Evolution of average eccentricity (solid lines) and inclination (in radians; dashed lines) in a disc of indestructible particles, inelastically bouncing off each other in collisions. The left- and right-hand-side panels show a low-excitation disc and high-excitation disc, respectively. Horizontal blue lines show the lowest grid bin values, i.e. the lowest values to which eccentricity and inclination can be damped in the model. Circles on solid curves indicate damping timescales predicted by a simple particle-in-a-box model. See Sect. 4.1.