CFD Online Discussion Forums - Momentum predictor

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)

- OpenFOAM (https://www.cfd-online.com/Forums/openfoam/)

- - Momentum predictor (https://www.cfd-online.com/Forums/openfoam/90500-momentum-predictor.html)

Momentum predictor

Hi all,
i have 2 questions about the solution procedure in interFoam.
The predicted fluxes are bulid in pEqn.H, if i'm not wrong:

Code:

volScalarField rAU(1.0/UEqn.A());

00003     surfaceScalarField rAUf(fvc::interpolate(rAU));

00004 

00005     U = rAU*UEqn.H();

00006     surfaceScalarField phiU

00007     (

00008         "phiU",

00009         (fvc::interpolate(U) & mesh.Sf())

00010       + fvc::ddtPhiCorr(rAU, rho, U, phi)

00011     );

00012 

00013     adjustPhi(phiU, U, p_rgh);

00014 

00015     phi = phiU +

00016     (

00017         fvc::interpolate(interface.sigmaK())*fvc::snGrad(alpha1)

00018       - ghf*fvc::snGrad(rho)

00019     )*rAUf*mesh.magSf();

what is the difference between switch on or switch off the momentum predictor in uEqn.H and how it affects the predicted velocities?. What does OF calculate in:

Code:

00019     if (pimple.momentumPredictor())

00020     {

00021         solve

00022         (

00023             UEqn

00024          ==

00025             fvc::reconstruct

00026             (

00027                 (

00028                     fvc::interpolate(interface.sigmaK())*fvc::snGrad(alpha1)

00029                   - ghf*fvc::snGrad(rho)

00030                   - fvc::snGrad(p_rgh)

00031                 ) * mesh.magSf()

00032             )

00033         );

00034     }

the second quesiton is: What does it mean
fvc::ddtPhiCorr(rAU, rho, U, phi) in the phiU calculation?

thanks a lot in advance

andrea

Andrea, predicted velocity is taken into account in H method (check Hrv Thesis). ddtPhiCorr is intended to apply a correction to Rhie-Chow interpolation due temporal scheme.

Regards.

Hi Santiago and thanks to your reply.
If i'm not wrong, H is a function of U (it takes into into account the velocities of the neighboring cells). So, if the momentum predictor is turned off, which is the velocity used to bulid up H?
Or in another way, are these lines (below) used to calculate a velocity or are used only to do an update of the coefficients of the matrices A and H?

00019 if (pimple.momentumPredictor())
00020 {
00021 solve
00022 (
00023 UEqn
00024 ==
00025 fvc::reconstruct
00026 (
00027 (
00028 fvc::interpolate(interface.sigmaK())*fvc::snGrad(alpha1)
00029 - ghf*fvc::snGrad(rho)
00030 - fvc::snGrad(p_rgh)
00031 ) * mesh.magSf()
00032 )
00033 );
00034 }

Andrea, if momentum predictor is turned off, U from previous time-step is used, if it is on once the solve method is executed U is refreshed and then used in H method.

Regards.

Ok, now it's totally clear!!

many thanks

andrea

Hi Santiago,
i'm still a little bit confused about the velocity reconstruction after the pressure solution.

U += rAU*fvc::reconstruct((phi - phiU)/rAUf);

Why phiU is substracted from the fluxes?
phiU is defined as the sum of the predicted velocity interpolated to the faces and the temporal correction. The contribution of H/A is included in the new velocity because of "+=", so which contribution is substructed? Is the temporal correction?

thanks again

andrea

Quote:

Originally Posted by Andrea_85 (Post 317067)

do you find a solution to this question? I'am the same question..i tried to compare it with the PISO algorithm but in the PISO algorithm the this U reconstruction is as follow:
U -= rUA*fvc::grad(p);

thanks for help