006, 07:02
Default where can I find this PhD thes
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where can I find this PhD thesisk. It sounds like Ive could use a thing or two from it?
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Old   April 24, 2006, 07:30
Default You can try the FOAM CFD resou
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Hrvoje Jasak
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You can try the FOAM CFD resource site http://www.foamcfd.org - there's quite a lot of stuff there including a bunch of PhD Theses done with OpenFOAM I managed to get in electronic form. Alternatively, you can try my personal web pages on http://www.h.jasak.dial.pipex.com.

Enjoy,

Hrv
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Old   April 2, 2008, 08:59
Default hi to all, I m a beginner i
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kanarya
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hi to all,

I m a beginner in OpenFoam and I want to program fluctuations with
tensor...
for example I want the following program to do with fluctuations.
magSqr(fvc::grad(U))+ sqr(gradU.component(tensor::YX) +
gradU.component(tensor::XY))

does someone can help me?
best regards
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Old   April 2, 2008, 09:01
Default hi to all, I m a beginner i
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kanarya
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hi to all,

I m a beginner in OpenFoam and I want to program fluctuations with
tensor...
for example I want the following program to do with fluctuations.
magSqr(fvc::grad(U))+ sqr(gradU.component(tensor::YX) +
gradU.component(tensor::XY))

can someone help me?
best regards
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