Add viscosity assembler with variable coefficients

[[Imported from SVN: r12873]]

dune/fufem/assemblers/localassemblers/variablecoefficientviscosityassembler.hh

+#ifndef VARIABLE_COEFFICIENT_VISCOSITY_ASSEMBLER_HH
+#define VARIABLE_COEFFICIENT_VISCOSITY_ASSEMBLER_HH
+
+#include "dune/fufem/assemblers/localassemblers/strainproductassembler.hh"
+#include "dune/fufem/functions/analyticgridfunction.hh"
+#include "dune/fufem/symmetrictensor.hh"
+
+/** \brief Local assembler for the viscous part of the linear visco-elastic Kelvin-Voigt material. With variable coefficients. */
+template <class GridType, class TrialLocalFE, class AnsatzLocalFE, class FunctionType>
+class VariableCoefficientViscosityAssembler
+    : public StrainProductAssembler< GridType, TrialLocalFE, AnsatzLocalFE, VariableCoefficientViscosityAssembler<GridType,TrialLocalFE,AnsatzLocalFE,FunctionType> >
+{
+    private:
+        typedef StrainProductAssembler< GridType, TrialLocalFE, AnsatzLocalFE, VariableCoefficientViscosityAssembler<GridType,TrialLocalFE,AnsatzLocalFE, FunctionType> > SPA;
+        using SPA::dim;
+        using typename SPA::ctype;
+        typedef typename FunctionType::RangeType RangeType;
+        typedef VirtualGridFunction<GridType, RangeType> GridFunction;
+
+    public:
+        using typename SPA::T;
+        using typename SPA::BoolMatrix;
+        using typename SPA::Element;
+        using typename SPA::LocalMatrix;
+
+    VariableCoefficientViscosityAssembler(const GridType& grid,
+                                          const FunctionType& muShear,
+                                          const FunctionType& muBulk,
+                                          const QuadratureRuleKey& muShearKey,
+                                          const QuadratureRuleKey& muBulkKey)
+      : SPA(muShearKey.sum(muBulkKey)),
+        muShearGridFunctionP_(makeGridFunction(grid, muShear)),
+        muBulkGridFunctionP_(makeGridFunction(grid, muBulk))
+    {}
+
+    SymmetricTensor<dim> strainToStress(const SymmetricTensor<dim>& strain,
+                                        const Element& element,
+                                        const Dune::FieldVector<ctype, dim>& position) const {
+            RangeType muBulk;
+            muBulkGridFunctionP_->evaluateLocal(element, position, muBulk);
+            RangeType muShear;
+            muShearGridFunctionP_->evaluateLocal(element, position, muShear);
+
+            SymmetricTensor<dim> stress = strain;
+            stress *= 2*muShear;
+            stress.addToDiag((muBulk-(2.0/3.0)*muShear) * strain.trace());
+            return stress;
+    }
+    protected:
+        Dune::shared_ptr<const GridFunction> muShearGridFunctionP_;
+        Dune::shared_ptr<const GridFunction> muBulkGridFunctionP_;
+};
+
+
+#endif