src/bcg.h

    Bell-Collela-Glaz advection scheme

    The function below implements the 2nd-order, unsplit, upwind scheme of Bell-Collela-Glaz, 1989. Given a centered scalar field f, a face vector field uf (possibly weighted by a face metric), a timestep dt and a source term field src, it fills the face vector field flux with the components of the advection fluxes of f.

    void tracer_fluxes (scalar f,
		    face vector uf,
		    face vector flux,
		    double dt,
		    (const) scalar src)
{

    We first compute the cell-centered gradient of f in a locally-allocated vector field.

      vector g[];
  gradients ({f}, {g});

    For each face, the flux is composed of two parts…

      foreach_face() {

    A normal component… (Note that we cheat a bit here, un should strictly be dt*(uf.x[i] + uf.x[i+1])/((fm.x[] + fm.x[i+1])*Delta) but this causes trouble with boundary conditions (when using narrow ‘1 ghost cell’ stencils)).

        double un = dt*uf.x[]/(fm.x[]*Delta + SEPS), s = sign(un);
    int i = -(s + 1.)/2.;
    double f2 = f[i] + (src[] + src[-1])*dt/4. + s*(1. - s*un)*g.x[i]*Delta/2.;

    and tangential components…

        #if dimension > 1
    if (fm.y[i] && fm.y[i,1]) {
      double vn = (uf.y[i] + uf.y[i,1])/(fm.y[i] + fm.y[i,1]);
      double fyy = vn < 0. ? f[i,1] - f[i] : f[i] - f[i,-1];
      f2 -= dt*vn*fyy/(2.*Delta);
    }
    #endif
    #if dimension > 2
    if (fm.z[i] && fm.z[i,0,1]) {
      double wn = (uf.z[i] + uf.z[i,0,1])/(fm.z[i] + fm.z[i,0,1]);
      double fzz = wn < 0. ? f[i,0,1] - f[i] : f[i] - f[i,0,-1];
      f2 -= dt*wn*fzz/(2.*Delta);
    }
    #endif

    flux.x[] = f2*uf.x[];
  }
}

    The function below uses the tracer_fluxes function to integrate the advection equation, using an explicit scheme with timestep dt, for each tracer in the list.

    void advection (scalar * tracers, face vector u, double dt,
		scalar * src = NULL)
{

    If src is not provided we set all the source terms to zero.

      scalar * psrc = src;
  if (!src)
    for (scalar s in tracers) {
      const scalar zero[] = 0.;
      src = list_append (src, zero);
    }
  assert (list_len (tracers) == list_len (src));

  scalar f, source;
  for (f,source in tracers,src) {
    face vector flux[];
    tracer_fluxes (f, u, flux, dt, source);
#if !EMBED
    foreach()
      foreach_dimension()
        f[] += dt*(flux.x[] - flux.x[1])/(Delta*cm[]);
#else // EMBED
    update_tracer (f, u, flux, dt);
#endif // EMBED
  }

  if (!psrc)
    free (src);
}

    Usage

    Tests