hbc2d_Radiation_LinearEuler2D Subroutine

  subroutine hbc2d_Radiation_LinearEuler2D(bc,mymodel)
    !! Radiation BC: zero acoustic perturbation in the exterior state; the
    !! sound speed (variable 4), background density (variable 5) and relaxation
    !! rate (variable 6) are copied from the interior side so the Riemann solver
    !! sees a consistent c and rho0 (impedance-matched, non-reflecting outflow)
    !! and the exterior state stays a faithful mirror of the interior one for
    !! diagnostics. The relaxation rate carries no flux, so its exterior value
    !! never enters the tendency.
    class(BoundaryCondition),intent(in) :: bc
    class(Model),intent(inout) :: mymodel
    ! Local
    integer :: n,i,iEl,j

    select type(m => mymodel)
    class is(LinearEuler2D_t)
      do n = 1,bc%nBoundaries
        iEl = bc%elements(n)
        j = bc%sides(n)
        do i = 1,m%solution%interp%N+1
          m%solution%extBoundary(i,j,iEl,1:3) = 0.0_prec
          m%solution%extBoundary(i,j,iEl,4) = m%solution%boundary(i,j,iEl,4) ! c preserved
          m%solution%extBoundary(i,j,iEl,5) = m%solution%boundary(i,j,iEl,5) ! rho0 preserved
          m%solution%extBoundary(i,j,iEl,6) = m%solution%boundary(i,j,iEl,6) ! sigma preserved
        enddo
      enddo
    endselect

  endsubroutine hbc2d_Radiation_LinearEuler2D