  The large time and length scales and, not least, the vast number of particles
involved in industrial-scale simulations inflate the computational costs of the
Discrete Element Method (DEM) excessively. Coarse grain models can help to
lower the computational demands significantly. However, for effects that
intrinsically depend on particle size, coarse grain models fail to correctly
predict the behaviour of the granular system. To solve this problem we have
developed a new technique based on the efficient combination of fine-scale and
coarse grain DEM models. The method is designed to capture the details of the
granular system in spatially confined sub-regions while keeping the
computational benefits of the coarse grain model where a lower resolution is
sufficient. To this end, our method establishes two-way coupling between
resolved and coarse grain parts of the system by volumetric passing of boundary
conditions. Even more, multiple levels of coarse-graining may be combined to
achieve an optimal balance between accuracy and speedup. This approach enables
us to reach large time and length scales while retaining specifics of crucial
regions. Furthermore, the presented model can be extended to coupled CFD-DEM
simulations, where the resolution of the CFD mesh may be changed adaptively as
well.
