# 3 discretisation solving questions

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 April 26, 2008, 10:21 Hello, I would be happy to #1 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 Hello, I would be happy to read some answer to these 3 things currently confusing me: 1. Whats the point of ddtSchemes subdictionary (userguide u-109), if actual discretisation scheme is what's written by constructing fvMatrix? 2. May I call OpenFOAM a implementation of *linear* FVM? Is there anything really quadratic (or more) about it? (what are cubicCorrection, fourth?) 3. Is it necessary to use fvm::{Su/Sp/SuSp}, if matrices are somehow preconditioned? For example, if momentum equation includes volume and body force (or some heat sources)? Thank you.

 May 3, 2008, 13:21 Ok, one question: are phi (sur #2 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 Ok, one question: are phi (surface field representing mass / volume flux) and U & mesh.Sf() supposed to be equal after PISO loop (U is velocity, of course)? Please answer to this!

 May 3, 2008, 20:30 Phi does not necessarily to be #3 Member   roy fokker Join Date: Mar 2009 Posts: 44 Rep Power: 9 Phi does not necessarily to be equal, however, it should obey the continuity after the 2 PISO correction steps, which is the key issue of the Pressure-Velocity correction procedure.

 May 4, 2008, 06:34 Another "mystery" related to t #4 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 Another "mystery" related to this is a steady solution of cavity - when using linear interpolation for convection, I get this ugly solution: but, the problem is, when use some limited scheme, say GammaV 1, they do not disappear! When solving in transient regime, this problem vanishes -- why? Could you explain, please?

 May 4, 2008, 11:26 Hi, I don't know the exactly p #5 Member   roy fokker Join Date: Mar 2009 Posts: 44 Rep Power: 9 Hi, I don't know the exactly problem you have, my suggestion might be wrong, but I just think something "might" affect your solution. Then continuity equation is: ddt(rho)+div(rho, U)=0 If you have compressible flow, phi should be rho*U which is interpolated to the face? Is your fluid density a constant? If not, then ddt(rho) might have an effect?

 May 4, 2008, 12:01 A simple case, where rho is co #6 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 A simple case, where rho is constant. Just for testing, so I can see if things work well, but they don't in fact. Currently I've managed to crash icoFoam (and my own solvers) (see "OpenFOAM crashes when input is a steady solution!!"). A more constructive question would be: how to calculate phi field, if *only* U field is given (say, for initial conditions)? U&Sf result is different from solved one...

 May 4, 2008, 17:28 Hi, I think the U field is int #7 Member   roy fokker Join Date: Mar 2009 Posts: 44 Rep Power: 9 Hi, I think the U field is interpolated to the control volume surface while calculating the phi. Since your rho is a constant, the phi is using the interpolated surface U times the surface area, that is why phi is coded as "surfaceScalarField" when used in the convection term (both in the pressure correction equation and the pseudo whole N-S equation for the PISO loop), I think a complete phi expression (for the compressible flow) should be: linearinterpolate(rho*U) & mesh.Sf() Correct me if I am wrong, thanks!

 May 5, 2008, 04:11 Well, I already did that. Sinc #8 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 Well, I already did that. Since my solver is incompressible, I used fvc::interpolate(U)&mesh.Sf() and that doesn't give me the same result for saved, converged U field as PISO loop does! So I ask again: why there is difference and how should I calculate phi from U?

 May 7, 2008, 04:17 To display difference field, I #9 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 To display difference field, I used div( interpolate_lin(U)&mesh.Sf() - phi ) and result looks like a fluctuations around zero. Also boundaries look problematic for some reason I don't know... Here's a sample:

 May 7, 2008, 04:27 (as Bernhard says: Warning - a #10 Senior Member   Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,783 Rep Power: 22 (as Bernhard says: Warning - advertising own stuff!) Have a look at my Thesis - the bit about pressure-velocity coupling, or try to dig out my CFD lecture material from somewhere. It will tell you what the difference between the interpolated velocity and face flux is and what to do if you insist on full consistency. For reference, this is to do with Rhie-Chow treatment. Hrv __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

 October 14, 2008, 06:11 Karlis, did you find out what #11 Senior Member   Ivan Flaminio Cozza Join Date: Mar 2009 Location: Torino, Piemonte, Italia Posts: 207 Rep Power: 10 Karlis, did you find out what happens if you choose cubicCorrection as interpolation scheme? I'm facing a similar and strange problem: I'm solving linearized Euler equations for a Gaussian acoustic pulse, and I tryied linear : gradSchemes { default Gauss linear; } divSchemes { default Gauss linear; // div(phi,utfold) Gauss limitedLinear 1; } laplacianSchemes { default none; } interpolationSchemes { default cubic; } and cubic schemes: gradSchemes { default Gauss cubic; } divSchemes { default Gauss cubic; // div(phi,utfold) Gauss limitedLinear 1; } laplacianSchemes { default none; } interpolationSchemes { default cubic; } and the results are the same!! I'm wondering what I'm wrong in it... Have a good day!

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