strainRateFunction

The viscosity is modeled as a function of strain rate.

transportModel  strainRateFunction;
    strainRateFunctionCoeffs
    {
        function polynomial ((a b) (c d));
    }

Corresponds to

$$\nu =a\times {\dot{\gamma }}^b+c\times {\dot{\gamma }}^d$$

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References

1. https://cfd.direct/openfoam/user-guide/v7-boundaries#x25-1840005.2.3.4
2. https://cfd.direct/openfoam/user-guide/v7-boundaries#x25-1840005.2.3.4