Please see this forum posting to see how to enable the hybrid model. Note that hybrid model is not fully tested, is not supported, and is in no way guaranteed to work. But you may be able to use a TFM phase for the fixed particles (with momentum equations disabled) and DEM for the other particles. It’s at least worth a try.
If you want to stick with pure DEM, you’d be modifying the source code, to keep particles in a particular phase fixed. This might be as simple as finding where the positions are updated and skipping this for a pre-designated phase, but there might be more to consider (conservation of energy and momentum, you have to do this in a way that’s physically reasonable)
– Charles
If you want to