staggeredupwindfluxvariables.hh

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#ifndef DUMUX_NAVIERSTOKES_STAGGERED_UPWINDVARIABLES_HH
#define DUMUX_NAVIERSTOKES_STAGGERED_UPWINDVARIABLES_HH
 
#include <array>
#include <optional>
 
#include <dumux/common/math.hh>
#include <dumux/common/exceptions.hh>
#include <dumux/common/parameters.hh>
#include <dumux/common/properties.hh>
 
#include <dumux/discretization/method.hh>
#include <dumux/freeflow/staggeredupwindmethods.hh>
#include "velocitygradients.hh"
 
namespace Dumux {
 
template<class TypeTag, int upwindSchemeOrder>
class StaggeredUpwindFluxVariables
{
    using GridVariables = GetPropType<TypeTag, Properties::GridVariables>;
 
    using GridVolumeVariables = typename GridVariables::GridVolumeVariables;
    using ElementVolumeVariables = typename GridVolumeVariables::LocalView;
    using VolumeVariables = typename GridVolumeVariables::VolumeVariables;
 
    using GridFluxVariablesCache = typename GridVariables::GridFluxVariablesCache;
    using FluxVariablesCache = typename GridFluxVariablesCache::FluxVariablesCache;
 
    using GridFaceVariables = typename GridVariables::GridFaceVariables;
    using ElementFaceVariables = typename GridFaceVariables::LocalView;
    using FaceVariables = typename GridFaceVariables::FaceVariables;
 
    using Problem = GetPropType<TypeTag, Properties::Problem>;
    using GridGeometry = GetPropType<TypeTag, Properties::GridGeometry>;
    using FVElementGeometry = typename GridGeometry::LocalView;
    using GridView = typename GridGeometry::GridView;
    using Element = typename GridView::template Codim<0>::Entity;
    using Scalar = GetPropType<TypeTag, Properties::Scalar>;
    using ModelTraits = GetPropType<TypeTag, Properties::ModelTraits>;
    using Indices = typename ModelTraits::Indices;
    using SubControlVolume = typename FVElementGeometry::SubControlVolume;
    using SubControlVolumeFace = typename FVElementGeometry::SubControlVolumeFace;
    using CellCenterPrimaryVariables = GetPropType<TypeTag, Properties::CellCenterPrimaryVariables>;
    using FacePrimaryVariables = GetPropType<TypeTag, Properties::FacePrimaryVariables>;
    using BoundaryTypes = GetPropType<TypeTag, Properties::BoundaryTypes>;
    using VelocityGradients = StaggeredVelocityGradients<Scalar, GridGeometry, BoundaryTypes, Indices>;
 
    using GlobalPosition = typename Element::Geometry::GlobalCoordinate;
    static constexpr bool useHigherOrder = upwindSchemeOrder > 1;
 
public:
    static FacePrimaryVariables computeUpwindedFrontalMomentum(const SubControlVolumeFace& scvf,
                                                               const ElementFaceVariables& elemFaceVars,
                                                               const ElementVolumeVariables& elemVolVars,
                                                               const GridFluxVariablesCache& gridFluxVarsCache,
                                                               const Scalar transportingVelocity)
    {
        const bool canHigherOrder = canFrontalSecondOrder_(scvf, transportingVelocity, std::integral_constant<bool, useHigherOrder>{});
 
        if (canHigherOrder)
        {
            const auto upwindingMomenta = getFrontalUpwindingMomenta_(scvf, elemFaceVars, elemVolVars[scvf.insideScvIdx()].density(),
                                                                      transportingVelocity, std::integral_constant<bool, useHigherOrder>{});
            return doFrontalMomentumUpwinding_(scvf, upwindingMomenta, transportingVelocity, gridFluxVarsCache);
        }
        else
        {
            const auto upwindingMomenta = getFrontalUpwindingMomenta_(scvf, elemFaceVars, elemVolVars[scvf.insideScvIdx()].density(),
                                                                      transportingVelocity, std::integral_constant<bool, false>{});
            return doFrontalMomentumUpwinding_(scvf, upwindingMomenta, transportingVelocity, gridFluxVarsCache);
        }
    }
 
    static FacePrimaryVariables computeUpwindedLateralMomentum(const Problem& problem,
                                                               const FVElementGeometry& fvGeometry,
                                                               const Element& element,
                                                               const SubControlVolumeFace& scvf,
                                                               const ElementVolumeVariables& elemVolVars,
                                                               const FaceVariables& faceVars,
                                                               const GridFluxVariablesCache& gridFluxVarsCache,
                                                               const int localSubFaceIdx,
                                                               const std::optional<BoundaryTypes>& currentScvfBoundaryTypes,
                                                               const std::optional<BoundaryTypes>& lateralFaceBoundaryTypes)
    {
        const auto eIdx = scvf.insideScvIdx();
        const auto& lateralFace = fvGeometry.scvf(eIdx, scvf.pairData(localSubFaceIdx).localLateralFaceIdx);
 
        // Get the transporting velocity, located at the scvf perpendicular to the current scvf where the dof
        // of interest is located.
        const Scalar transportingVelocity = faceVars.velocityLateralInside(localSubFaceIdx);
 
        // Check whether the own or the neighboring element is upstream.
        const bool selfIsUpstream = ( lateralFace.directionSign() == sign(transportingVelocity) );
 
        const bool canHigherOrder = canLateralSecondOrder_(scvf, selfIsUpstream, localSubFaceIdx, std::integral_constant<bool, useHigherOrder>{});
 
        if (canHigherOrder)
        {
            const auto parallelUpwindingMomenta = getLateralUpwindingMomenta_(problem, fvGeometry, element, scvf, elemVolVars, faceVars,
                                                                              transportingVelocity, localSubFaceIdx, currentScvfBoundaryTypes,
                                                                              lateralFaceBoundaryTypes, std::integral_constant<bool, useHigherOrder>{});
            return doLateralMomentumUpwinding_(fvGeometry, scvf, parallelUpwindingMomenta, transportingVelocity, localSubFaceIdx, gridFluxVarsCache);
        }
        else
        {
            const auto parallelUpwindingMomenta = getLateralUpwindingMomenta_(problem, fvGeometry, element, scvf, elemVolVars, faceVars,
                                                                              transportingVelocity, localSubFaceIdx, currentScvfBoundaryTypes,
                                                                              lateralFaceBoundaryTypes, std::integral_constant<bool, false>{});
            return doLateralMomentumUpwinding_(fvGeometry, scvf, parallelUpwindingMomenta, transportingVelocity, localSubFaceIdx, gridFluxVarsCache);
        }
    }
 
private:
 
    static bool canFrontalSecondOrder_(const SubControlVolumeFace& ownScvf,
                                       const Scalar transportingVelocity,
                                       std::false_type)
    { return false; }
 
    static bool canFrontalSecondOrder_(const SubControlVolumeFace& ownScvf,
                                       const Scalar transportingVelocity,
                                       std::true_type)
    {
        // Depending on selfIsUpstream I have to check if I have a forward or a backward neighbor to retrieve
        const bool selfIsUpstream = ownScvf.directionSign() != sign(transportingVelocity);
        return selfIsUpstream ? ownScvf.hasForwardNeighbor(0) : ownScvf.hasBackwardNeighbor(0);
    }
 
    static std::array<Scalar, 2> getFrontalUpwindingMomenta_(const SubControlVolumeFace& scvf,
                                                             const ElementFaceVariables& elemFaceVars,
                                                             const Scalar density,
                                                             const Scalar transportingVelocity,
                                                             std::false_type)
    {
        const Scalar momentumSelf = elemFaceVars[scvf].velocitySelf() * density;
        const Scalar momentumOpposite = elemFaceVars[scvf].velocityOpposite() * density;
        const bool selfIsUpstream = scvf.directionSign() != sign(transportingVelocity);
 
        return selfIsUpstream ? std::array<Scalar, 2>{momentumOpposite, momentumSelf}
                              : std::array<Scalar, 2>{momentumSelf, momentumOpposite};
    }
 
    static std::array<Scalar, 3> getFrontalUpwindingMomenta_(const SubControlVolumeFace& scvf,
                                                             const ElementFaceVariables& elemFaceVars,
                                                             const Scalar density,
                                                             const Scalar transportingVelocity,
                                                             std::true_type)
    {
        const bool selfIsUpstream = scvf.directionSign() != sign(transportingVelocity);
        std::array<Scalar, 3> momenta;
 
        if (selfIsUpstream)
        {
            momenta[0] = elemFaceVars[scvf].velocityOpposite() * density;
            momenta[1] = elemFaceVars[scvf].velocitySelf() * density;
            momenta[2] = elemFaceVars[scvf].velocityForward(0) * density;
        }
        else
        {
            momenta[0] = elemFaceVars[scvf].velocitySelf() * density;
            momenta[1] = elemFaceVars[scvf].velocityOpposite() * density;
            momenta[2] = elemFaceVars[scvf].velocityBackward(0) * density;
        }
 
        return momenta;
    }
 
    static Scalar doFrontalMomentumUpwinding_(const SubControlVolumeFace& scvf,
                                              const std::array<Scalar, 2>& momenta,
                                              const Scalar transportingVelocity,
                                              const GridFluxVariablesCache& gridFluxVarsCache)
    {
        const auto& upwindScheme = gridFluxVarsCache.staggeredUpwindMethods();
        return upwindScheme.upwind(momenta[0], momenta[1]);
    }
 
    template<bool enable = useHigherOrder, std::enable_if_t<enable, int> = 0>
    static Scalar doFrontalMomentumUpwinding_(const SubControlVolumeFace& scvf,
                                              const std::array<Scalar, 3>& momenta,
                                              const Scalar transportingVelocity,
                                              const GridFluxVariablesCache& gridFluxVarsCache)
    {
        const auto& upwindScheme = gridFluxVarsCache.staggeredUpwindMethods();
        const bool selfIsUpstream = scvf.directionSign() != sign(transportingVelocity);
        std::array<Scalar,3> distances = getFrontalDistances_(scvf, selfIsUpstream);
        return upwindScheme.tvd(momenta, distances, selfIsUpstream, upwindScheme.tvdApproach());
    }
 
    template<bool enable = useHigherOrder, std::enable_if_t<enable, int> = 0>
    static std::array<Scalar, 3> getFrontalDistances_(const SubControlVolumeFace& ownScvf,
                                                      const bool selfIsUpstream)
    {
        // Depending on selfIsUpstream the downstream and the (up)upstream distances are saved.
        // distances {upstream to downstream distance, up-upstream to upstream distance, downstream staggered cell size}
        std::array<Scalar, 3> distances;
 
        if (selfIsUpstream)
        {
            distances[0] = ownScvf.selfToOppositeDistance();
            distances[1] = ownScvf.axisData().inAxisForwardDistances[0];
            distances[2] = 0.5 * (ownScvf.axisData().selfToOppositeDistance + ownScvf.axisData().inAxisBackwardDistances[0]);
        }
        else
        {
            distances[0] = ownScvf.selfToOppositeDistance();
            distances[1] = ownScvf.axisData().inAxisBackwardDistances[0];
            distances[2] = 0.5 * (ownScvf.axisData().selfToOppositeDistance + ownScvf.axisData().inAxisForwardDistances[0]);
        }
        return distances;
    }
 
    static bool canLateralSecondOrder_(const SubControlVolumeFace& ownScvf,
                                       const bool selfIsUpstream,
                                       const int localSubFaceIdx,
                                       std::true_type)
    {
        if (selfIsUpstream)
        {
            // The self velocity is upstream. The downstream velocity can be assigned or retrieved
            // from the boundary, even if there is no parallel neighbor.
            return true;
        }
        else
        {
            // The self velocity is downstream. If there is no parallel neighbor I cannot use a second order approximation.
            return ownScvf.hasParallelNeighbor(localSubFaceIdx, 0);
        }
    }
 
    static bool canLateralSecondOrder_(const SubControlVolumeFace& ownScvf,
                                       const bool selfIsUpstream,
                                       const int localSubFaceIdx,
                                       std::false_type)
    {   return false; }
 
 
    static std::array<Scalar, 3> getLateralUpwindingMomenta_(const Problem& problem,
                                                             const FVElementGeometry& fvGeometry,
                                                             const Element& element,
                                                             const SubControlVolumeFace& ownScvf,
                                                             const ElementVolumeVariables& elemVolVars,
                                                             const FaceVariables& faceVars,
                                                             const Scalar transportingVelocity,
                                                             const int localSubFaceIdx,
                                                             const std::optional<BoundaryTypes>& currentScvfBoundaryTypes,
                                                             const std::optional<BoundaryTypes>& lateralFaceBoundaryTypes,
                                                             std::true_type)
    {
        const SubControlVolumeFace& lateralFace = fvGeometry.scvf(ownScvf.insideScvIdx(), ownScvf.pairData(localSubFaceIdx).localLateralFaceIdx);
 
        // Get the volume variables of the own and the neighboring element
        const auto& insideVolVars = elemVolVars[lateralFace.insideScvIdx()];
        const auto& outsideVolVars = elemVolVars[lateralFace.outsideScvIdx()];
 
        // If the lateral face lies on a boundary, we assume that the parallel velocity on the boundary is actually known,
        // thus we always use this value for the computation of the transported momentum.
        if (!ownScvf.hasParallelNeighbor(localSubFaceIdx, 0))
        {
            const Scalar boundaryMomentum = getParallelVelocityFromBoundary_(problem, element, fvGeometry, ownScvf,
                                                                             faceVars, currentScvfBoundaryTypes, lateralFaceBoundaryTypes,
                                                                             localSubFaceIdx) * insideVolVars.density();
 
             return std::array<Scalar, 3>{boundaryMomentum, boundaryMomentum, boundaryMomentum};
        }
 
        // Check whether the own or the neighboring element is upstream.
        const bool selfIsUpstream = lateralFace.directionSign() == sign(transportingVelocity);
 
        std::array<Scalar, 3> momenta;
 
        if (selfIsUpstream)
        {
            momenta[1] = faceVars.velocitySelf() * insideVolVars.density();
 
            if (ownScvf.hasParallelNeighbor(localSubFaceIdx, 0))
                momenta[0] = faceVars.velocityParallel(localSubFaceIdx, 0) * insideVolVars.density();
            else
                momenta[0] = getParallelVelocityFromBoundary_(problem, element, fvGeometry, ownScvf,
                                                              faceVars, currentScvfBoundaryTypes, lateralFaceBoundaryTypes,
                                                              localSubFaceIdx) * insideVolVars.density();
 
            // The local index of the faces that is opposite to localSubFaceIdx
            const int oppositeSubFaceIdx = localSubFaceIdx % 2 ? localSubFaceIdx - 1 : localSubFaceIdx + 1;
 
            // The "upstream-upstream" velocity is retrieved from the other parallel neighbor or from the boundary.
            if (ownScvf.hasParallelNeighbor(oppositeSubFaceIdx, 0))
                momenta[2] = faceVars.velocityParallel(oppositeSubFaceIdx, 0) * insideVolVars.density();
            else
                momenta[2] = getParallelVelocityFromOppositeBoundary_(problem, element, fvGeometry, ownScvf,
                                                                      faceVars, currentScvfBoundaryTypes,
                                                                      oppositeSubFaceIdx) * insideVolVars.density();
        }
        else
        {
            momenta[0] = faceVars.velocitySelf() * outsideVolVars.density();
            momenta[1] = faceVars.velocityParallel(localSubFaceIdx, 0) * outsideVolVars.density();
 
            // If there is another parallel neighbor I can assign the "upstream-upstream" velocity, otherwise I retrieve it from the boundary.
            if (ownScvf.hasParallelNeighbor(localSubFaceIdx, 1))
                momenta[2] = faceVars.velocityParallel(localSubFaceIdx, 1) * outsideVolVars.density();
            else
            {
                const Element& elementParallel = fvGeometry.gridGeometry().element(lateralFace.outsideScvIdx());
                const SubControlVolumeFace& firstParallelScvf = fvGeometry.scvf(lateralFace.outsideScvIdx(), ownScvf.localFaceIdx());
 
                momenta[2] = getParallelVelocityFromOppositeBoundary_(problem, elementParallel, fvGeometry, firstParallelScvf,
                                                                      faceVars, problem.boundaryTypes(elementParallel, firstParallelScvf),
                                                                      localSubFaceIdx) * outsideVolVars.density();
            }
        }
 
        return momenta;
    }
 
    static std::array<Scalar, 2> getLateralUpwindingMomenta_(const Problem& problem,
                                                             const FVElementGeometry& fvGeometry,
                                                             const Element& element,
                                                             const SubControlVolumeFace& ownScvf,
                                                             const ElementVolumeVariables& elemVolVars,
                                                             const FaceVariables& faceVars,
                                                             const Scalar transportingVelocity,
                                                             const int localSubFaceIdx,
                                                             const std::optional<BoundaryTypes>& currentScvfBoundaryTypes,
                                                             const std::optional<BoundaryTypes>& lateralFaceBoundaryTypes,
                                                             std::false_type)
    {
         // Check whether the own or the neighboring element is upstream.
        const SubControlVolumeFace& lateralFace = fvGeometry.scvf(ownScvf.insideScvIdx(), ownScvf.pairData(localSubFaceIdx).localLateralFaceIdx);
 
        // Get the volume variables of the own and the neighboring element
        const auto& insideVolVars = elemVolVars[lateralFace.insideScvIdx()];
        const auto& outsideVolVars = elemVolVars[lateralFace.outsideScvIdx()];
 
        const Scalar momentumParallel = ownScvf.hasParallelNeighbor(localSubFaceIdx, 0)
                                      ? faceVars.velocityParallel(localSubFaceIdx, 0) * outsideVolVars.density()
                                      : (getParallelVelocityFromBoundary_(problem, element, fvGeometry, ownScvf,
                                                                          faceVars, currentScvfBoundaryTypes, lateralFaceBoundaryTypes,
                                                                          localSubFaceIdx)
                                      * insideVolVars.density());
 
        // If the lateral face lies on a boundary, we assume that the parallel velocity on the boundary is actually known,
        // thus we always use this value for the computation of the transported momentum.
        if (!ownScvf.hasParallelNeighbor(localSubFaceIdx, 0))
            return std::array<Scalar, 2>{momentumParallel, momentumParallel};
 
        const bool selfIsUpstream = lateralFace.directionSign() == sign(transportingVelocity);
        const Scalar momentumSelf = faceVars.velocitySelf() * insideVolVars.density();
 
        return selfIsUpstream ? std::array<Scalar, 2>{momentumParallel, momentumSelf}
                              : std::array<Scalar, 2>{momentumSelf, momentumParallel};
    }
 
    static Scalar doLateralMomentumUpwinding_(const FVElementGeometry& fvGeometry,
                                              const SubControlVolumeFace& scvf,
                                              const std::array<Scalar, 2>& momenta,
                                              const Scalar transportingVelocity,
                                              const int localSubFaceIdx,
                                              const GridFluxVariablesCache& gridFluxVarsCache)
    {
        return doFrontalMomentumUpwinding_(scvf, momenta, transportingVelocity, gridFluxVarsCache);
    }
 
    static Scalar doLateralMomentumUpwinding_(const FVElementGeometry& fvGeometry,
                                              const SubControlVolumeFace& scvf,
                                              const std::array<Scalar, 3>& momenta,
                                              const Scalar transportingVelocity,
                                              const int localSubFaceIdx,
                                              const GridFluxVariablesCache& gridFluxVarsCache)
    {
        const auto eIdx = scvf.insideScvIdx();
        const auto& lateralFace = fvGeometry.scvf(eIdx, scvf.pairData(localSubFaceIdx).localLateralFaceIdx);
 
        const bool selfIsUpstream = ( lateralFace.directionSign() == sign(transportingVelocity) );
        const auto& upwindScheme = gridFluxVarsCache.staggeredUpwindMethods();
        std::array<Scalar,3> distances = getLateralDistances_(scvf, localSubFaceIdx, selfIsUpstream);
        return upwindScheme.tvd(momenta, distances, selfIsUpstream, upwindScheme.tvdApproach());
    }
 
    template<bool enable = useHigherOrder, std::enable_if_t<enable, int> = 0>
    static std::array<Scalar, 3> getLateralDistances_(const SubControlVolumeFace& ownScvf,
                                                      const int localSubFaceIdx,
                                                      const bool selfIsUpstream)
    {
        // distances {upstream to downstream distance, up-upstream to upstream distance, downstream staggered cell size}
        std::array<Scalar, 3> distances;
 
        if (selfIsUpstream)
        {
            // The local index of the faces that is opposite to localSubFaceIdx
            const int oppositeSubFaceIdx = localSubFaceIdx % 2 ? localSubFaceIdx - 1 : localSubFaceIdx + 1;
 
            distances[0] = ownScvf.parallelDofsDistance(localSubFaceIdx, 0);
            distances[1] = ownScvf.parallelDofsDistance(oppositeSubFaceIdx, 0);
            if (ownScvf.hasParallelNeighbor(localSubFaceIdx, 0))
                distances[2] = ownScvf.pairData(localSubFaceIdx).parallelCellWidths[0];
            else
                distances[2] = ownScvf.area() / 2.0;
        }
        else
        {
            distances[0] = ownScvf.parallelDofsDistance(localSubFaceIdx, 0);
            distances[1] = ownScvf.parallelDofsDistance(localSubFaceIdx, 1);
            distances[2] = ownScvf.pairData(localSubFaceIdx).parallelCellWidths[0];
        }
 
        return distances;
    }
 
    static Scalar getParallelVelocityFromBoundary_(const Problem& problem,
                                                   const Element& element,
                                                   const FVElementGeometry& fvGeometry,
                                                   const SubControlVolumeFace& scvf,
                                                   const FaceVariables& faceVars,
                                                   const std::optional<BoundaryTypes>& currentScvfBoundaryTypes,
                                                   const std::optional<BoundaryTypes>& lateralFaceBoundaryTypes,
                                                   const int localSubFaceIdx)
    {
        // Find out what boundary type is set on the lateral face
        const bool useZeroGradient = lateralFaceBoundaryTypes && (lateralFaceBoundaryTypes->isSymmetry()
                                                                  || lateralFaceBoundaryTypes->isDirichlet(Indices::pressureIdx));
        const bool lateralFaceHasBJS = lateralFaceBoundaryTypes && lateralFaceBoundaryTypes->isBeaversJoseph(Indices::velocity(scvf.directionIndex()));
        const bool lateralFaceHasDirichletVelocity = lateralFaceBoundaryTypes && lateralFaceBoundaryTypes->isDirichlet(Indices::velocity(scvf.directionIndex()));
        const Scalar velocitySelf = faceVars.velocitySelf();
 
        // If there is a Dirichlet condition for the pressure we assume zero gradient for the velocity,
        // so the velocity at the boundary equal to that on the scvf.
        if (useZeroGradient)
            return velocitySelf;
 
        if (lateralFaceHasBJS)
            return VelocityGradients::beaversJosephVelocityAtLateralScvf(problem, element, fvGeometry, scvf,  faceVars,
                                                                         currentScvfBoundaryTypes, lateralFaceBoundaryTypes, localSubFaceIdx);
 
        else if(lateralFaceHasDirichletVelocity)
        {
            //     ________________
            //     ---------------o                 || frontal face of staggered half-control-volume
            //     |      ||      # current scvf    #  ghostFace of interest, lies on boundary
            //     |      ||      #                 x  dof position
            //     |      ||      x~~~~> vel.Self   -- element boundaries
            //     |      ||      #                 __ domain boundary
            //     |      ||      #                 o  position at which the boundary conditions will be evaluated
            //     ---------------#
 
            const auto ghostFace = makeParallelGhostFace_(scvf, localSubFaceIdx);
            return problem.dirichlet(element, ghostFace)[Indices::velocity(scvf.directionIndex())];
        }
        else
        {
            // Neumann conditions are not well implemented
            DUNE_THROW(Dune::InvalidStateException, "Something went wrong with the boundary conditions for the momentum equations at global position " << fvGeometry.scvf(scvf.insideScvIdx(), scvf.pairData(localSubFaceIdx).localLateralFaceIdx).center());
        }
    }
 
    static Scalar getParallelVelocityFromOppositeBoundary_(const Problem& problem,
                                                           const Element& element,
                                                           const FVElementGeometry& fvGeometry,
                                                           const SubControlVolumeFace& scvf,
                                                           const FaceVariables& faceVars,
                                                           const std::optional<BoundaryTypes>& currentScvfBoundaryTypes,
                                                           const int localOppositeSubFaceIdx)
    {
        // A ghost subface at the boundary is created, featuring the location of the sub face's center
        const SubControlVolumeFace& lateralOppositeScvf = fvGeometry.scvf(scvf.insideScvIdx(), scvf.pairData(localOppositeSubFaceIdx).localLateralFaceIdx);
        GlobalPosition center = scvf.pairData(localOppositeSubFaceIdx).lateralStaggeredFaceCenter + lateralOppositeScvf.center();
        center *= 0.5;
 
        //          lateral face               #  lateralFace currently being assembled
        //    --------########                 || frontal face of staggered half-control-volume
        //    |      ||      | current scvf    %  lateralOppositeBoundaryFace of interest, lies on boundary
        //    |      ||      |                 x  dof position
        //    |      ||      x~~~~> vel.Self   -- element boundaries
        //    |      ||      |                 __ domain boundary
        //    |      ||      |                 o  position at which the boundary conditions will be evaluated
        //    --------%%%c%%%o
        //    ________________                 c  center of opposite boundary face
 
 
        // Get the boundary types of the lateral opposite boundary face
        const auto lateralOppositeFaceBoundaryTypes = problem.boundaryTypes(element, lateralOppositeScvf.makeBoundaryFace(center));
 
        return getParallelVelocityFromBoundary_(problem, element, fvGeometry, scvf,
                                                faceVars, currentScvfBoundaryTypes, lateralOppositeFaceBoundaryTypes,
                                                localOppositeSubFaceIdx);
    }
 
    static SubControlVolumeFace makeParallelGhostFace_(const SubControlVolumeFace& ownScvf, const int localSubFaceIdx)
    {
        return ownScvf.makeBoundaryFace(ownScvf.pairData(localSubFaceIdx).lateralStaggeredFaceCenter);
    };
};
 
} // end namespace Dumux
 
#endif // DUMUX_NAVIERSTOKES_STAGGERED_UPWINDVARIABLES_HH