Implement DiffusivityConstantTortuosity that reads tortuosity from command line

dumux/material/fluidmatrixinteractions/diffusivityconstanttortuosity.hh

+// -*- mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+// vi: set et ts=4 sw=4 sts=4:
+/*****************************************************************************
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+ *****************************************************************************/
+/*!
+ * \file
+ *
+ * \brief   Relation for the saturation-dependent effective diffusion coefficient
+ */
+#ifndef DIFFUSIVITY_CONSTANT_TORTUOSITY_HH
+#define DIFFUSIVITY_CONSTANT_TORTUOSITY_HH
+
+#include <dumux/common/parameters.hh>
+
+namespace Dumux
+{
+/*!
+ * \ingroup fluidmatrixinteractionslaws
+ *
+ * \brief Relation for the saturation-dependent effective diffusion coefficient
+ *
+ * The material law is:
+ * \f[
+ *  D_\text{eff,pm} = \phi * S_w * \tau * D
+ * \f]
+ *
+ * with a constant tau.
+ *
+ * The default value is 0.5, empirically obtained in Carman 1937:
+ * <i>Fluid flow through granular beds</i> \cite carman1937
+ * Additionally, Bear 1972 \cite bear1972 mentions values 0.4 and in the
+ * range of 0.56 to 0.8.
+ */
+template<class Scalar>
+class DiffusivityConstantTortuosity
+{
+public:
+    /*!
+     * \brief Returns the effective diffusion coefficient \f$\mathrm{[m^2/s]}\f$ based
+     *        on a constant tortuosity value
+     *
+     * \param porosity The porosity
+     * \param saturation The saturation of the wetting phase
+     * \param diffCoeff The diffusion coefficient of the phase in \f$\mathrm{[m^2/s]}\f$
+     */
+    static Scalar effectiveDiffusivity(const Scalar porosity,
+                                       const Scalar saturation,
+                                       const Scalar diffCoeff)
+
+    {
+        static const Scalar tau = getParam<Scalar>("SpatialParams.Tortuosity", 0.5);
+
+        return porosity * saturation * tau * diffCoeff;
+    }
+};
+
+} // end namespace Dumux
+
+#endif // DIFFUSIVITY_CONSTANT_TORTUOSITY_HH