advectionDiffusionPatchDistMethod: Added more description including specification of relaxation

src/finiteVolume/fvMesh/wallDist/patchDistMethods/advectionDiffusion/advectionDiffusionPatchDistMethod.H

@@ -29,6 +29,17 @@ Description
     boundary by solving the Eikonal equation in advection form with diffusion
     smoothing.
 
+    If the diffusion coefficient is set to 0 this method is exact in principle
+    but the numerical schemes used are not rendering the scheme approximate, but
+    more accurate than the Poisson method.  Also many models relying on the
+    distance to the wall benefit from this field being smooth and monotonic so
+    the addition of diffusion smoothing improves both the convergence and
+    stability of the solution of the Eikonal equation and the behavior of the
+    models using the distance field generated.  However, it is not clear what
+    the optimum value for the diffusion coefficient epsilon should be; a
+    default value of 0.1 is provided but higher values may be preferable under
+    some circumstances.
+
     Convergence is accelerated by first generating an approximate solution
     using one of the simpler methods, e.g. Poisson.  The method used for
     this prediction step is specified in the 'advectionDiffusionCoeffs'
@@ -90,6 +101,18 @@ Description
             relTol          0;
         }
 
+        relaxationFactors
+        {
+            equations
+            {
+                .
+                .
+                yWall           1;
+                .
+                .
+            }
+        }
+
     or for more complex cases:
 
         yPsi
@@ -115,6 +138,18 @@ Description
             tolerance       1e-4;
             relTol          0;
         }
+
+        relaxationFactors
+        {
+            equations
+            {
+                .
+                .
+                yWall           0.7;
+                .
+                .
+            }
+        }
     \endverbatim
 
 SeeAlso