mg2d: account for effect of the ~u term on the relaxation factor

The maximum allowed time step in the presence of a -K * u term goes like
2 (dx ** 2) / (4 + K (dx ** 2))

1 files changed, 14 insertions, 0 deletions

diff --git a/ell_grid_solve.h b/ell_grid_solve.h
index a12e9c3..4ee6cb4 100644
--- a/ell_grid_solve.h
+++ b/ell_grid_solve.h
@@ -65,10 +65,24 @@ typedef struct EGSInternal EGSInternal;
 
 typedef struct EGSRelaxContext {
     /**
+     * The "time step" for relaxation is calculated as
+     * Δt = r_m * r_f * step[0] * step[1]
+     *
+     * Where
+     *  - r_m is relax_multiplier if specified, 1.0 otherwise
+     *  - r_f is relax_factor if specified, a default fd_stencil-dependent value
+     *    otherwise
+     */
+    /**
      * The time stepping factor in relaxation.
      */
     double relax_factor;
 
+    /**
+     * Multiplier for the time stepping factor.
+     */
+    double relax_multiplier;
+
     int64_t count_correct;
     int64_t time_correct;
 } EGSRelaxContext;