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Boundary conditions in OpenFOAM

Solver Settings - fvSolution File

  • Sub-dictionary SIMPLE in file fvSolution is important.
  • nNonOrthogonalCorrectors is number of pressure equation steps (zero means one step)
  • Parameters pMinpMax, etc., are limits allowed for variables
  • In sub-dictionary relaxationFactors there are under-relaxation factors for all variables.

fvSolution file for subsonic computations:

 

solvers
{
    p
    {
        solver          GAMG;
        tolerance       1e-8;
        relTol          0.001;
        smoother        GaussSeidel;
        cacheAgglomeration true;
        nCellsInCoarsestLevel 20;
        agglomerator    faceAreaPair;
        mergeLevels     1;
    }

    U
    {
        solver          smoothSolver;
        smoother        GaussSeidel;
        nSweeps         2;
        tolerance       1e-20;
        relTol          0.1;
    }

    k
    {
        solver          smoothSolver;
        smoother        GaussSeidel;
        nSweeps         2;
        tolerance       1e-20;
        relTol          0.1;
    }

    "(omega|epsilon)"
    {
        solver          smoothSolver;
        smoother        GaussSeidel;
        nSweeps         2;
        tolerance       1e-20;
        relTol          0.1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;
    numberIterationsStator 1;
    numberIterationsRotor 1;
    UMax      500;
    pMin      pMin   [0 2 -2 0 0 0 0]    -1000;
    pMax      pMax   [0 2 -2 0 0 0 0]     1000;
}

relaxationFactors
{
    fields
    {
        p               0.2;
    }
    equations
    {
        U               0.4;
        k               0.3;
        epsilon         0.3;
        omega           0.3;
    }
}

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System of Linear Equations

Finite Volume Method converts the system of differential equations to the system of linear equations:

img194 1

Such a linear algebra problem can be solved with following methods implemented in OpenFOAM:

Linear system solver method:

  • GAMG (Geometric-Algebraic Multi-Grid) for both symmetric and asymmetric matrices
  • PBiCG (Preconditioned Biconjugate Gradient ) for asymmetric matrices
  • PCG (Preconditioned Conjugate Gradient) for symmetric matrices
  • smoothSolver (solver using a smoother for both symmetric and asymmetric matrices )
  • ICCG (Incomplete Cholesky preconditioned PCG solver, i.e. PBiCG with DIC )8.2
  • BICCG (Diagonal Incomplete LU preconditioned PBiCG solver, i.e. PCG with DILU)8.3

Method preconditioner:

  • DILU (Diagonal Incomplete LU decomposition)
  • DIC (Diagonal incomplete-Cholesky) for symmetric matrices
  • FDIC (Faster diagonal incomplete-Cholesky) for symmetric matrices
  • diagonal (Diagonal)
  • GAMG (Geometric-Algebraic Multi-Grid)
  • none (No preconditioning)
  • DICGaussSeidel, GaussSeidel, nonBlockingGaussSeidel, symGaussSeidel (for symmetric matrices)

For more details see e.g. [9].