# Governing equation for simpleFoam

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 April 24, 2006, 04:40 Hi, Im doing my thesis work an #1 newbee Guest   Posts: n/a 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; fumiya and 6863523 like this.

 April 24, 2006, 04:50 Hi! For the first part of y #2 Member   Andreas Hauffe Join Date: Mar 2009 Location: Dresden, Germany Posts: 36 Rep Power: 10 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. Andreas

 April 24, 2006, 05:06 Hi! Ups, I think your right #3 Member   Andreas Hauffe Join Date: Mar 2009 Location: Dresden, Germany Posts: 36 Rep Power: 10 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? Andreas

 April 24, 2006, 05:11 phi is in general the flux (rh #4 Member   stefan Join Date: Mar 2009 Posts: 96 Rep Power: 10 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

 April 24, 2006, 05:22 > what does turbulence->divR(U #5 Senior Member   Gavin Tabor Join Date: Mar 2009 Posts: 181 Rep Power: 10 > 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 Mehrez likes this.

 April 24, 2006, 05:42 Hi! As far as I understood #6 Member   Andreas Hauffe Join Date: Mar 2009 Location: Dresden, Germany Posts: 36 Rep Power: 10 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 Andreas

 April 24, 2006, 05:52 Hi! Next mistake. It should #7 Member   Andreas Hauffe Join Date: Mar 2009 Location: Dresden, Germany Posts: 36 Rep Power: 10 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. Andreas

 April 24, 2006, 05:58 Thanks a lot! This was big #8 newbee Guest   Posts: n/a 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));

 April 24, 2006, 06:12 No, there is no mistake in Hrv #9 Senior Member   Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,797 Rep Power: 23 No, there is no mistake in Hrv's Thesis on page 71. Hrv __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

 April 24, 2006, 06:25 Hi! In this case I want to #10 Member   Andreas Hauffe Join Date: Mar 2009 Location: Dresden, Germany Posts: 36 Rep Power: 10 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? Andreas

 April 24, 2006, 06:32 Hmm, my mistake in fact: it sh #11 Senior Member   Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,797 Rep Power: 23 Hmm, my mistake in fact: it should say div{(overline{Uprime Uprime})} right? and I am missing the div. Will fix it asap. Hrv __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

 April 24, 2006, 06:40 Hi! I think div{(overline{U #12 Member   Andreas Hauffe Join Date: Mar 2009 Location: Dresden, Germany Posts: 36 Rep Power: 10 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! Andreas

 April 24, 2006, 07:02 where can I find this PhD thes #13 newbee Guest   Posts: n/a where can I find this PhD thesisk. It sounds like Ive could use a thing or two from it?

 April 24, 2006, 07:30 You can try the FOAM CFD resou #14 Senior Member   Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,797 Rep Power: 23 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 __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

 April 2, 2008, 08:59 hi to all, I m a beginner i #15 kanarya Guest   Posts: n/a 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

 April 2, 2008, 09:01 hi to all, I m a beginner i #16 kanarya Guest   Posts: n/a 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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