Flow1D.h

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//! @file Flow1D.h
 
// This file is part of Cantera. See License.txt in the top-level directory or
// at https://cantera.org/license.txt for license and copyright information.
 
#ifndef CT_FLOW1D_H
#define CT_FLOW1D_H
 
#include "Domain1D.h"
#include "OneDim.h"
#include "cantera/base/Array.h"
#include "cantera/base/Solution.h"
#include "cantera/thermo/ThermoPhase.h"
#include "cantera/kinetics/Kinetics.h"
 
namespace Cantera
{
 
//------------------------------------------
//   constants
//------------------------------------------
 
//! Offsets of solution components in the 1D solution array.
enum offset
{
    c_offset_U   //! axial velocity [m/s]
    , c_offset_V //! strain rate
    , c_offset_T //! temperature [kelvin]
    , c_offset_L //! (1/r)dP/dr
    , c_offset_E //! electric field
    , c_offset_Uo //! oxidizer axial velocity [m/s]
    , c_offset_Y //! mass fractions
};
 
class Transport;
 
//! @defgroup flowGroup Flow Domains
//! One-dimensional flow domains.
//! @ingroup onedGroup
 
/**
 *  This class represents 1D flow domains that satisfy the one-dimensional
 *  similarity solution for chemically-reacting, axisymmetric flows.
 *  @ingroup flowGroup
 */
class Flow1D : public Domain1D
{
public:
    //! Create a new flow domain.
    //! @param phase  Solution object used to evaluate all thermodynamic, kinetic, and
    //!     transport properties
    //! @param id  name of flow domain
    //! @param points  initial number of grid points
    Flow1D(shared_ptr<Solution> phase, const string& id="", size_t points=1);
 
    ~Flow1D();
 
    string domainType() const override;
 
    //! @name Problem Specification
    //! @{
 
    void setupGrid(span<const double> z) override;
 
    void resetBadValues(span<double> x) override;
 
    //! Access the phase object used to compute thermodynamic properties for points in
    //! this domain.
    ThermoPhase& phase() {
        return *m_thermo;
    }
 
    //! Access the Kinetics object used to compute reaction rates for points in this
    //! domain.
    Kinetics& kinetics() {
        return *m_kin;
    }
 
protected:
    void _setKinetics(shared_ptr<Kinetics> kin) override;
    void _setTransport(shared_ptr<Transport> trans) override;
 
public:
    //! Set transport model by name.
    //! @param model  String specifying model name.
    //! @since New in %Cantera 3.0.
    void setTransportModel(const string& model) override;
 
    //! Retrieve transport model
    //! @since New in %Cantera 3.0.
    string transportModel() const;
 
    //! Enable thermal diffusion, also known as Soret diffusion.
    //! Requires that multicomponent transport properties be
    //! enabled to carry out calculations.
    void enableSoret(bool withSoret) {
        m_do_soret = withSoret;
    }
 
    //! Indicates if thermal diffusion (Soret effect) term is being calculated.
    bool withSoret() const {
        return m_do_soret;
    }
 
    //! Compute species diffusive fluxes with respect to
    //! their mass fraction gradients (fluxGradientBasis = ThermoBasis::mass)
    //! or mole fraction gradients (fluxGradientBasis = ThermoBasis::molar, default)
    //! when using the mixture-averaged transport model.
    //! @param fluxGradientBasis  set flux computation to mass or mole basis
    //! @since New in %Cantera 3.1.
    void setFluxGradientBasis(ThermoBasis fluxGradientBasis);
 
    //! Compute species diffusive fluxes with respect to
    //! their mass fraction gradients (fluxGradientBasis = ThermoBasis::mass)
    //! or mole fraction gradients (fluxGradientBasis = ThermoBasis::molar, default)
    //! when using the mixture-averaged transport model.
    //! @return the basis used for flux computation (mass or mole fraction gradients)
    //! @since New in %Cantera 3.1.
    ThermoBasis fluxGradientBasis() const {
        return m_fluxGradientBasis;
    }
 
    //! Set the pressure. Since the flow equations are for the limit of small
    //! Mach number, the pressure is very nearly constant throughout the flow.
    void setPressure(double p) {
        m_press = p;
    }
 
    //! The current pressure [Pa].
    double pressure() const {
        return m_press;
    }
 
    //! Write the initial solution estimate into array x.
    void _getInitialSoln(span<double> x) override;
 
    void _finalize(span<const double> x) override;
 
    /**
     * Set fixed temperature profile.
     * Sometimes it is desired to carry out the simulation using a specified
     * temperature profile, rather than computing it by solving the energy
     * equation.
     * @param zfixed  Vector containing locations where profile is specified.
     * @param tfixed  Vector containing specified temperatures.
     */
    void setFixedTempProfile(span<const double> zfixed, span<const double> tfixed) {
        m_zfix.assign(zfixed.begin(), zfixed.end());
        m_tfix.assign(tfixed.begin(), tfixed.end());
    }
 
    /**
     * Set the temperature fixed point at grid point j, and disable the energy
     * equation so that the solution will be held to this value.
     */
    void setTemperature(size_t j, double t) {
        m_fixedtemp[j] = t;
        m_do_energy[j] = false;
    }
 
    //! The fixed temperature value at point j.
    double T_fixed(size_t j) const {
        return m_fixedtemp[j];
    }
 
    //! @}
 
    string componentName(size_t n) const override;
 
    size_t componentIndex(const string& name, bool checkAlias=true) const override;
 
    bool hasComponent(const string& name, bool checkAlias=true) const override;
 
    //! Returns true if the specified component is an active part of the solver state
    virtual bool componentActive(size_t n) const;
 
    void updateState(size_t loc) override;
    void show(span<const double> x) override;
 
    void getValues(const string& component, span<double> values) const override;
    void setValues(const string& component, span<const double> values) override;
    void getResiduals(const string& component, span<double> values) const override;
    void setProfile(const string& component, span<const double> pos,
                    span<const double> values) override;
    void setFlatProfile(const string& component, double value) override;
 
    shared_ptr<SolutionArray> toArray(bool normalize=false) override;
    void fromArray(const shared_ptr<SolutionArray>& arr) override;
 
    //! Set flow configuration for freely-propagating flames, using an internal point
    //! with a fixed temperature as the condition to determine the inlet mass flux.
    void setFreeFlow() {
        m_dovisc = false;
        m_isFree = true;
        m_usesLambda = false;
    }
 
    //! Set flow configuration for axisymmetric counterflow flames, using specified
    //! inlet mass fluxes.
    void setAxisymmetricFlow() {
        m_dovisc = true;
        m_isFree = false;
        m_usesLambda = true;
    }
 
    //! Set flow configuration for burner-stabilized flames, using specified inlet mass
    //! fluxes.
    void setUnstrainedFlow() {
        m_dovisc = false;
        m_isFree = false;
        m_usesLambda = false;
    }
 
    //! Specify that the energy equation should be solved at point `j`.
    //! The converse of this method is fixTemperature().
    //! @param j  Point at which to enable the energy equation. `npos` means all points.
    void solveEnergyEqn(size_t j=npos);
 
    /**
     * Check if energy is enabled for entire domain.
     * @todo Should be simplified by removing the ability of solving the energy equation
     *      at some arbitrary subset of grid points while holding it fixed at others.
     * @since New in %Cantera 3.2
     */
    bool allOfEnergyEnabled() {
        return std::all_of(m_do_energy.begin(), m_do_energy.end(),
                           [](bool v) { return v; });
    }
 
    /**
     * Check if energy is disabled for entire domain.
     * @todo Should be simplified by removing the ability of solving the energy equation
     *      at some arbitrary subset of grid points while holding it fixed at others.
     * @since New in %Cantera 3.2
     */
    bool noneOfEnergyEnabled() {
        return std::none_of(m_do_energy.begin(), m_do_energy.end(),
                            [](bool v) { return v; });
    }
 
    /**
     * Set energy enabled flag for entire domain.
     * @since New in %Cantera 3.2
     */
    void setEnergyEnabled(bool flag) {
        if (flag) {
            solveEnergyEqn();
        } else {
            fixTemperature();
        }
    }
 
    //! Set to solve electric field in a point (used by IonFlow specialization)
    virtual void solveElectricField();
 
    //! Set to fix voltage in a point (used by IonFlow specialization)
    virtual void fixElectricField();
 
    //! Retrieve flag indicating whether electric field is solved or not (used by
    //! IonFlow specialization)
    virtual bool doElectricField() const;
 
    //! Turn radiation on / off.
    void enableRadiation(bool doRadiation) {
        m_do_radiation = doRadiation;
    }
 
    //! Returns `true` if the radiation term in the energy equation is enabled
    bool radiationEnabled() const {
        return m_do_radiation;
    }
 
    //! Return radiative heat loss at grid point j
    double radiativeHeatLoss(size_t j) const {
        return m_qdotRadiation[j];
    }
 
    //! Set the emissivities for the boundary values
    /*!
     * Reads the emissivities for the left and right boundary values in the
     * radiative term and writes them into the variables, which are used for the
     * calculation.
     */
    void setBoundaryEmissivities(double e_left, double e_right);
 
    //! Return emissivity at left boundary
    double leftEmissivity() const {
        return m_epsilon_left;
    }
 
    //! Return emissivity at right boundary
    double rightEmissivity() const {
        return m_epsilon_right;
    }
 
    //! Specify that the the temperature should be held fixed at point `j`.
    //! The converse of this method is enableEnergyEqn().
    //! @param j  Point at which to specify a fixed temperature. `npos` means all
    //!           points.
    void fixTemperature(size_t j=npos);
 
    /**
     * @name Two-Point control method
     *
     * In this method two control points are designated in the 1D domain, and the value
     * of the temperature at these points is fixed. The values of the control points are
     * imposed and thus serve as a boundary condition that affects the solution of the
     * governing equations in the 1D domain. The imposition of fixed points in the
     * domain means that the original set of governing equations' boundary conditions
     * would over-determine the problem. Thus, the boundary conditions are changed to
     * reflect the fact that the control points are serving as internal boundary
     * conditions.
     *
     * The imposition of the two internal boundary conditions requires that two other
     * boundary conditions be changed. The first is the boundary condition for the
     * continuity equation at the left boundary, which is changed to be a value that is
     * derived from the solution at the left boundary. The second is the continuity
     * boundary condition at the right boundary, which is also determined from the flow
     * solution by using the oxidizer axial velocity equation variable to compute the
     * mass flux at the right boundary.
     *
     * This method is based on the work of Nishioka et al. @cite nishioka1996 .
     */
    //! @{
 
    //! Returns the temperature at the left control point
    double leftControlPointTemperature() const;
 
    //! Returns the z-coordinate of the left control point
    double leftControlPointCoordinate() const;
 
    //! Sets the temperature of the left control point
    void setLeftControlPointTemperature(double temperature);
 
    //! Sets the coordinate of the left control point
    void setLeftControlPointCoordinate(double z_left);
 
    //! Returns the temperature at the right control point
    double rightControlPointTemperature() const;
 
    //! Returns the z-coordinate of the right control point
    double rightControlPointCoordinate() const;
 
    //! Sets the temperature of the right control point
    void setRightControlPointTemperature(double temperature);
 
    //! Sets the coordinate of the right control point
    void setRightControlPointCoordinate(double z_right);
 
    //! Sets the status of the two-point control
    void enableTwoPointControl(bool twoPointControl);
 
    //! Returns the status of the two-point control
    bool twoPointControlEnabled() const {
        return m_twoPointControl;
    }
    //! @}
 
    //! `true` if the energy equation is solved at point `j` or `false` if a fixed
    //! temperature condition is imposed.
    bool doEnergy(size_t j) {
        return m_do_energy[j];
    }
 
    //! Change the grid size. Called after grid refinement.
    void resize(size_t components, size_t points) override;
 
    //! Set the gas object state to be consistent with the solution at point j.
    void setGas(span<const double> x, size_t j);
 
    //! Set the gas state to be consistent with the solution at the midpoint
    //! between j and j + 1.
    void setGasAtMidpoint(span<const double> x, size_t j);
 
    //! Get the density [kg/m³] at point `j`
    double density(size_t j) const {
        return m_rho[j];
    }
 
    /**
     * Retrieve flag indicating whether flow is freely propagating.
     * The flow is unstrained and the axial mass flow rate is not specified.
     * For free flame propagation, the axial velocity is determined by the solver.
     * @since New in %Cantera 3.0
     */
    bool isFree() const {
        return m_isFree;
    }
 
    /**
     * Retrieve flag indicating whether flow uses radial momentum.
     * If `true`, radial momentum equation for @f$ V @f$ as well as
     * @f$ d\Lambda/dz = 0 @f$ are solved; if `false`, @f$ \Lambda(z) = 0 @f$ and
     * @f$ V(z) = 0 @f$ by definition.
     * @since New in %Cantera 3.0
     */
    bool isStrained() const {
        return m_usesLambda;
    }
 
    //! Specify if the viscosity term should be included in the momentum equation
    void setViscosityFlag(bool dovisc) {
        m_dovisc = dovisc;
    }
 
    /**
     * Evaluate the residual functions for axisymmetric stagnation flow.
     * If jGlobal == npos, the residual function is evaluated at all grid points.
     * Otherwise, the residual function is only evaluated at grid points j-1, j,
     * and j+1. This option is used to efficiently evaluate the Jacobian numerically.
     *
     * These residuals at all the boundary grid points are evaluated using a default
     * boundary condition that may be modified by a boundary object that is attached
     * to the domain. The boundary object connected will modify these equations by
     * subtracting the boundary object's values for V, T, mdot, etc. As a result,
     * these residual equations will force the solution variables to the values of
     * the connected boundary object.
     *
     *  @param jGlobal  Global grid point at which to update the residual
     *  @param[in] xGlobal  Global state vector
     *  @param[out] rsdGlobal  Global residual vector
     *  @param[out] diagGlobal  Global boolean mask indicating whether each solution
     *      component has a time derivative (1) or not (0).
     *  @param[in] rdt  Reciprocal of the timestep (`rdt=0` implies steady-state.)
     */
    void eval(size_t jGlobal, span<const double> xGlobal, span<double> rsdGlobal,
              span<int> diagGlobal, double rdt) override;
 
    //! Index of the species on the left boundary with the largest mass fraction
    size_t leftExcessSpecies() const {
        return m_kExcessLeft;
    }
 
    //! Index of the species on the right boundary with the largest mass fraction
    size_t rightExcessSpecies() const {
        return m_kExcessRight;
    }
 
protected:
    AnyMap getMeta() const override;
    void setMeta(const AnyMap& state) override;
 
    //! @name Updates of cached properties
    //! These methods are called by eval() to update cached properties and data that are
    //! used for the evaluation of the governing equations.
    //! @{
 
    /**
     * Update the thermodynamic properties from point j0 to point j1
     * (inclusive), based on solution x.
     *
     * The gas state is set to be consistent with the solution at the
     * points from j0 to j1.
     *
     * Properties that are computed and cached are:
     * * #m_rho (density)
     * * #m_wtm (mean molecular weight)
     * * #m_cp (specific heat capacity)
     * * #m_hk (species specific enthalpies)
     * * #m_wdot (species production rates)
     */
    void updateThermo(span<const double> x, size_t j0, size_t j1) {
        for (size_t j = j0; j <= j1; j++) {
            setGas(x,j);
            m_rho[j] = m_thermo->density();
            m_wtm[j] = m_thermo->meanMolecularWeight();
            m_cp[j] = m_thermo->cp_mass();
            m_thermo->getPartialMolarEnthalpies(span<double>(&m_hk(0, j), m_nsp));
            m_kin->getNetProductionRates(span<double>(&m_wdot(0, j), m_nsp));
        }
    }
 
    /**
     * Update the transport properties at grid points in the range from `j0`
     * to `j1`, based on solution `x`. Evaluates the solution at the midpoint
     * between `j` and `j + 1` to compute the transport properties. For example,
     * the viscosity at element `j`  is the viscosity evaluated at the midpoint
     * between `j` and `j + 1`.
     */
    virtual void updateTransport(span<const double> x, size_t j0, size_t j1);
 
    //! Update the diffusive mass fluxes.
    virtual void updateDiffFluxes(span<const double> x, size_t j0, size_t j1);
 
    //! Update the properties (thermo, transport, and diffusion flux).
    //! This function is called in eval after the points which need
    //! to be updated are defined.
    virtual void updateProperties(size_t jg, span<const double> x,
                                  size_t jmin, size_t jmax);
 
    /**
     * Computes the radiative heat loss vector over points jmin to jmax and stores
     * the data in the qdotRadiation variable.
     *
     * The simple radiation model used was established by Liu and Rogg
     * @cite liu1991. This model considers the radiation of CO2 and H2O.
     *
     * This model uses the optically thin limit and the gray-gas approximation to
     * simply calculate a volume specified heat flux out of the Planck absorption
     * coefficients, the boundary emissivities and the temperature. Polynomial lines
     * calculate the species Planck coefficients for H2O and CO2. The data for the
     * lines are taken from the RADCAL program @cite RADCAL.
     * The coefficients for the polynomials are taken from
     * [TNF Workshop](https://tnfworkshop.org/radiation/) material.
     */
    void computeRadiation(span<const double> x, size_t jmin, size_t jmax);
 
    //! @}
 
    //! @name Governing Equations
    //! Methods called by eval() to calculate residuals for individual governing
    //! equations.
    //! @{
 
    /**
     * Evaluate the continuity equation residual.
     *
     * @f[
     *     \frac{d(\rho u)}{dz} + 2\rho V = 0
     * @f]
     *
     * Axisymmetric flame:
     *  The continuity equation propagates information from right-to-left.
     *  The @f$ \rho u @f$ at point 0 is dependent on @f$ \rho u @f$ at point 1,
     *  but not on @f$ \dot{m} @f$ from the inlet.
     *
     * Freely-propagating flame:
     *  The continuity equation propagates information away from a fixed temperature
     *  point that is set in the domain.
     *
     * Unstrained flame:
     *  A specified mass flux; the main example being burner-stabilized flames.
     *
     * The default boundary condition for the continuity equation is
     * (@f$ u = 0 @f$) at the right boundary. Because the equation is a first order
     * equation, only one boundary condition is needed.
     *
     * @param[in] x  Local domain state vector, includes variables like temperature,
     *               density, etc.
     * @param[out] rsd  Local domain residual vector that stores the continuity
     *                  equation residuals.
     * @param[out] diag  Local domain diagonal matrix that controls whether an entry
     *                   has a time-derivative (used by the solver).
     * @param[in] rdt  Reciprocal of the timestep.
     * @param[in] jmin  The index for the starting point in the local domain grid.
     * @param[in] jmax  The index for the ending point in the local domain grid.
     */
    virtual void evalContinuity(span<const double> x, span<double> rsd, span<int> diag,
                                double rdt, size_t jmin, size_t jmax);
 
    /**
     * Evaluate the momentum equation residual.
     *
     * @f[
     *    \rho u \frac{dV}{dz} + \rho V^2 =
     *    \frac{d}{dz}\left( \mu \frac{dV}{dz} \right) - \Lambda
     * @f]
     *
     * The radial momentum equation is used for axisymmetric flows, and incorporates
     * terms for time and spatial variations of radial velocity (@f$ V @f$). The
     * default boundary condition is zero radial velocity (@f$ V @f$) at the left
     * and right boundary.
     *
     * For argument explanation, see evalContinuity().
     */
    virtual void evalMomentum(span<const double> x, span<double> rsd, span<int> diag,
                              double rdt, size_t jmin, size_t jmax);
 
    /**
     * Evaluate the radial pressure gradient equation residual.
     *
     * @f[
     *    \frac{d\Lambda}{dz} = 0
     * @f]
     *
     * The radial pressure gradient @f$ \Lambda @f$ serves as an eigenvalue that allows
     * the momentum and continuity equations to be simultaneously satisfied in
     * axisymmetric flows. This equation propagates information from
     * left-to-right. The default boundary condition is @f$ \Lambda = 0 @f$
     * at the left boundary. The equation is first order and so only one
     * boundary condition is needed.
     *
     * For argument explanation, see evalContinuity().
     */
    virtual void evalLambda(span<const double> x, span<double> rsd, span<int> diag,
                            double rdt, size_t jmin, size_t jmax);
 
    /**
     * Evaluate the energy equation residual.
     *
     * @f[
     *   \rho c_p u \frac{dT}{dz} =
     *   \frac{d}{dz}\left( \Lambda \frac{dT}{dz} \right)
     *   - \sum_k h_kW_k\dot{\omega}_k
     *   - \sum_k  j_k \frac{dh_k}{dz}
     * @f]
     *
     * The energy equation includes contributions from
     * chemical reactions and diffusion. Default is zero temperature (@f$ T @f$)
     * at the left and right boundaries. These boundary values are updated by the
     * specific boundary object connected to the domain.
     *
     * For argument explanation, see evalContinuity().
     */
    virtual void evalEnergy(span<const double> x, span<double> rsd, span<int> diag,
                            double rdt, size_t jmin, size_t jmax);
 
    /**
     * Evaluate the species equations' residuals.
     *
     * @f[
     *    \rho u \frac{dY_k}{dz} + \frac{dj_k}{dz} = W_k\dot{\omega}_k
     * @f]
     *
     * The species equations include terms for temporal and spatial variations
     * of species mass fractions (@f$ Y_k @f$). The default boundary condition is zero
     * flux for species at the left and right boundary.
     *
     * For argument explanation, see evalContinuity().
     */
    virtual void evalSpecies(span<const double> x, span<double> rsd, span<int> diag,
                             double rdt, size_t jmin, size_t jmax);
 
    /**
     * Evaluate the electric field equation residual to be zero everywhere.
     *
     * The electric field equation is implemented in the IonFlow class. The default
     * boundary condition is zero electric field (@f$ E @f$) at the boundary,
     * and @f$ E @f$ is zero within the domain.
     *
     * For argument explanation, see evalContinuity().
     */
    virtual void evalElectricField(span<const double> x, span<double> rsd,
                                   span<int> diag, double rdt, size_t jmin,
                                   size_t jmax);
 
    //! @} End of Governing Equations
 
    /**
     * Evaluate the oxidizer axial velocity equation residual.
     *
     * The function calculates the oxidizer axial velocity equation as
     * @f[
     *    \frac{dU_{o}}{dz} = 0
     * @f]
     *
     * This equation serves as a dummy equation that is used only in the context of
     * two-point flame control, and serves as the way for two interior control points to
     * be specified while maintaining block tridiagonal structure. The default boundary
     * condition is @f$ U_o = 0 @f$ at the right and zero flux at the left boundary.
     *
     * For argument explanation, see evalContinuity().
     */
    virtual void evalUo(span<const double> x, span<double> rsd, span<int> diag,
                        double rdt, size_t jmin, size_t jmax);
 
    //! @name Solution components
    //! @{
 
    //! Get the temperature at point `j` from the local state vector `x`.
    double T(span<const double> x, size_t j) const {
        return x[index(c_offset_T, j)];
    }
    //! Get the temperature at point `j` from the local state vector `x`.
    double& T(span<double> x, size_t j) {
        return x[index(c_offset_T, j)];
    }
 
    //! Get the temperature at point `j` from the previous time step.
    double T_prev(size_t j) const {
        return prevSoln(c_offset_T, j);
    }
 
    //! Get the axial mass flux [kg/m²/s] at point `j` from the local state vector `x`.
    double rho_u(span<const double> x, size_t j) const {
        return m_rho[j]*x[index(c_offset_U, j)];
    }
 
    //! Get the axial velocity [m/s] at point `j` from the local state vector `x`.
    double u(span<const double> x, size_t j) const {
        return x[index(c_offset_U, j)];
    }
 
    //! Get the spread rate (tangential velocity gradient) [1/s] at point `j` from the
    //! local state vector `x`.
    double V(span<const double> x, size_t j) const {
        return x[index(c_offset_V, j)];
    }
 
    //! Get the spread rate [1/s] at point `j` from the previous time step.
    double V_prev(size_t j) const {
        return prevSoln(c_offset_V, j);
    }
 
    //! Get the radial pressure gradient [N/m⁴] at point `j` from the local state vector
    //! `x`
    double Lambda(span<const double> x, size_t j) const {
        return x[index(c_offset_L, j)];
    }
 
    //! Get the oxidizer inlet velocity [m/s] linked to point `j` from the local state
    //! vector `x`.
    //!
    //! @see evalUo()
    double Uo(span<const double> x, size_t j) const {
        return x[index(c_offset_Uo, j)];
    }
 
    //! Get the mass fraction of species `k` at point `j` from the local state vector
    //! `x`.
    double Y(span<const double> x, size_t k, size_t j) const {
        return x[index(c_offset_Y + k, j)];
    }
 
    //! Get the mass fraction of species `k` at point `j` from the local state vector
    //! `x`.
    double& Y(span<double> x, size_t k, size_t j) {
        return x[index(c_offset_Y + k, j)];
    }
 
    //! Get the array of mass fractions at point `j` from the local state vector `x`.
    span<double> Y(span<double> x, size_t j) {
        return span<double>(&x[index(c_offset_Y, j)], m_nsp);
    }
    span<const double> Y(span<const double> x, size_t j) {
        return span<const double>(&x[index(c_offset_Y, j)], m_nsp);
    }
 
    //! Get the mass fraction of species `k` at point `j` from the previous time step.
    double Y_prev(size_t k, size_t j) const {
        return prevSoln(c_offset_Y + k, j);
    }
 
    //! Get the mole fraction of species `k` at point `j` from the local state vector
    //! `x`.
    double X(span<const double> x, size_t k, size_t j) const {
        return m_wtm[j]*Y(x,k,j)/m_wt[k];
    }
 
    //! Get the diffusive mass flux [kg/m²/s] of species `k` at point `j`
    double flux(size_t k, size_t j) const {
        return m_flux(k, j);
    }
    //! @}
 
    //! @name Convective spatial derivatives
    //!
    //! These methods use upwind differencing to calculate spatial derivatives
    //! for velocity, species mass fractions, and temperature. Upwind differencing
    //! is a numerical discretization method that considers the direction of the
    //! flow to improve stability.
    //! @{
 
    /**
     * Calculates the spatial derivative of velocity V with respect to z at point j
     * using upwind differencing.
     *
     * For more details on the upwinding scheme, see the
     * [science reference documentation](../reference/onedim/discretization.html#upwinding).
     *
     * @f[
     *   \frac{\partial V}{\partial z} \bigg|_{j} \approx \frac{V_{\ell} -
     *     V_{\ell-1}}{z_{\ell} - z_{\ell-1}}
     * @f]
     *
     * Where the value of @f$ \ell @f$ is determined by the sign of the axial velocity.
     * If the axial velocity is positive, the value of @f$ \ell @f$ is j. If the axial
     * velocity is negative, the value of @f$ \ell @f$ is j + 1. A positive velocity
     * means that the flow is moving left-to-right.
     *
     * @param[in] x  The local domain state vector.
     * @param[in] j  The grid point index at which the derivative is computed.
     */
    double dVdz(span<const double> x, size_t j) const {
        size_t jloc = (u(x, j) > 0.0 ? j : j + 1);
        return (V(x, jloc) - V(x, jloc-1))/m_dz[jloc-1];
    }
 
    /**
     * Calculates the spatial derivative of the species mass fraction @f$ Y_k @f$ with
     * respect to z for species k at point j using upwind differencing.
     *
     * For details on the upwinding scheme, see dVdz().
     *
     * @param[in] x  The local domain state vector.
     * @param[in] k  The species index.
     * @param[in] j  The grid point index at which the derivative is computed.
     */
    double dYdz(span<const double> x, size_t k, size_t j) const {
        size_t jloc = (u(x, j) > 0.0 ? j : j + 1);
        return (Y(x, k, jloc) - Y(x, k, jloc-1))/m_dz[jloc-1];
    }
 
    /**
     * Calculates the spatial derivative of temperature T with respect to z at point
     * j using upwind differencing.
     *
     * For details on the upwinding scheme, see dVdz().
     *
     * @param[in] x  The local domain state vector.
     * @param[in] j  The grid point index at which the derivative is computed.
     */
    double dTdz(span<const double> x, size_t j) const {
        size_t jloc = (u(x, j) > 0.0 ? j : j + 1);
        return (T(x, jloc) - T(x, jloc-1))/m_dz[jloc-1];
    }
    //! @}
 
    /**
     * Compute the shear term from the momentum equation using a central
     * three-point differencing scheme.
     *
     * The term to be discretized is:
     * @f[
     *   \frac{d}{dz}\left(\mu \frac{dV}{dz}\right)
     * @f]
     *
     * For more details on the discretization scheme used for the second derivative,
     * see the
     * [documentation](../reference/onedim/discretization.html#second-derivative-term).
     *
     * @f[
     *  \frac{d}{dz}\left(\mu \frac{dV}{dz}\right) \approx
     *   \frac{\mu_{j+1/2} \frac{V_{j+1} - V_j}{z_{j+1} - z_j} -
     *   \mu_{j-1/2} \frac{V_j - V_{j-1}}{z_j - z_{j-1}}}{\frac{z_{j+1} - z_{j-1}}{2}}
     * @f]
     *
     * @param[in] x  The local domain state vector.
     * @param[in] j  The grid point index at which the derivative is computed.
     */
    double shear(span<const double> x, size_t j) const {
        double A_left = m_visc[j-1]*(V(x, j) - V(x, j-1)) / (z(j) - z(j-1));
        double A_right = m_visc[j]*(V(x, j+1) - V(x, j)) / (z(j+1) - z(j));
        return 2.0*(A_right - A_left) / (z(j+1) - z(j-1));
    }
 
    /**
     * Compute the conduction term from the energy equation using a central
     * three-point differencing scheme.
     *
     * For the details about the discretization, see shear().
     *
     * @param[in] x  The local domain state vector.
     * @param[in] j  The grid point index at which the derivative is computed.
     */
    double conduction(span<const double> x, size_t j) const {
        double A_left = m_tcon[j-1]*(T(x, j) - T(x, j-1)) / (z(j) - z(j-1));
        double A_right = m_tcon[j]*(T(x, j+1) - T(x, j)) / (z(j+1) - z(j));
        return -2.0*(A_right - A_left) / (z(j+1) - z(j-1));
    }
 
    /**
     * Array access mapping for a 3D array stored in a 1D vector. Used for
     * accessing data in the #m_multidiff member variable.
     *
     * @param[in] k  First species index.
     * @param[in] j  The grid point index.
     * @param[in] m  The second species index.
     */
    size_t mindex(size_t k, size_t j, size_t m) {
        return m*m_nsp*m_nsp + m_nsp*j + k;
    }
 
    /**
     * Compute the spatial derivative of species specific molar enthalpies using upwind
     * differencing. Updates all species molar enthalpies for all species at point j.
     * Updates the #m_dhk_dz 2D array.
     *
     * For details on the upwinding scheme, see dVdz().
     *
     * @param[in] x  The local domain state vector.
     * @param[in] j  The index at which the derivative is computed.
     */
    virtual void grad_hk(span<const double> x, size_t j);
 
    //---------------------------------------------------------
    //             member data
    //---------------------------------------------------------
 
    //! Grid spacing. Element `j` holds the value of `z(j+1) - z(j)`.
    vector<double> m_dz;
 
    // mixture thermo properties
    vector<double> m_rho; //!< Density at each grid point
    vector<double> m_wtm; //!< Mean molecular weight at each grid point
    vector<double> m_wt; //!< Molecular weight of each species
    vector<double> m_cp; //!< Specific heat capacity at each grid point
 
    // transport properties
    vector<double> m_visc; //!< Dynamic viscosity at each grid point [Pa∙s]
    vector<double> m_tcon; //!< Thermal conductivity at each grid point [W/m/K]
 
    //! Coefficient used in diffusion calculations for each species at each grid point.
    //!
    //! The value stored is different depending on the transport model (multicomponent
    //! versus mixture averaged) and flux gradient basis (mass or molar). Vector size is
    //! #m_nsp × #m_points, where `m_diff[k + j*m_nsp]` contains the value for species
    //! `k` at point `j`.
    vector<double> m_diff;
 
    //! Vector of size #m_nsp × #m_nsp × #m_points for saving multicomponent
    //! diffusion coefficients. Order of elements is defined by mindex().
    vector<double> m_multidiff;
 
    //! Array of size #m_nsp by #m_points for saving thermal diffusion coefficients
    Array2D m_dthermal;
 
    //! Array of size #m_nsp by #m_points for saving diffusive mass fluxes
    Array2D m_flux;
 
    //! Array of size #m_nsp by #m_points for saving molar enthalpies
    Array2D m_hk;
 
    //! Array of size #m_nsp by #m_points-1 for saving enthalpy fluxes
    Array2D m_dhk_dz;
 
    //! Array of size #m_nsp by #m_points for saving species production rates
    Array2D m_wdot;
 
    size_t m_nsp; //!< Number of species in the mechanism
 
    //! Phase object used for calculating thermodynamic properties
    ThermoPhase* m_thermo = nullptr;
 
    //! Kinetics object used for calculating species production rates
    Kinetics* m_kin = nullptr;
 
    //! Transport object used for calculating transport properties
    Transport* m_trans = nullptr;
 
    //! Emissivity of the surface to the left of the domain. Used for calculating
    //! radiative heat loss.
    double m_epsilon_left = 0.0;
 
    //! Emissivity of the surface to the right of the domain. Used for calculating
    //! radiative heat loss.
    double m_epsilon_right = 0.0;
 
    //! Indices within the ThermoPhase of the radiating species. First index is
    //! for CO2, second is for H2O.
    vector<size_t> m_kRadiating;
 
    //! @name flags
    //! @{
 
    //! For each point in the domain, `true` if energy equation is solved or `false` if
    //! temperature is held constant.
    //! @see doEnergy, fixTemperature
    vector<bool> m_do_energy;
 
    //! `true` if the Soret diffusion term should be calculated.
    bool m_do_soret = false;
 
    //! Determines whether diffusive fluxes are computed using gradients of mass
    //! fraction or mole fraction.
    //! @see setFluxGradientBasis, fluxGradientBasis
    ThermoBasis m_fluxGradientBasis = ThermoBasis::molar;
 
    //! `true` if transport fluxes are computed using the multicomponent diffusion
    //! coefficients, or `false` if mixture-averaged diffusion coefficients are used.
    bool m_do_multicomponent = false;
 
    //! Determines whether radiative heat loss is calculated.
    //! @see enableRadiation, radiationEnabled, computeRadiation
    bool m_do_radiation = false;
 
    //! Determines whether the viscosity term in the momentum equation is calculated
    //! @see setViscosityFlag, setFreeFlow, setAxisymmetricFlow, setUnstrainedFlow,
    //!      updateTransport, shear
    bool m_dovisc;
 
    //! Flag that is `true` for freely propagating flames anchored by a temperature
    //! fixed point.
    //! @see setFreeFlow, setAxisymmetricFlow, setUnstrainedFlow
    bool m_isFree;
 
    //! Flag that is `true` for counterflow configurations that use the pressure
    //! eigenvalue @f$ \Lambda @f$ in the radial momentum equation.
    //! @see setFreeFlow, setAxisymmetricFlow, setUnstrainedFlow
    bool m_usesLambda;
 
    //! Flag for activating two-point flame control
    bool m_twoPointControl = false;
    //! @}
 
    //! radiative heat loss vector
    vector<double> m_qdotRadiation;
 
    // fixed T and Y values
    //! Fixed values of the temperature at each grid point that are used when solving
    //! with the energy equation disabled.
    //!
    //! Values are interpolated from profiles specified with the setFixedTempProfile
    //! method as part of _finalize().
    vector<double> m_fixedtemp;
 
    //! Relative coordinates used to specify a fixed temperature profile.
    //!
    //! 0 corresponds to the left edge of the domain and 1 corresponds to the right edge
    //! of the domain. Length is the same as the #m_tfix array.
    //! @see setFixedTempProfile, _finalize
    vector<double> m_zfix;
 
    //! Fixed temperature values at the relative coordinates specified in #m_zfix.
    //! @see setFixedTempProfile, _finalize
    vector<double> m_tfix;
 
    //! Index of species with a large mass fraction at the left boundary, for which the
    //! mass fraction may be calculated as 1 minus the sum of the other mass fractions
    size_t m_kExcessLeft = 0;
 
    //! Index of species with a large mass fraction at the right boundary, for which the
    //! mass fraction may be calculated as 1 minus the sum of the other mass fractions
    size_t m_kExcessRight = 0;
 
    //! Location of the left control point when two-point control is enabled
    double m_zLeft = Undef;
 
    //! Temperature of the left control point when two-point control is enabled
    double m_tLeft = Undef;
 
    //! Location of the right control point when two-point control is enabled
    double m_zRight = Undef;
 
    //! Temperature of the right control point when two-point control is enabled
    double m_tRight = Undef;
 
public:
    //! Location of the point where temperature is fixed
    double m_zfixed = Undef;
 
    //! Temperature at the point used to fix the flame location
    double m_tfixed = -1.0;
 
private:
    //! Holds the average of the species mass fractions between grid points j and j+1.
    //! Used when building a gas state at the grid midpoints for evaluating transport
    //! properties at the midpoints.
    vector<double> m_ybar;
};
 
}
 
#endif