During an OpenFOAM simulation it is sometimes necessary to update a numerical scheme, for example, switching from first order to second order schemes to improve accuracy vs stability.
In order to perform these modifications automatically, we can add a function object to our controlDict, as shown in the following example, where after iteration 100 the fvSchemes is automatically updated:
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functions
{
fileUpdate1
{
type timeActivatedFileUpdate;
libs ("libutilityFunctionObjects.so");
writeControl timeStep;
writeInterval 1;
fileToUpdate "$FOAM_CASE/system/fvSchemes";
timeVsFile
(
// start with first order
(-1 "$FOAM_CASE/system/fvSchemes_1")
// after 100 iterations use second order
(100 "$FOAM_CASE/system/fvSchemes_2")
);
}
}
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In addition to the previous function object, we also need to make 2 copies of the “fvSchemes” file and rename them as “fvSchemes_1” and “fvSchemes_2”, both saved in the system directory.
In fvSchemes_1, divergence schemes are set to first order bounded “upwind” scheme:
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divSchemes
{
default none;
div(phi,U) bounded Gauss upwind;
div(phi,k) bounded Gauss upwind;
div(phi,omega) bounded Gauss upwind;
div((nuEff*dev(T(grad(U))))) Gauss linear;
div(phi,nuTilda) bounded Gauss upwind;
}
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And in fv_Schemes_2, divergence schemes are set to second order bounded “linearUpwind” scheme:
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divSchemes
{
default none;
div(phi,U) bounded Gauss linearUpwind grad(U);
div(phi,k) bounded Gauss linearUpwind default;
div(phi,omega) bounded Gauss linearUpwind default;
div((nuEff*dev(T(grad(U))))) Gauss linear;
div(phi,nuTilda) bounded Gauss linearUpwind default;
}
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In this way OpenFOAM when iteration 100 is reached, will use second order accuracy, so that when the number of grid points doubles, the discretization error will be reduced by a factor of 4.
