BoundaryFlux.hpp

// Distributed under the MIT License.
// See LICENSE.txt for details.

#pragma once

#include <algorithm>
#include <array>
#include <cstddef>

#include "DataStructures/DataBox/DataBoxTag.hpp"
#include "DataStructures/DataBox/Prefixes.hpp"
#include "DataStructures/Tensor/Tensor.hpp"
#include "DataStructures/Variables.hpp"
#include "Domain/Mesh.hpp"
#include "NumericalAlgorithms/DiscontinuousGalerkin/BoundarySchemes/FirstOrder/BoundaryData.hpp"
#include "NumericalAlgorithms/DiscontinuousGalerkin/LiftFlux.hpp"
#include "NumericalAlgorithms/DiscontinuousGalerkin/MortarHelpers.hpp"
#include "NumericalAlgorithms/DiscontinuousGalerkin/NumericalFluxes/NumericalFluxHelpers.hpp"
#include "NumericalAlgorithms/DiscontinuousGalerkin/Protocols.hpp"
#include "NumericalAlgorithms/Spectral/Spectral.hpp"
#include "Utilities/ProtocolHelpers.hpp"
#include "Utilities/Requires.hpp"
#include "Utilities/TMPL.hpp"

namespace dg {
/// Functionality related to the first-order DG scheme.
namespace FirstOrderScheme {

/*!
 * \brief Compute the boundary flux contribution and lift it to the volume.
 *
 * This computes the numerical flux minus the flux dotted into the interface
 * normal, projects the result to the face mesh if necessary, and then lifts it
 * to the volume (still presented only on the face mesh as all other points are
 * zero).
 *
 * Projection must happen after the numerical flux calculation so that the
 * elements on either side of the mortar calculate the same result.  Projection
 * must happen before flux lifting because we want the factor of the magnitude
 * of the unit normal added during the lift to cancel the Jacobian factor in
 * integrals to preserve conservation; this only happens if the two operations
 * are done on the same grid.
 */
template <size_t FaceDim, typename NumericalFluxType, typename AllFieldsTags,
          typename AllExtraDataTags,
          Requires<tt::conforms_to_v<NumericalFluxType,
                                     dg::protocols::NumericalFlux>> = nullptr>
Variables<typename NumericalFluxType::variables_tags> boundary_flux(
    const dg::SimpleBoundaryData<AllFieldsTags, AllExtraDataTags>&
        local_boundary_data,
    const dg::SimpleBoundaryData<AllFieldsTags, AllExtraDataTags>&
        remote_boundary_data,
    const NumericalFluxType& numerical_flux_computer,
    const Scalar<DataVector>& magnitude_of_face_normal,
    const size_t extent_perpendicular_to_boundary,
    const Mesh<FaceDim>& face_mesh, const Mesh<FaceDim>& mortar_mesh,
    const MortarSize<FaceDim>& mortar_size) noexcept {
  using variables_tags = typename NumericalFluxType::variables_tags;

  // Compute the Tags::NormalDotNumericalFlux from local and remote data
  Variables<db::wrap_tags_in<::Tags::NormalDotNumericalFlux, variables_tags>>
      normal_dot_numerical_fluxes{mortar_mesh.number_of_grid_points()};
  dg::NumericalFluxes::normal_dot_numerical_fluxes(
      make_not_null(&normal_dot_numerical_fluxes), numerical_flux_computer,
      local_boundary_data, remote_boundary_data);

  // Subtract the local Tags::NormalDotFlux
  tmpl::for_each<variables_tags>([&normal_dot_numerical_fluxes,
                                  &local_boundary_data](
                                     const auto tag_v) noexcept {
    using tag = tmpl::type_from<decltype(tag_v)>;
    auto& numerical_flux =
        get<::Tags::NormalDotNumericalFlux<tag>>(normal_dot_numerical_fluxes);
    const auto& local_flux =
        get<::Tags::NormalDotFlux<tag>>(local_boundary_data.field_data);
    for (size_t i = 0; i < numerical_flux.size(); ++i) {
      numerical_flux[i] -= local_flux[i];
    }
  });

  // Project from the mortar back to the face if needed
  auto projected_fluxes =
      needs_projection(face_mesh, mortar_mesh, mortar_size)
          ? project_from_mortar(normal_dot_numerical_fluxes, face_mesh,
                                mortar_mesh, mortar_size)
          : std::move(normal_dot_numerical_fluxes);

  // Lift flux to the volume. We still only need to provide it on the face
  // because it is zero everywhere else.
  return lift_flux(std::move(projected_fluxes),
                   extent_perpendicular_to_boundary, magnitude_of_face_normal);
}

}  // namespace FirstOrderScheme
}  // namespace dg