  A method for adaptive model order reduction for nonsmooth discrete element
simulation is developed and analysed in numerical experiments. Regions of the
granular media that collectively move as rigid bodies are substituted with
rigid bodies of the corresponding shape and mass distribution. The method also
support particles merging with articulated multibody systems. A model
approximation error is defined and used to derive conditions for when and where
to apply reduction and refinement back into particles and smaller rigid bodies.
Three methods for refinement are proposed and tested: prediction from contact
events, trial solutions computed in the background and using split sensors. The
computational performance can be increased by 5 - 50 times for model reduction
level between 70 - 95%.
