FEMPoissonSolver.h

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// Class FEMPoissonSolver
//   Solves the poisson equation using finite element methods and Conjugate
//   Gradient
 
#ifndef IPPL_FEMPOISSONSOLVER_H
#define IPPL_FEMPOISSONSOLVER_H
 
#include "LinearSolvers/PCG.h"
#include "Poisson.h"
 
namespace ippl {
 
    template <typename Tlhs, unsigned Dim, unsigned numElemDOFs>
    struct EvalFunctor {
        const Vector<Tlhs, Dim> DPhiInvT;
        const Tlhs absDetDPhi;
 
        EvalFunctor(Vector<Tlhs, Dim> DPhiInvT, Tlhs absDetDPhi)
            : DPhiInvT(DPhiInvT)
            , absDetDPhi(absDetDPhi) {}
 
        KOKKOS_FUNCTION auto operator()(
            const size_t& i, const size_t& j,
            const Vector<Vector<Tlhs, Dim>, numElemDOFs>& grad_b_q_k) const {
            return dot((DPhiInvT * grad_b_q_k[j]), (DPhiInvT * grad_b_q_k[i])).apply() * absDetDPhi;
        }
    };

    template <typename FieldLHS, typename FieldRHS = FieldLHS, unsigned Order = 1, unsigned QuadNumNodes = 5>
    class FEMPoissonSolver : public Poisson<FieldLHS, FieldRHS> {
        constexpr static unsigned Dim = FieldLHS::dim;
        using Tlhs                    = typename FieldLHS::value_type;
 
    public:
        using Base = Poisson<FieldLHS, FieldRHS>;
        using typename Base::lhs_type, typename Base::rhs_type;
        using MeshType = typename FieldRHS::Mesh_t;
 
        // PCG (Preconditioned Conjugate Gradient) is the solver algorithm used
        using PCGSolverAlgorithm_t =
            CG<lhs_type, lhs_type, lhs_type, lhs_type, lhs_type, FieldLHS, FieldRHS>;
 
        // FEM Space types
        using ElementType =
            std::conditional_t<Dim == 1, ippl::EdgeElement<Tlhs>,
                               std::conditional_t<Dim == 2, ippl::QuadrilateralElement<Tlhs>,
                                                  ippl::HexahedralElement<Tlhs>>>;
 
        using QuadratureType = GaussJacobiQuadrature<Tlhs, QuadNumNodes, ElementType>;
 
        using LagrangeType = LagrangeSpace<Tlhs, Dim, Order, ElementType, QuadratureType, FieldLHS, FieldRHS>;
 
        // default constructor (compatibility with Alpine)
        FEMPoissonSolver() 
            : Base()
            , refElement_m()
            , quadrature_m(refElement_m, 0.0, 0.0)
            , lagrangeSpace_m(*(new MeshType(NDIndex<Dim>(Vector<unsigned, Dim>(0)), Vector<Tlhs, Dim>(0),
                                Vector<Tlhs, Dim>(0))), refElement_m, quadrature_m)
        {}
 
        FEMPoissonSolver(lhs_type& lhs, rhs_type& rhs)
            : Base(lhs, rhs)
            , refElement_m()
            , quadrature_m(refElement_m, 0.0, 0.0)
            , lagrangeSpace_m(rhs.get_mesh(), refElement_m, quadrature_m, rhs.getLayout()) {
            static_assert(std::is_floating_point<Tlhs>::value, "Not a floating point type");
            setDefaultParameters();
 
            // start a timer
            static IpplTimings::TimerRef init = IpplTimings::getTimer("initFEM");
            IpplTimings::startTimer(init);
            
            rhs.fillHalo();
 
            lagrangeSpace_m.evaluateLoadVector(rhs);
 
            rhs.fillHalo();
            
            IpplTimings::stopTimer(init);
        }
 
        void setRhs(rhs_type& rhs) override {
            Base::setRhs(rhs);
 
            lagrangeSpace_m.initialize(rhs.get_mesh(), rhs.getLayout());
 
            rhs.fillHalo();
 
            lagrangeSpace_m.evaluateLoadVector(rhs);
 
            rhs.fillHalo();
        }

        void solve() override {
            // start a timer
            static IpplTimings::TimerRef solve = IpplTimings::getTimer("solve");
            IpplTimings::startTimer(solve);
 
            const Vector<size_t, Dim> zeroNdIndex = Vector<size_t, Dim>(0);
 
            // We can pass the zeroNdIndex here, since the transformation jacobian does not depend
            // on translation
            const auto firstElementVertexPoints =
                lagrangeSpace_m.getElementMeshVertexPoints(zeroNdIndex);
 
            // Compute Inverse Transpose Transformation Jacobian ()
            const Vector<Tlhs, Dim> DPhiInvT =
                refElement_m.getInverseTransposeTransformationJacobian(firstElementVertexPoints);
 
            // Compute absolute value of the determinant of the transformation jacobian (|det D
            // Phi_K|)
            const Tlhs absDetDPhi = Kokkos::abs(
                refElement_m.getDeterminantOfTransformationJacobian(firstElementVertexPoints));
 
            EvalFunctor<Tlhs, Dim, LagrangeType::numElementDOFs> poissonEquationEval(
                DPhiInvT, absDetDPhi);
 
            // get BC type of our RHS
            BConds<FieldRHS, Dim>& bcField = (this->rhs_mp)->getFieldBC();
            FieldBC bcType = bcField[0]->getBCType();
 
            const auto algoOperator = [poissonEquationEval, &bcField, this](rhs_type field) -> lhs_type {
                // set appropriate BCs for the field as the info gets lost in the CG iteration
                field.setFieldBC(bcField);
 
                field.fillHalo();
 
                auto return_field = lagrangeSpace_m.evaluateAx(field, poissonEquationEval);
 
                return return_field;
            };
 
            pcg_algo_m.setOperator(algoOperator);
 
            // send boundary values to RHS (load vector) i.e. lifting (Dirichlet BCs)
            if (bcType == CONSTANT_FACE) {
                *(this->rhs_mp) = *(this->rhs_mp) -
                    lagrangeSpace_m.evaluateAx_lift(*(this->rhs_mp), poissonEquationEval);
            }
 
            // start a timer
            static IpplTimings::TimerRef pcgTimer = IpplTimings::getTimer("pcg");
            IpplTimings::startTimer(pcgTimer);
 
            pcg_algo_m(*(this->lhs_mp), *(this->rhs_mp), this->params_m);
 
            (this->lhs_mp)->fillHalo();
 
            IpplTimings::stopTimer(pcgTimer);
 
            int output = this->params_m.template get<int>("output_type");
            if (output & Base::GRAD) {
                *(this->grad_mp) = -grad(*(this->lhs_mp));
            }
 
            IpplTimings::stopTimer(solve);
        }

        int getIterationCount() { return pcg_algo_m.getIterationCount(); }

        Tlhs getResidue() const { return pcg_algo_m.getResidue(); }

        template <typename F>
        Tlhs getL2Error(const F& analytic) {
            Tlhs error_norm = this->lagrangeSpace_m.computeErrorL2(*(this->lhs_mp), analytic);
            return error_norm;
        }

        Tlhs getAvg(bool Vol = false) {
            Tlhs avg = this->lagrangeSpace_m.computeAvg(*(this->lhs_mp));
            if (Vol) {
                lhs_type unit((this->lhs_mp)->get_mesh(), (this->lhs_mp)->getLayout());
                unit = 1.0;
                Tlhs vol = this->lagrangeSpace_m.computeAvg(unit);
                return avg/vol;
            } else {
                return avg;
            }
        }
 
    protected:
        PCGSolverAlgorithm_t pcg_algo_m;
 
        virtual void setDefaultParameters() override {
            this->params_m.add("max_iterations", 1000);
            this->params_m.add("tolerance", (Tlhs)1e-13);
        }
 
        ElementType refElement_m;
        QuadratureType quadrature_m;
        LagrangeType lagrangeSpace_m;
    };
 
}  // namespace ippl
 
#endif