Accretion Simulation with SideFX Houdini
This video demonstrates using Houdini grains to simulate accretion. The grain source is an array of spheres positioned over jittered points created from a disc shaped volume. Since the grains clump together and are simultaneously drawn towards a central point, they give rise to a nice accretion effect.
I've detailed the work involved in a blog post: http://flexmonkey.blogspot.co.uk/2016/10/simulating-accretion-with-houdinis.html