# Momentum predictor

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 July 12, 2011, 10:25 Momentum predictor #1 Senior Member   Andrea Ferrari Join Date: Dec 2010 Posts: 296 Rep Power: 8 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

 July 19, 2011, 18:52 #2 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 430 Rep Power: 16 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. Tushar@cfd likes this. __________________ Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC) - CONICET/UNL Tel: 54-342-4511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe - Argentina. http://www.cimec.org.ar

 July 20, 2011, 03:49 #3 Senior Member   Andrea Ferrari Join Date: Dec 2010 Posts: 296 Rep Power: 8 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 }

 July 20, 2011, 09:29 #4 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 430 Rep Power: 16 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. lth and kaifu like this. __________________ Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC) - CONICET/UNL Tel: 54-342-4511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe - Argentina. http://www.cimec.org.ar

 July 20, 2011, 09:35 #5 Senior Member   Andrea Ferrari Join Date: Dec 2010 Posts: 296 Rep Power: 8 Ok, now it's totally clear!! many thanks andrea

 July 22, 2011, 07:04 #6 Senior Member   Andrea Ferrari Join Date: Dec 2010 Posts: 296 Rep Power: 8 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 giack likes this.

March 11, 2013, 05:09
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 Originally Posted by Andrea_85 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
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:

thanks for help

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