[spatialparams] Add gravity to the pm spatial params

dumux/material/spatialparams/fv1p.hh

@@ -90,7 +90,12 @@ class FVSpatialParamsOneP
 public:
     FVSpatialParamsOneP(std::shared_ptr<const FVGridGeometry> fvGridGeometry)
     : fvGridGeometry_(fvGridGeometry)
+    , gravity_(0.0)
     {
+        const bool enableGravity = getParam<bool>("Problem.EnableGravity");
+        if (enableGravity)
+            gravity_[dimWorld-1]  = -9.81;
+
         /* \brief default forchheimer coefficient
          * Source: Ward, J.C. 1964 Turbulent flow in porous media. ASCE J. Hydraul. Div 90 \cite ward1964 .
          *        Actually the Forchheimer coefficient is also a function of the dimensions of the
@@ -100,6 +105,19 @@ public:
         forchCoeffDefault_ = getParam<Scalar>("SpatialParams.ForchCoeff", 0.55);
     }
 
+    /*!
+     * \brief Returns the acceleration due to gravity \f$\mathrm{[m/s^2]}\f$.
+     *
+     * The default behaviour is a constant gravity vector;
+     * if the <tt>Problem.EnableGravity</tt> parameter is true,
+     * \f$\boldsymbol{g} = ( 0,\dots,\ -9.81)^T \f$,
+     * else \f$\boldsymbol{g} = ( 0,\dots, 0)^T \f$.
+     *
+     * \param pos the spatial position at which to evaulate the gravity vector
+     */
+    const GlobalPosition& gravity(const GlobalPosition &pos) const
+    { return gravity_; }
+
     /*!
      * \brief Harmonic average of a discontinuous scalar field at discontinuity interface
      *        (for compatibility reasons with the function below)
@@ -322,6 +340,7 @@ protected:
 
 private:
     std::shared_ptr<const FVGridGeometry> fvGridGeometry_;
+    GlobalPosition gravity_; //!< The gravity vector
     Scalar forchCoeffDefault_;
 };