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- - **How to solve in simpleFoam with a volumesourceterm implicity **
(*https://www.cfd-online.com/Forums/openfoam-solving/57807-how-solve-simplefoam-volumesourceterm-implicity.html*)

Hey openFoam experts,
i wanHey openFoam experts,
i want to solve the following in openfoam navier-stokes-equation like in simpleFoam, tmp<fvvectormatrix> UEqn ( fvm::div(phi, U) + turbulence->divR(U) ); UEqn().relax(); solve(UEqn() == -fvc::grad(p)-alpha);//sourceterm with alpha and U (velocity of flow) are vector/volVectorFields. My goal is to solve the sourceterm alpha implicity and tried with tmp<fvvectormatrix> UEqn ( fvm::div(phi, U) + turbulence->divR(U) + fvm::Sp(alpha / U, U) // implicit sourceterm ); UEqn().relax(); solve(UEqn() == -fvc::grad(p)); But i get an error like FOAM FATAL ERROR : incompatible fields for operation [U] + [alpha]#0 Foam::error::printStack(Foam::-Ostream&) in "/users/studi8/OpenFOAM/OpenFOAM-1.4.1/lib/linuxGccDPOpt/libOpenFOAM.so" Have someone any idea, how to solve these problem implicity anyway? Regards, Khaled |

problem:
dU/dt = U + a
expproblem:
dU/dt = U + a explicit: (U^n - U^(n-1)/dt = U^(n-1) + a implicit: (U^n - U^(n-1)/dt = U^n + a explicit/implicit has nothing to do with the evaluation of a here, only with how U is evaluated. So your question doesnt make any sense. ...unless a really is a = a(U) but thats not obvious from your question. if the influence of a is big there is a common linearizing 'cheat' that goes something like this: dU/dt = U + a*1 = U + a*(U/U) discretized as: (U^n - U^(n-1)/dt = U^(n-1) + a*(U^n/U^(n-1)) |

...interrupted...continuing:
...interrupted...continuing:
however if a and U is a vector you will have a/U (vector/vector = ?) further (a/U)*U should still be a vector so (a/U) should either be a scalar or a matrix and in your case it is probably a matrix, so you have to construct this matrix, which is probably diagonal and a_i/U_i. And as far as I know there is no implementation of implicit treatment of fvm::Sp(<tensorfield>, <vectorfield>) |

Hi Niklas,
it seems so. ItīHi Niklas,
it seems so. Itīs a pity. I try it explicit like tmp<fvvectormatrix> UEqn ( fvm::div(phi, U) + turbulence->divR(U) ); UEqn().relax(); solve(UEqn() == -fvc::grad(p)-alpha);//sourceterm Thanks, Khaled |

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