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#1 |
Member
Nikhil
Join Date: May 2020
Location: Freiburg
Posts: 43
Rep Power: 6 ![]() |
Hallo Foamers,
I have a small doudt. In UEqn.H, in solvers where momentum predictor setting exists in pimple loop. If momentum predictor is set to no then the Right-hand side of the momentum equation is not solved (skipped because of if loop). But, I dont understand, in any case, we need to solve whole momentum equation to get U right? . Why we skip gravity and dp term ? what does this mean ![]() Code:
tmp<fvVectorMatrix> tUEqn ( fvm::ddt(rho, U) + fvm::div(phi, U) + MRF.DDt(rho, U) + turbulence->divDevTau(U) == fvOptions(rho, U) ); fvVectorMatrix& UEqn = tUEqn.ref(); UEqn.relax(); fvOptions.constrain(UEqn); // In here if (pimple.momentumPredictor()) { solve ( UEqn == fvc::reconstruct ( ( - ghf*fvc::snGrad(rho) - fvc::snGrad(p_rgh) )*mesh.magSf() ) ); fvOptions.correct(U); K = 0.5*magSqr(U); } |
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#2 |
Senior Member
Cyprien
Join Date: Feb 2010
Location: Stanford University
Posts: 299
Rep Power: 18 ![]() |
Dear Nikhil,
When you turn off momentumPrediction, you do no solve the momentum equation. If you look at the predictor-corrector solution algorithms used in CFD (PISO, SIMPLE, PIMPLE...), basically you do the following sequence: - Discretize the momentum equation and form a matrix for U (optionnaly inverse this matrix to have a first guess of U) - Form a matrix for p. This matrix depends on the coefficients of the matrix for U. Inverse the matrix for p. - Correct the value of U point-wised based on the updated pressure field. - Iterate until you converge. - March in time So, when you turn off momentumPrediction, you do update the coefficients in the matrix for U, but you do not inverse this matrix. Cheers, Cyprien
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www.cypriensoulaine.com/openfoam |
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#3 |
Member
Nikhil
Join Date: May 2020
Location: Freiburg
Posts: 43
Rep Power: 6 ![]() |
Hallo Cyprien,
Thanks for the explanation. Yah! now it's crystal clear. cheers, nick. |
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Tags |
momentum equation, piso, ueqn.h |
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