# Is good initial guess field is neccessary ?

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 October 10, 2012, 13:28 Is good initial guess field is neccessary ? #1 Member   M Mallikarjuna Reddy Join Date: Jul 2012 Posts: 91 Rep Power: 14 Hi foamers, i developed my own solver for predicting velocity profile(U) and particle concentration profile(T) for suspension flow (fluid+particles). And in my momentum equation, the viscosity is not constant, it is function of particle volume fraction(T). And i written the momentum equation as follows: tmp UEqn ( fvm::div(phi, U) - fvm::laplacian(nu*pow(1-T/0.68,-1.82), U) ); UEqn().relax(); sources.constrain(UEqn()); solve(UEqn() == -fvc::grad(p)); where T is the particle volume fraction (dimensionless scalar field). For predicting particle volume fraction(T), the governing equation written as: solve ( fvm::div(phi, T) - fvm::laplacian(0.62*1.82*pow(a,2)*g*pow(T,2)*pow(0 .68-T,-1), T) - fvm::laplacian(0.41*pow(a,2)*g*T, T) == fvc::laplacian(0.41*pow(a,2)*T*T, g) ); Where g = local shear rate = sqrt(2*E:E) = 1.4142*sqrt(magSqr(symm(fvc::grad(U)))) when i run my case (simple channel flow) by using my solver, initially it is giving the following error. error message:- Create time Create mesh for time = 0 Reading transportProperties Reading field p Reading field U Reading field T Reading/calculating face flux field phi No field sources present SIMPLE: convergence criteria field p tolerance 0.01 field U tolerance 0.001 field T tolerance 1e-05 Starting time loop Time = 1 DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 4.00548e-06, No Iterations 4 DILUPBiCG: Solving for Uy, Initial residual = 0, Final residual = 0, No Iterations 0 #0 Foam::errorprintStack(Foam::Ostream&) in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libOpenFOAM.so" #1 Foam::sigFpe::sigHandler(int) in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libOpenFOAM.so" #2 Uninterpreted: #3 FoamDILUPreconditioner::calcReciprocalD(Foam::Fi eld&, Foam::lduMatrix const&) in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libOpenFOAM.so" #4 FoamDILUPreconditionerDILUPreconditioner(Foam: :lduMatrix::solver const&, Foam::dictionary const&) in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libOpenFOAM.so" #5 Foam::lduMatrixpreconditioner::addasymMatrixCons tructorToTable::New(Foam ::lduMatrix::solver const&, Foam::dictionary const&) in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libOpenFOAM.so" #6 Foam::lduMatrixpreconditioner::New(Foam::lduMatr ix::solver const&, Foam::dictionary const&) in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libOpenFOAM.so" #7 FoamPBiCG::solve(Foam::Field&, Foam::Field const&, unsigned char) const in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libOpenFOAM.so" #8 Foam::fvMatrix::solve(Foam::dictionary const&) in "/opt/openfoam211/platforms/linuxGccDPOpt/lib/libfiniteVolume.so" #9 Foam::fvMatrix::solve() in "/home/malli_reddy/OpenFOAM/malli_reddy-2.1.1/platforms/linuxGccDPOpt/bin/myDiffusiveFoam" #10 in "/home/malli_reddy/OpenFOAM/malli_reddy-2.1.1/platforms/linuxGccDPOpt/bin/myDiffusiveFoam" #11 __libc_start_main in "/lib/i386-linux-gnu/libc.so.6" #12 in "/home/malli_reddy/OpenFOAM/malli_reddy-2.1.1/platforms/linuxGccDPOpt/bin/myDiffusiveFoam" Floating point exception malli_reddy@ubuntu:~/OpenFOAM/malli_reddy-2.1.1/PROJECT/myDiffusive\$ But when i give good initial guess (by running the same case by using another solver in which coefficients are constants), then the case is running and giving good results. My geometry is 2D channel flow. when i run my case with checkMesh, it given no errors. And i used SIMPLE scheme for pressure velocity coupling. And for laplacian() Gauss linear corrected; div() Gauss linear; To generalize my solver should i shift to any other schemes? Thanks in advance cheers Mallikarjuna

 October 11, 2012, 01:44 #2 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 21 My guess would be, that the first time you solve the equation for T, g=0, which is causing problems in the matrix equation you are constructing. When you have a non-constant initial velocity, g evaluates to non-zero value. Maybe you can calculate g at the end of a time-step and initialize it with a non-zero value?

 October 11, 2012, 02:08 #3 Member   M Mallikarjuna Reddy Join Date: Jul 2012 Posts: 91 Rep Power: 14 Hi Bernhard, Thanks for quick reply. For g i given the following boundary conditions:- dimensions [0 0 -1 0 0 0 0]; internalField uniform 10; boundaryField { leftWall { type calculated; } fixedWalls { type calculated; } rightWall { type calculated; } frontAndBack { type empty; } } Initially i given uniform 0 for internal fields. According to your suggestion i changed it to 10. But the result is same (giving error). I given non zero for internal fields only. Could you please tell me how to give non zero for boundary patches also. Thanks Regards Mallikarjuna

 October 11, 2012, 02:13 #4 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 21 I am not sure how you defined g in the solver, but do you really need an 0/g file? Cannot you define this field just in the solver? Also does it not conflict with gravity? How do you initialize T?

 October 11, 2012, 02:28 #5 Member   M Mallikarjuna Reddy Join Date: Jul 2012 Posts: 91 Rep Power: 14 Hi bernhard, I can define g in the solver. But i am using groovyBC for T. In that i need g. For that i defined g as volScalarField. I don't know actually, whether it conflict with gravity or not. I initialized T as: dimensions [0 0 0 0 0 0 0]; internalField uniform 0.02; boundaryField { leftWall { type fixedValue; value uniform 0.4; } fixedWalls { type groovyBC; g g; gradientExpression "-0.41*T*snGrad(g)/(g*(0.41+T*1.1284*pow(0.68-T, -1)))"; fractionExpression "0"; evaluateDuringConstruction 0; variables ""; timelines ( ); lookuptables ( ); } rightWall { type zeroGradient; } frontAndBack { type empty; } } Thanks Regards Mallikarjuna

 October 17, 2012, 12:58 #6 Member   M Mallikarjuna Reddy Join Date: Jul 2012 Posts: 91 Rep Power: 14 Hi Bernhard, I don't know whether i should dig here or not. I am thinking this thread is still alive. I have not solved my problem. Is there any suggestions about it? Thanks Regards Mallikarjuna