verticalDamping: Expanded documentation

src/fvOptions/sources/derived/verticalDamping/verticalDamping.H

@@ -30,6 +30,24 @@ Description
     vertical motions of an interface in the region approaching an outlet so that
     no reflections are generated.
 
+    Damping is achieved by applying a force to the momentum equation
+    proportional to the momentum of the flow in the direction of gravity. The
+    constant of proportionality is given by a coefficient, lambda, which has
+    units of inverse-time. In the absence of any other forces this would
+    generate an exponential decay of the vertical velocity.
+        \f[
+            \frac{d (m u_z)}{d t} = - \lambda m u_z
+        \f]
+        \f[
+            u_z = u_{z0} e^{- \lambda t}
+        \f]
+
+    Lambda should be set based on the desired level of damping and the residence
+    time of a perturbation through the damping zone. For example, if waves
+    moving at 2 [m/s] are travelling through a damping zone 8 [m] in length,
+    then the residence time is 4 [s]. If it is deemed necessary to damp for 5
+    time-scales, then lambda should be set to equal 5/(4 [s]) = 1.2 [1/s].
+
 Usage
     Example usage:
     \verbatim