reduce_sum Subroutine

   subroutine reduce_sum(local_value, global_value, flow)
      real(rk), intent(in) :: local_value
      real(rk), intent(out) :: global_value
      type(flow_mpi_t), intent(in) :: flow

      integer :: ierr
      real(rk) :: send_val, recv_val

      send_val = local_value
      call MPI_Allreduce(send_val, recv_val, 1, MPI_DOUBLE_PRECISION, MPI_SUM, flow%comm, ierr)
      if (ierr /= MPI_SUCCESS) call fatal_error('output', 'MPI failure in reduce_sum')
      global_value = recv_val

   end subroutine reduce_sum