TpfaCompressible.hpp

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/*
  Copyright 2010 SINTEF ICT, Applied Mathematics.

  This file is part of the Open Porous Media project (OPM).

  OPM is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  OPM is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with OPM.  If not, see <http://www.gnu.org/licenses/>.
*/

#ifndef OPM_TPFACOMPRESSIBLE_HEADER_INCLUDED
#define OPM_TPFACOMPRESSIBLE_HEADER_INCLUDED


#include <opm/porsol/mimetic/TpfaCompressibleAssembler.hpp>
#include <opm/porsol/blackoil/BlackoilFluid.hpp>
#include <opm/core/linalg/LinearSolverIstl.hpp>
#include <opm/porsol/common/BoundaryConditions.hpp>

#include <opm/common/ErrorMacros.hpp>
#include <opm/core/utility/SparseTable.hpp>

#include <array>
#include <iostream>
#include <memory>

namespace Opm
{


    template <class GridInterface,
              class RockInterface,
              class FluidInterface,
              class WellsInterface,
              class BCInterface>
    class TpfaCompressible
    {
    public:
        typedef TpfaCompressibleAssembler PressureAssembler;

        TpfaCompressible()
            : pgrid_(0), prock_(0), pfluid_(0)
        {
        }


        void init(const Opm::parameter::ParameterGroup& param)
        {
            // Initialize inflow mixture to a fixed, user-provided mix.
            typename FluidInterface::CompVec mix(0.0);
            const int nc = FluidInterface::numComponents;
            double inflow_mixture_gas = param.getDefault("inflow_mixture_gas", 1.0);
            double inflow_mixture_oil = param.getDefault("inflow_mixture_oil", 0.0);
            switch (nc) {
            case 2:
                mix[FluidInterface::Gas] = inflow_mixture_gas;
                mix[FluidInterface::Oil] = inflow_mixture_oil;
                break;
            case 3: {
                double inflow_mixture_water = param.getDefault("inflow_mixture_water", 0.0);
                mix[FluidInterface::Water] = inflow_mixture_water;
                mix[FluidInterface::Gas] = inflow_mixture_gas;
                mix[FluidInterface::Oil] = inflow_mixture_oil;
                break;
            }
            default:
                OPM_THROW(std::runtime_error, "Unhandled number of components: " << nc);
            }
            inflow_mixture_ = mix;
            linsolver_.reset(new LinearSolverIstl(param));
            flux_rel_tol_ = param.getDefault("flux_rel_tol", 1e-5);
            press_rel_tol_ = param.getDefault("press_rel_tol", 1e-5);
            max_num_iter_ = param.getDefault("max_num_iter", 15);
            max_relative_voldiscr_ = param.getDefault("max_relative_voldiscr", 0.15);
            relax_time_voldiscr_ = param.getDefault("relax_time_voldiscr", 0.0);
            relax_weight_pressure_iteration_ = param.getDefault("relax_weight_pressure_iteration", 1.0);
            experimental_jacobian_ = param.getDefault("experimental_jacobian", false);
            nonlinear_residual_tolerance_ = param.getDefault("nonlinear_residual_tolerance", 0.0);
            output_residual_ = param.getDefault("output_residual", false);
            voldisc_factor_ = param.getDefault("voldisc_factor", 1.0);
        }




        typename FluidInterface::CompVec inflowMixture() const
        {
            return inflow_mixture_;
        }




        void setup(const GridInterface&         grid,
                   const RockInterface&         rock,
                   const FluidInterface&        fluid,
                   const WellsInterface&        wells,
                   const typename GridInterface::Vector& grav,
                   const BCInterface& bc,
                   const std::vector<typename FluidInterface::PhaseVec>* face_pressure = 0)
        {
            pgrid_ = &grid;
            prock_ = &rock;
            pfluid_ = &fluid;
            pwells_ = &wells;
            gravity_ = grav;

            // Extract perm tensors.
            const double* perm = &(rock.permeability(0)(0,0));
            poro_.clear();
            poro_.resize(grid.numCells(), 1.0);
            for (int i = 0; i < grid.numCells(); ++i) {
                poro_[i] = rock.porosity(i);
            }
            // Initialize 
            psolver_.init(grid, wells, perm, &poro_[0], grav);

            // Build bctypes_ and bcvalues_.
            int num_faces = grid.numFaces();
            bctypes_.clear();
            bctypes_.resize(num_faces, PressureAssembler::FBC_UNSET);
            bcvalues_.clear();
            bcvalues_.resize(num_faces, 0.0);
            for (int face = 0; face < num_faces; ++face) {
                int bid = pgrid_->boundaryId(face);
                if (bid == 0) {
                    bctypes_[face] = PressureAssembler::FBC_UNSET;
                    continue;
                }
                FlowBC face_bc = bc.flowCond(bid);
                if (face_bc.isDirichlet()) {
                    bctypes_[face] = PressureAssembler::FBC_PRESSURE;
                    bcvalues_[face] = face_bc.pressure();
                    if (face_bc.pressure() < 0.0) {
                       bcvalues_[face] = (*face_pressure)[face][FluidInterface::Liquid];
                    }
                } else if (face_bc.isNeumann()) {
                    bctypes_[face] = PressureAssembler::FBC_FLUX;
                    bcvalues_[face] = face_bc.outflux(); // TODO: may have to switch sign here depending on orientation.
                    if (bcvalues_[face] != 0.0) {
                        OPM_THROW(std::runtime_error, "Nonzero Neumann conditions not yet properly implemented "
                              "(signs must be fixed, also face pressures are not correctly computed for this case)");
                    }
                } else {
                    OPM_THROW(std::runtime_error, "Unhandled boundary condition type.");
                }
            }

            // Setup unchanging well data structures.
            perf_wells_.clear();
            perf_cells_.clear();
            perf_A_.clear();
            perf_mob_.clear();
            perf_sat_.clear();
            int num_wells = pwells_->numWells();
            for (int well = 0; well < num_wells; ++well) {
                int num_perf = pwells_->numPerforations(well);
                for (int perf = 0; perf < num_perf; ++perf) {
                    int cell = pwells_->wellCell(well, perf);
                    perf_wells_.push_back(well);
                    perf_cells_.push_back(cell);
                }
            }
            int num_perf = perf_wells_.size();
            perf_A_.resize(num_perf*numPhases*numComponents);
            perf_mob_.resize(num_perf*numPhases);
            perf_sat_.resize(num_perf);
        }




        double volumeDiscrepancyLimit() const
        {
            return max_relative_voldiscr_;
        }




        const std::vector<double>& faceTransmissibilities()
        {
            return psolver_.faceTransmissibilities();
        }




        bool volumeDiscrepancyAcceptable(const std::vector<typename FluidInterface::PhaseVec>& cell_pressure,
                                         const std::vector<typename FluidInterface::PhaseVec>& face_pressure,
                                         const std::vector<double>& well_perf_pressure,
                                         const std::vector<typename FluidInterface::CompVec>& cell_z,
                                         const double dt)
        {
            computeFluidProps(cell_pressure, face_pressure, well_perf_pressure, cell_z, dt);
            double rel_voldiscr = *std::max_element(fp_.relvoldiscr.begin(), fp_.relvoldiscr.end());
            if (rel_voldiscr > max_relative_voldiscr_) {
                std::cout << "    Relative volume discrepancy too large: " << rel_voldiscr << std::endl;
                return false;
            } else {
                std::cout << "    Relative volume discrepancy ok: " << rel_voldiscr << std::endl;
                return true;
            }
        }

        enum ReturnCode { SolveOk, VolumeDiscrepancyTooLarge, FailedToConverge };


        ReturnCode solve(std::vector<typename FluidInterface::PhaseVec>& cell_pressure,
                         std::vector<typename FluidInterface::PhaseVec>& face_pressure,
                         const std::vector<typename FluidInterface::CompVec>& cell_z,
                         std::vector<double>& face_flux,
                         std::vector<double>& well_bhp_pressures,
                         std::vector<double>& well_perf_pressures,
                         std::vector<double>& well_perf_fluxes,
                         const std::vector<double>& src,
                         const double dt)
        {
            // Set up initial state.
            // \TODO We are currently using Liquid phase pressure,
            //       what is correct with capillary pressure?
            SolverState state;
            int num_cells = cell_pressure.size();
            int num_faces = face_pressure.size();
            state.cell_pressure.resize(num_cells);
            for (int cell = 0; cell < num_cells; ++cell) {
                state.cell_pressure[cell] = cell_pressure[cell][FluidInterface::Liquid];
            }
            state.face_pressure.resize(num_faces);
            for (int face = 0; face < num_faces; ++face) {
                state.face_pressure[face] = face_pressure[face][FluidInterface::Liquid];
            }
            state.face_flux.clear();
            state.face_flux.resize(num_faces);
            state.well_bhp_pressure = well_bhp_pressures;
            state.well_perf_pressure = well_perf_pressures;
            state.well_perf_flux = well_perf_fluxes;

            // Run solver.
            ReturnCode retcode;
            if (nonlinear_residual_tolerance_ == 0.0) {
                // Old version.
                retcode = solveImpl(cell_z, src, dt, state);
            } else {
                // New version using residual reduction as
                // convergence criterion.
                retcode = solveImplNew(cell_z, src, dt, state);
            }

            // Copy results to output variables.
            if (retcode == SolveOk) {
                // \TODO Currently all phase pressures will be equal,
                //       what is correct with capillary pressure?
                for (int cell = 0; cell < num_cells; ++cell) {
                    cell_pressure[cell] = state.cell_pressure[cell];
                }
                for (int face = 0; face < num_faces; ++face) {
                    face_pressure[face] = state.face_pressure[face];
                }
                face_flux = state.face_flux;
                well_bhp_pressures = state.well_bhp_pressure;
                well_perf_pressures = state.well_perf_pressure;
                well_perf_fluxes = state.well_perf_flux;
            }
            return retcode;
        }



        double stableStepIMPES()
        {
            return psolver_.explicitTimestepLimit(&fp_.face_data.state_matrix[0][0][0],
                                                  &fp_.face_data.mobility[0][0],
                                                  &fp_.face_data.mobility_deriv[0][0][0],
                                                  &(pfluid_->surfaceDensities()[0]));
        }



        void doStepIMPES(std::vector<typename FluidInterface::CompVec>& cell_z,
                         const double dt)
        {
            psolver_.explicitTransport(dt, &(cell_z[0][0]));
        }



    private:
        const GridInterface* pgrid_;
        const RockInterface* prock_;
        const FluidInterface* pfluid_;
        const WellsInterface* pwells_;
        typename GridInterface::Vector gravity_;
        // typename FluidInterface::FluidData fp_;
        Opm::AllFluidData fp_;
        std::vector<double> poro_;
        PressureAssembler psolver_;
        std::unique_ptr<LinearSolverIstl> linsolver_;
        std::vector<PressureAssembler::FlowBCTypes> bctypes_;
        std::vector<double> bcvalues_;

        typename FluidInterface::CompVec inflow_mixture_;
        double flux_rel_tol_;
        double press_rel_tol_;
        int max_num_iter_;
        double max_relative_voldiscr_;
        double relax_time_voldiscr_;
        double relax_weight_pressure_iteration_;
        bool experimental_jacobian_;
        bool output_residual_;
        double nonlinear_residual_tolerance_;
        double voldisc_factor_;

        typedef typename FluidInterface::PhaseVec PhaseVec;
        typedef typename FluidInterface::CompVec CompVec;
        enum { numPhases = FluidInterface::numPhases,
               numComponents = FluidInterface::numComponents };

        std::vector<int> perf_wells_;
        std::vector<int> perf_cells_;
        std::vector<double> perf_A_;   // Flat storage.
        std::vector<double> perf_mob_; // Flat storage.
        std::vector<PhaseVec> perf_sat_;
        std::vector<double> perf_gpot_; // Flat storage.

        // Only intended to avoid extra allocations.
        mutable std::vector<PhaseVec> helper_cell_pressure_;
        mutable std::vector<PhaseVec> helper_face_pressure_;

        struct SolverState
        {
            std::vector<double> cell_pressure;
            std::vector<double> face_pressure;
            std::vector<double> face_flux;
            std::vector<double> well_bhp_pressure;
            std::vector<double> well_perf_pressure;
            std::vector<double> well_perf_flux;
        };


        void computeFluidProps(const std::vector<typename FluidInterface::PhaseVec>& phase_pressure,
                               const std::vector<typename FluidInterface::PhaseVec>& phase_pressure_face,
                               const std::vector<double>& well_perf_pressure,
                               const std::vector<typename FluidInterface::CompVec>& cell_z,
                               const double dt)
        {
            fp_.computeNew(*pgrid_, *prock_, *pfluid_, gravity_, phase_pressure, phase_pressure_face, cell_z, inflow_mixture_, dt);
            // Properties at well perforations.
            // \TODO only need to recompute this once per pressure update.
            // No, that is false, at production perforations the cell z is
            // used, which may change every step.
            unsigned int perfcount = 0;
            int num_wells = pwells_->numWells();
            for (int well = 0; well < num_wells; ++well) {
                bool inj = pwells_->type(well) == WellsInterface::Injector;
                int num_perf = pwells_->numPerforations(well);
                for (int perf = 0; perf < num_perf; ++perf) {
                    int cell = pwells_->wellCell(well, perf);
                    // \TODO handle capillary in perforation pressure below?
                    PhaseVec well_pressure = inj ? PhaseVec(well_perf_pressure[perfcount]) : phase_pressure[cell];
                    CompVec well_mixture = inj ? pwells_->injectionMixture(cell) : cell_z[cell];
                    typename FluidInterface::FluidState state = pfluid_->computeState(well_pressure, well_mixture);
                    std::copy(&state.phase_to_comp_[0][0], &state.phase_to_comp_[0][0] + numComponents*numPhases,
                              &perf_A_[perfcount*numPhases*numComponents]);
                    std::copy(state.mobility_.begin(), state.mobility_.end(),
                              &perf_mob_[perfcount*numPhases]);
                    perf_sat_[perfcount] = state.saturation_;
                    ++perfcount;
                }
            }
            assert(perfcount == perf_wells_.size());
        }



        // Convert scalar pressures to phase pressures, then call computeFluidProps().
        void computeFluidPropsScalarPress(const std::vector<double>& cell_pressure_scalar,
                                          const std::vector<double>& face_pressure_scalar,
                                          const std::vector<double>& well_perf_pressure,
                                          const std::vector<typename FluidInterface::CompVec>& cell_z,
                                          const double dt)
        {
            // Copy to phase pressures. \TODO handle capillary pressure.
            int num_cells = pgrid_->numCells();
            int num_faces = pgrid_->numFaces();
            helper_cell_pressure_.resize(num_cells);
            helper_face_pressure_.resize(num_faces);
            for (int cell = 0; cell < num_cells; ++cell) {
                helper_cell_pressure_[cell] = cell_pressure_scalar[cell];
            }
            for (int face = 0; face < num_faces; ++face) {
                helper_face_pressure_[face] = face_pressure_scalar[face];
            }
            computeFluidProps(helper_cell_pressure_, helper_face_pressure_, well_perf_pressure, cell_z, dt);
        }



        // Compute res = Ax - b.
        void computeLinearResidual(const PressureAssembler::LinearSystem& s, std::vector<double>& res)
        {
            res.resize(s.n);
            for (int row = 0; row < s.n; ++row) {
                res[row] = -s.b[row];
                for (int i = s.ia[row]; i < s.ia[row + 1]; ++i) {
                    res[row] += s.sa[i]*s.x[s.ja[i]];
                }
            }
        }



        // Compute residual and Jacobian of the new formulation.
        void computeResidualJacobian(const std::vector<double>& initial_voldiscr,
                                     const std::vector<double>& cell_pressure_scalar,
                                     const std::vector<double>& cell_pressure_scalar_initial,
                                     const std::vector<double>& well_bhp,
                                     const std::vector<double>& src,
                                     const double dt,
                                     PressureAssembler::LinearSystem& linsys,
                                     std::vector<double>& res)
        {
            std::vector<double> zero(initial_voldiscr.size(), 0.0);
            // Assemble system matrix and rhs.
            psolver_.assemble(&src[0],
                              &bctypes_[0],
                              &bcvalues_[0],
                              dt,
                              &fp_.cell_data.total_compressibility[0],
                              &zero[0],
                              &fp_.cell_data.state_matrix[0][0][0],
                              &fp_.face_data.state_matrix[0][0][0],
                              &perf_A_[0],
                              &fp_.face_data.mobility[0][0],
                              &perf_mob_[0],
                              &cell_pressure_scalar_initial[0],
                              &fp_.face_data.gravity_potential[0][0],
                              &perf_gpot_[0],
                              &(pfluid_->surfaceDensities()[0]));
            psolver_.linearSystem(linsys);
            // The linear system is for direct evaluation, we want a residual based approach.
            // First we compute the residual for the original code.
            int num_cells = pgrid_->numCells();
            std::copy(cell_pressure_scalar.begin(), cell_pressure_scalar.end(), linsys.x);
            std::copy(well_bhp.begin(), well_bhp.end(), linsys.x + num_cells);
            computeLinearResidual(linsys, res);
            // Then we compute the residual we actually want by subtracting terms that do not
            // appear in the new formulation and adding the new terms.
            for (int cell = 0; cell < num_cells; ++cell) {
                double tc = fp_.cell_data.total_compressibility[cell];
                double dres = tc*(cell_pressure_scalar[cell] - cell_pressure_scalar_initial[cell]);
                dres -= 1.0 - fp_.cell_data.total_phase_volume_density[cell];
                dres *= pgrid_->cellVolume(cell)*prock_->porosity(cell)/dt;
                res[cell] -= dres;
            }
            // Change the jacobian by adding/subtracting the necessary terms.
            for (int cell = 0; cell < num_cells; ++cell) {
                for (int i = linsys.ia[cell]; i < linsys.ia[cell + 1]; ++i) {
                    if  (linsys.ja[i] == cell) {
                        double tc = fp_.cell_data.total_compressibility[cell];
                        linsys.sa[i] -= tc*pgrid_->cellVolume(cell)*prock_->porosity(cell)/dt;
                        double ex = fp_.cell_data.experimental_term[cell];
                        linsys.sa[i] += ex*pgrid_->cellVolume(cell)*prock_->porosity(cell)/dt;
                    }
                }
            }
        }



        // Compute the well potentials. Assumes that the perforation variables
        // have been set properly: perf_[wells_|cells_|A_].
        void computeWellPotentials(std::vector<double>& wellperf_gpot) const
        {
            int num_perf = perf_cells_.size();
            wellperf_gpot.resize(num_perf*numPhases);
            for (int perf = 0; perf < num_perf; ++perf) {
                int well = perf_wells_[perf];
                int cell = perf_cells_[perf];
                typename GridInterface::Vector pos = pgrid_->cellCentroid(cell);
                // With wells, we assume that gravity is in the z-direction.
                assert(gravity_[0] == 0.0 && gravity_[1] == 0.0);
                double depth_delta = pos[2] - pwells_->referenceDepth(well);
                double gh = gravity_[2]*depth_delta;
                // At is already transposed since in Fortran order.
                const double* At = &perf_A_[perf*numPhases*numComponents];
                PhaseVec rho = pfluid_->phaseDensities(At);
                for (int phase = 0; phase < numPhases; ++phase) {
                    // Gravity potential is (by phase) \rho_\alpha g h
                    wellperf_gpot[numPhases*perf + phase] = rho[phase]*gh;
                }
            }
        }



        // Compute the relative changes in fluxes and pressures.
        static std::pair<double, double>
        computeFluxPressChanges(const std::vector<double>& face_flux,
                                const std::vector<double>& well_perf_fluxes,
                                const std::vector<double>& cell_pressure_scalar,
                                const std::vector<double>& start_face_flux,
                                const std::vector<double>& start_perf_flux,
                                const std::vector<double>& start_cell_press)
        {
            int num_faces = face_flux.size();
            int num_perf = well_perf_fluxes.size();
            int num_cells = cell_pressure_scalar.size();
            double max_flux_face = std::max(std::fabs(*std::min_element(face_flux.begin(), face_flux.end())),
                                            std::fabs(*std::max_element(face_flux.begin(), face_flux.end())));
            double max_flux_perf = num_perf == 0 ? 0.0
                : std::max(std::fabs(*std::min_element(well_perf_fluxes.begin(), well_perf_fluxes.end())),
                           std::fabs(*std::max_element(well_perf_fluxes.begin(), well_perf_fluxes.end())));
            double max_flux = std::max(max_flux_face, max_flux_perf);
            double max_press = std::max(std::fabs(*std::min_element(cell_pressure_scalar.begin(),
                                                                    cell_pressure_scalar.end())),
                                        std::fabs(*std::max_element(cell_pressure_scalar.begin(),
                                                                    cell_pressure_scalar.end())));
            double flux_change_infnorm = 0.0;
            double press_change_infnorm = 0.0;
            for (int face = 0; face < num_faces; ++face) {
                flux_change_infnorm = std::max(flux_change_infnorm,
                                               std::fabs(face_flux[face] - start_face_flux[face]));
            }
            for (int perf = 0; perf < num_perf; ++perf) {
                flux_change_infnorm = std::max(flux_change_infnorm,
                                               std::fabs(well_perf_fluxes[perf] - start_perf_flux[perf]));
            }
            for (int cell = 0; cell < num_cells; ++cell) {
                press_change_infnorm = std::max(press_change_infnorm,
                                                std::fabs(cell_pressure_scalar[cell] - start_cell_press[cell]));
            }
            double flux_rel_difference = flux_change_infnorm/max_flux;
            double press_rel_difference = press_change_infnorm/max_press;
            return std::make_pair(flux_rel_difference, press_rel_difference);
        }



        // Compute well perforation pressures.
        void computeWellPerfPressures(const std::vector<double>& well_perf_fluxes,
                                      const std::vector<double>& well_bhp,
                                      const std::vector<double>& wellperf_gpot,
                                      std::vector<double>& well_perf_pressures) const
        {
            // Compute averaged saturations for each well. This code
            // assumes that flow is either in or out of any single
            // well, not both.
            int num_perf = well_perf_fluxes.size();
            std::vector<PhaseVec> well_sat(pwells_->numWells(), PhaseVec(0.0));
            std::vector<double> well_flux(pwells_->numWells(), 0.0);
            for (int perf = 0; perf < num_perf; ++perf) {
                int well = perf_wells_[perf];
                double flux = well_perf_fluxes[perf];
                well_flux[well] += flux;
                PhaseVec tmp = perf_sat_[perf];
                tmp *= flux;
                well_sat[well] += tmp;
            }
            for (int well = 0; well < pwells_->numWells(); ++well) {
                well_sat[well] *= 1.0/well_flux[well];
            }

            // Compute well_perf_pressures
            for (int perf = 0; perf < num_perf; ++perf) {
                well_perf_pressures[perf] = well_bhp[perf_wells_[perf]];
                PhaseVec sat = well_sat[perf_wells_[perf]];
                for (int phase = 0; phase < numPhases; ++phase) {
                    well_perf_pressures[perf]
                        += sat[phase]*wellperf_gpot[numPhases*perf + phase];
                }
            }
        }



        // Sets the initial volume discrepancy, applying relaxation if requested.
        bool getInitialVolumeDiscrepancy(const double dt, std::vector<double>& voldiscr_initial)
        {
            voldiscr_initial = fp_.voldiscr;
            double rel_voldiscr = *std::max_element(fp_.relvoldiscr.begin(), fp_.relvoldiscr.end());
            if (rel_voldiscr > max_relative_voldiscr_) {
                std::cout << "Initial relative volume discrepancy too large: " << rel_voldiscr << std::endl;
                return false;
            }
            if (relax_time_voldiscr_ > 0.0) {
                double relax = std::min(1.0,dt/relax_time_voldiscr_);
                std::transform(voldiscr_initial.begin(), voldiscr_initial.end(), voldiscr_initial.begin(),
                               std::binder1st<std::multiplies<double> >(std::multiplies<double>() , relax));
            }
            if (voldisc_factor_ != 1.0) {
                std::transform(voldiscr_initial.begin(), voldiscr_initial.end(), voldiscr_initial.begin(),
                               std::binder1st<std::multiplies<double> >(std::multiplies<double>(), voldisc_factor_));
            }
            return true;
        }



        // Modifies cell pressures, face pressures and face fluxes of
        // state such that
        //    state <- weight*state + (1 - weight)*start_state
        // So weight == 1.0 does nothing.
        void relaxState(const double weight,
                        const SolverState& start_state,
                        SolverState& state)
        {
            int num_cells = pgrid_->numCells();
            int num_faces = pgrid_->numFaces();
            for (int cell = 0; cell < num_cells; ++cell) {
                state.cell_pressure[cell] = weight*state.cell_pressure[cell] + (1.0-weight)*start_state.cell_pressure[cell];
            }
            for (int face = 0; face < num_faces; ++face) {
                state.face_pressure[face] = weight*state.face_pressure[face] + (1.0-weight)*start_state.face_pressure[face];
                state.face_flux[face] = weight*state.face_flux[face] + (1.0-weight)*start_state.face_flux[face];
            }
        }



        // Implements the main nonlinear loop of the pressure solver.
        ReturnCode solveImpl(const std::vector<typename FluidInterface::CompVec>& cell_z,
                             const std::vector<double>& src,
                             const double dt,
                             SolverState& state)
        {
            int num_cells = pgrid_->numCells();
            std::vector<double> cell_pressure_initial = state.cell_pressure;
            std::vector<double> voldiscr_initial;
            SolverState start_state;

            // ------------  Main iteration loop -------------
            for (int iter = 0; iter < max_num_iter_; ++iter) {
                start_state = state;
                // (Re-)compute fluid properties.
                computeFluidPropsScalarPress(state.cell_pressure, state.face_pressure, state.well_perf_pressure, cell_z, dt);

                // Initialization for the first iteration only.
                if (iter == 0) {
                    bool voldisc_ok = getInitialVolumeDiscrepancy(dt, voldiscr_initial);
                    if (!voldisc_ok) {
                        return VolumeDiscrepancyTooLarge;
                    }

                    // perf_gpot_ is computed once per pressure solve,
                    // while perf_A_, perf_mob_ are recoomputed
                    // for every iteration.
                    computeWellPotentials(perf_gpot_);
                }

                if (experimental_jacobian_) {
                    // Compute residual and jacobian.
                    PressureAssembler::LinearSystem s;
                    std::vector<double> residual;
                    computeResidualJacobian(voldiscr_initial, state.cell_pressure, cell_pressure_initial,
                                            state.well_bhp_pressure, src, dt, s, residual);

                    if (output_residual_) {
                        // Temporary hack to get output of residual.
                        static int psolve_iter = -1;
                        if (iter == 0) {
                            ++psolve_iter;
                        }
                        std::ostringstream oss;
                        oss << "residual-" << psolve_iter << '-' << iter << ".dat";
                        std::ofstream outres(oss.str().c_str());
                        std::copy(residual.begin(), residual.end(), std::ostream_iterator<double>(outres, "\n"));
                    }

                    // Solve system for dp, that is, we use residual as the rhs.
                    LinearSolverIstl::LinearSolverReport result
                        = linsolver_->solve(s.n, s.nnz, s.ia, s.ja, s.sa, &residual[0], s.x);
                    if (!result.converged) {
                        OPM_THROW(std::runtime_error, "Linear solver failed to converge in " << result.iterations << " iterations.\n"
                              << "Residual reduction achieved is " << result.residual_reduction << '\n');
                    }
                    // Set x so that the call to computePressuresAndFluxes() will work.
                    // Recall that x now contains dp, and we want it to contain p - dp
                    for (int cell = 0; cell < num_cells; ++cell) {
                        s.x[cell] = state.cell_pressure[cell] - s.x[cell];
                    }
                    for (int well = 0; well < pwells_->numWells(); ++well) {
                        s.x[num_cells + well] = state.well_bhp_pressure[well] - s.x[num_cells + well]; 
                    }
                } else {
                    // Assemble system matrix and rhs.
                    psolver_.assemble(&src[0],
                                      &bctypes_[0],
                                      &bcvalues_[0],
                                      dt,
                                      &fp_.cell_data.total_compressibility[0],
                                      &voldiscr_initial[0],
                                      &fp_.cell_data.state_matrix[0][0][0],
                                      &fp_.face_data.state_matrix[0][0][0],
                                      &perf_A_[0],
                                      &fp_.face_data.mobility[0][0],
                                      &perf_mob_[0],
                                      &cell_pressure_initial[0],
                                      &fp_.face_data.gravity_potential[0][0],
                                      &perf_gpot_[0],
                                      &(pfluid_->surfaceDensities()[0]));
                    PressureAssembler::LinearSystem s;
                    psolver_.linearSystem(s);
                    // Solve system.
                    LinearSolverIstl::LinearSolverReport res = linsolver_->solve(s.n, s.nnz, s.ia, s.ja, s.sa, s.b, s.x);
                    if (!res.converged) {
                        OPM_THROW(std::runtime_error, "Linear solver failed to converge in " << res.iterations << " iterations.\n"
                              << "Residual reduction achieved is " << res.residual_reduction << '\n');
                    }
                }

                // Get pressures and face fluxes.
                psolver_.computePressuresAndFluxes(state.cell_pressure, state.face_pressure, state.face_flux,
                                                   state.well_bhp_pressure, state.well_perf_flux);

                // Relaxation
                if (relax_weight_pressure_iteration_ != 1.0) {
                    relaxState(relax_weight_pressure_iteration_, start_state, state);
                }

                // Compute state.well_perf_pressure.
                computeWellPerfPressures(state.well_perf_flux, state.well_bhp_pressure,
                                         perf_gpot_, state.well_perf_pressure);

                // Compute relative changes for pressure and flux.
                std::pair<double, double> rel_changes
                    = computeFluxPressChanges(state.face_flux, state.well_perf_flux, state.cell_pressure,
                                              start_state.face_flux, start_state.well_perf_flux, start_state.cell_pressure);
                double flux_rel_difference = rel_changes.first;
                double press_rel_difference = rel_changes.second;

                // Test for convergence.
                if (iter == 0) {
                    std::cout << "Iteration      Rel. flux change     Rel. pressure change\n";
                }
                std::cout.precision(5);
                std::cout << std::setw(6) << iter
                          << std::setw(24) << flux_rel_difference
                          << std::setw(24) << press_rel_difference << std::endl;
                std::cout.precision(16);

                if (flux_rel_difference < flux_rel_tol_ || press_rel_difference < press_rel_tol_) {
                    std::cout << "Pressure solver converged. Number of iterations: " << iter + 1 << '\n' << std::endl;
                    return SolveOk;
                }
            }

            return FailedToConverge;
        }




        // Implements the main nonlinear loop of the pressure solver.
        ReturnCode solveImplNew(const std::vector<typename FluidInterface::CompVec>& cell_z,
                                const std::vector<double>& src,
                                const double dt,
                                SolverState& state)
        {
            int num_cells = pgrid_->numCells();
            int num_faces = pgrid_->numFaces();
            std::vector<double> cell_pressure_initial = state.cell_pressure;
            std::vector<double> voldiscr_initial;
            SolverState start_state;

            // ------------  Main iteration loop -------------
            for (int iter = 0; iter < max_num_iter_; ++iter) {
                start_state = state;
                // (Re-)compute fluid properties.
                computeFluidPropsScalarPress(state.cell_pressure, state.face_pressure, state.well_perf_pressure, cell_z, dt);

                // Initialization for the first iteration only.
                if (iter == 0) {
                    bool voldisc_ok = getInitialVolumeDiscrepancy(dt, voldiscr_initial);
                    if (!voldisc_ok) {
                        return VolumeDiscrepancyTooLarge;
                    }

                    // perf_gpot_ is computed once per pressure solve,
                    // while perf_A_, perf_mob_ are recoomputed
                    // for every iteration.
                    computeWellPotentials(perf_gpot_);
                }

                // Compute residual and jacobian.
                PressureAssembler::LinearSystem s;
                std::vector<double> residual;
                computeResidualJacobian(voldiscr_initial, state.cell_pressure, cell_pressure_initial,
                                        state.well_bhp_pressure, src, dt, s, residual);
                if (output_residual_) {
                    // Temporary hack to get output of residual.
                    static int psolve_iter = -1;
                    if (iter == 0) {
                        ++psolve_iter;
                    }
                    std::ostringstream oss;
                    oss << "residual-" << psolve_iter << '-' << iter << ".dat";
                    std::ofstream outres(oss.str().c_str());
                    std::copy(residual.begin(), residual.end(), std::ostream_iterator<double>(outres, "\n"));
                }

                // Find the maxnorm of the residual.
                double maxres = std::max(std::fabs(*std::max_element(residual.begin(), residual.end())),
                                         std::fabs(*std::min_element(residual.begin(), residual.end())));

                // Solve system for dp, that is, we use residual as the rhs.
                LinearSolverIstl::LinearSolverReport result
                    = linsolver_->solve(s.n, s.nnz, s.ia, s.ja, s.sa, &residual[0], s.x);
                if (!result.converged) {
                    OPM_THROW(std::runtime_error, "Linear solver failed to converge in " << result.iterations << " iterations.\n"
                          << "Residual reduction achieved is " << result.residual_reduction << '\n');
                }

                // Set x so that the call to computePressuresAndFluxes() will work.
                // Recall that x now contains dp, and we want it to contain p - dp
                for (int cell = 0; cell < num_cells; ++cell) {
                    s.x[cell] = state.cell_pressure[cell] - s.x[cell];
                }
                for (int well = 0; well < pwells_->numWells(); ++well) {
                    s.x[num_cells + well] = state.well_bhp_pressure[well] - s.x[num_cells + well]; 
                }

                // Get pressures and face fluxes.
                psolver_.computePressuresAndFluxes(state.cell_pressure, state.face_pressure, state.face_flux,
                                                   state.well_bhp_pressure, state.well_perf_flux);

                // Relaxation
                if (relax_weight_pressure_iteration_ != 1.0) {
                    double ww = relax_weight_pressure_iteration_;
                    for (int cell = 0; cell < num_cells; ++cell) {
                        state.cell_pressure[cell] = ww*state.cell_pressure[cell] + (1.0-ww)*start_state.cell_pressure[cell];
                    }
                    if (iter > 0) {
                        for (int face = 0; face < num_faces; ++face) {
                            state.face_pressure[face] = ww*state.face_pressure[face] + (1.0-ww)*start_state.face_pressure[face];
                            state.face_flux[face] = ww*state.face_flux[face] + (1.0-ww)*start_state.face_flux[face];
                        }
                    }
                }

                // Compute state.well_perf_pressure.
                computeWellPerfPressures(state.well_perf_flux, state.well_bhp_pressure,
                                         perf_gpot_, state.well_perf_pressure);

                // Compute relative changes for pressure and flux.
                std::pair<double, double> rel_changes
                    = computeFluxPressChanges(state.face_flux, state.well_perf_flux, state.cell_pressure,
                                              start_state.face_flux, start_state.well_perf_flux, start_state.cell_pressure);
                double flux_rel_difference = rel_changes.first;
                double press_rel_difference = rel_changes.second;

                // Test for convergence.
                if (iter == 0) {
                    std::cout << "Iteration      Rel. flux change     Rel. pressure change             Residual\n";
                }
                std::cout.precision(5);
                std::cout << std::setw(6) << iter
                          << std::setw(24) << flux_rel_difference
                          << std::setw(24) << press_rel_difference
                          << std::setw(24) << maxres << std::endl;
                std::cout.precision(16);

                if (maxres < nonlinear_residual_tolerance_) {
                    std::cout << "Pressure solver converged. Number of iterations: " << iter + 1 << '\n' << std::endl;
                    return SolveOk;
                }
            }

            return FailedToConverge;
        }


    };


} // namespace Opm



#endif // OPM_TPFACOMPRESSIBLE_HEADER_INCLUDED