WIP: [material][constant] Put volume work energy into storage term / internal energy

Harmonize enthalpy/internal energy of Constant component with the SimpleH2O implementation.

This is how it's done in SimpleH2O.

@holle Can you comment on this? Numerically it seems to make a difference if the consitutive law, i.e. dh = du + pdv is inserted for u or for h. It seems to me easier to account for the volume work in the volume integral.

As this has been only recently introduced in master !2421 (merged)/!2423 (merged) we can still change it easily.

Edit/Update: Constant component seems to be correct and SimpleH2O is the one that has a bug (wrong enthalpy and thus internal energy). So we shouldn't merge this but fix SimpleH2O instead.

dumux/material/components/constant.hh

@@ -158,13 +158,14 @@ public:
      * \brief Specific internal energy of the component \f$\mathrm{[J/kg]}\f$ as a liquid.
      * \param temperature temperature of phase in \f$\mathrm{[K]}\f$
      * \param pressure pressure of phase in \f$\mathrm{[Pa]}\f$
+     * \note Account for volume work in the internal energy which appears
+     *       in the storage term of the energy balance
      */
     static Scalar liquidInternalEnergy(Scalar temperature, Scalar pressure)
     {
-        // u = c * dT for incompressible fluids
-        const Scalar heatCapacity = liquidHeatCapacity(temperature, pressure);
-        static const Scalar tRef = getParamFromGroup<Scalar>(std::to_string(id), "Component.ReferenceTemperature", 293.15);
-        return heatCapacity * (temperature - tRef);
+        const Scalar h = liquidEnthalpy(temperature, pressure);
+        const Scalar rho = liquidDensity(temperature, pressure);
+        return h - pressure / rho;
     }
 
     /*!
@@ -175,9 +176,9 @@ public:
      */
     static Scalar liquidEnthalpy(Scalar temperature, Scalar pressure)
     {
-        const Scalar u = liquidInternalEnergy(temperature, pressure);
-        const Scalar rho = liquidDensity(temperature, pressure);
-        return u + pressure / rho;
+        const Scalar heatCapacity = liquidHeatCapacity(temperature, pressure);
+        static const Scalar tRef = getParamFromGroup<Scalar>(std::to_string(id), "Component.ReferenceTemperature", 293.15);
+        return heatCapacity * (temperature - tRef);
     }
 
     /*!