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July 23, 2012, 17:03 
Natural Convection with icoFoam

#1 
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Hey Foamers
I have added energy equation to icoFoam, and I am solving natural convection in cavity. The problem is that as I am using smaller and smaller time steps, the velocities become smaller and smaller, and I can not get a results which is independent of time step. So do you have any ideas to help me getting time step independent results? 

July 24, 2012, 04:23 

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dear mahdi
do you add temperature truly? it seams that your solver has a problem. you should attach your solver. it is hard to make decision without looking to your solver 

July 24, 2012, 10:43 

#3 
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Dear Niyaz
this is my solver /**\ =========  \\ / F ield  OpenFOAM: The Open Source CFD Toolbox \\ / O peration  \\ / A nd  Copyright (C) 19912010 OpenCFD Ltd. \\/ M anipulation   License This file is part of OpenFOAM. OpenFOAM is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. OpenFOAM is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenFOAM. If not, see <http://www.gnu.org/licenses/>. Application icoFoam Description Transient solver for incompressible, laminar flow of Newtonian fluids. \**/ #include "fvCFD.H" // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // int main(int argc, char *argv[]) { #include "setRootCase.H" #include "createTime.H" #include "createMesh.H" #include "createFields.H" #include "initContinuityErrs.H" // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // Info<< "\nStarting time loop\n" << endl; while (runTime.loop()) { Info<< "Time = " << runTime.timeName() << nl << endl; #include "readPISOControls.H" #include "CourantNo.H" fvVectorMatrix UEqn ( (Density)*fvm::ddt(U) + (Density)*fvm::div(phi, U)  (Visc)*(fvm::laplacian(U)) ); solve(UEqn == GravityVector*g*Density*(BetaT*(TTNot))  (fvc::grad(p))); //solve(UEqn == fvc::grad(p)); //  PISO loop for (int corr=0; corr<nCorr; corr++) { volScalarField rUA = 1.0/UEqn.A(); U = rUA*UEqn.H(); phi = (fvc::interpolate(U) & mesh.Sf()) + fvc::ddtPhiCorr(rUA, U, phi); adjustPhi(phi, U, p); for (int nonOrth=0; nonOrth<=nNonOrthCorr; nonOrth++) { fvScalarMatrix pEqn ( fvm::laplacian(rUA, p) == fvc::div(phi) ); pEqn.setReference(pRefCell, pRefValue); pEqn.solve(); if (nonOrth == nNonOrthCorr) { phi = pEqn.flux(); } } #include "continuityErrs.H" U = rUA*fvc::grad(p); U.correctBoundaryConditions(); } fvScalarMatrix TEqn ( Density*Cp*fvm::ddt(T) + Density*Cp*fvm::div(phi, T)  Cond*(fvm::laplacian(T)) ); TEqn.solve(); //F = Gravity*(1Beta*(TTemAmb)); /////////// fvScalarMatrix CEqn ( fvm::ddt(C) + fvm::div(phi, C) ); CEqn.solve(); /////////// // forAll( h, celli) // { // T[celli] = (h[celli]  1)/Ste.value(); //T[celli] = (h[celli]  L.value())/Cp.value(); // h[celli] = 1 + (Ste.value()*T[celli]); // h[celli] = (Cp.value()*T[celli]) + L.value(); // } /////////// //F = (GravityVector*Pr*T)(GravityVector*Le*C); //F = gl*GravityVector*g*Density*(1BetaT*(TTNot)); F = GravityVector*g*Density*(BetaT*(TTNot)); runTime.write(); Info<< "ExecutionTime = " << runTime.elapsedCpuTime() << " s" << " ClockTime = " << runTime.elapsedClockTime() << " s" << nl << endl; } Info<< "End\n" << endl; return 0; } // ************************************************** *********************** // 

July 24, 2012, 11:26 

#4 
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your changes are strange.
for U: fvVectorMatrix UEqn ( fvm::ddt(U) + fvm::div(phi, U)  fvm::laplacian(nu, U) == beta*(T  T0)*g ); for T: solve ( fvm::ddt(T) + fvm::div(phi, T)  fvm::laplacian(DT, T) ); where nu is viscosity and DT is conductivity your equation seems have a problem. 

July 24, 2012, 15:41 

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solve(UEqn == GravityVector*g*Density*(BetaT*(TTNot))  (fvc::grad(p)));
I think that before the GravityVector, you should have () 

July 24, 2012, 15:45 

#7 
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Thank you. But it will not fix the problem. It will only change the direction of U and V.


July 24, 2012, 16:09 

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Can you show a contour of two results?
your case is a bit strange. 

July 24, 2012, 16:10 

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at first, I suggest you to solve a forced convection to prove the exactness of the solver then add boussinesqe term to equation.


July 26, 2012, 14:07 

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I fixed the problem.
The key is in order to get a proper behavior of velocitypressure coupling in PISO algorithm, I should include the source term inside the velocity equation such that fvVectorMatrix UEqn ( fvm::ddt(U) +fvm::div(phi,U) fvm::laplacian(ModifVisc,U) == GravityVector*betaT*(TTnot) ); solve(UEqn == fvc::grad(p) ); 

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