[PL/TH2M] Separate non-const solid phase vol.frac.

The propertie belongs into the material models.
Computations in near future will require only constant
solid phase volume fraction, so the switch is added
to separate constant and non-constant parts.

ProcessLib/TH2M/TH2MFEM-impl.h

@@ -137,9 +137,6 @@ void TH2MLocalAssembler<ShapeFunctionDisplacement, ShapeFunctionPressure,
         auto const& Nu = ip_data.N_u;
         auto const& gradNu = ip_data.dNdx_u;
 
-        auto const& m = Invariants::identity2;
-        auto const mT = m.transpose();
-
         auto const x_coord =
             NumLib::interpolateXCoordinate<ShapeFunctionDisplacement,
                                            ShapeMatricesTypeDisplacement>(
@@ -238,17 +235,30 @@ void TH2MLocalAssembler<ShapeFunctionDisplacement, ShapeFunctionPressure,
         auto const phi_0 = medium.property(MPL::PropertyType::porosity)
                                .template value<double>(vars, pos, t, dt);
 
-        // solid phase volume fraction
-        double const div_u = mT * eps;
         auto const phi_S_0 = 1. - phi_0;
+
+#define NON_CONSTANT_SOLID_PHASE_VOLUME_FRACTION
+
+#ifdef NON_CONSTANT_SOLID_PHASE_VOLUME_FRACTION
+        auto const& m = Invariants::identity2;
+        double const div_u = m.transpose() * eps;
+
         const double phi_S = phi_S_0 * (1. + ip_data.thermal_volume_strain -
                                         ip_data.alpha_B * div_u);
+#else   // NON_CONSTANT_SOLID_PHASE_VOLUME_FRACTION
+        const double phi_S = phi_S_0;
+#endif  // NON_CONSTANT_SOLID_PHASE_VOLUME_FRACTION
+
         // porosity
         ip_data.phi = 1. - phi_S;
 
         // solid phase density
+#ifdef NON_CONSTANT_SOLID_PHASE_VOLUME_FRACTION
         auto const rhoSR = rho_ref_SR * (1. - ip_data.thermal_volume_strain +
                                          (ip_data.alpha_B - 1.) * div_u);
+#else   // NON_CONSTANT_SOLID_PHASE_VOLUME_FRACTION
+        auto const rhoSR = rho_ref_SR;
+#endif  // NON_CONSTANT_SOLID_PHASE_VOLUME_FRACTION
 
         ip_data.updateConstitutiveRelation(vars, t, pos, dt, T - T_dot * dt);