Differences between DEM and DPM
This simulation of a cyclone is done with the internal particle solver of OpenFoam called MPPICFoam.
We can see here a particle material with a density of 1000 kg/m³ which could be something like green hardwood. These particles get conveyed by an airstream through a cyclone which seperates the particle due to centrifugal force.
The MPPICFoam solver is used to solve more or less dens particle flows in a CFD simulation. The solver was designed for simulation which could not be simulated in DEM. However due to the increase in CPU power they are simulateable in a coupled DEM-CFD Simulation now. Like I demonstrated on the suction excavator which handled also a lot of particles.
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DPM Solver, which stands for Dens Particle Modelling, in OpenFoam will in general abstract the particle interaction with very simplified models. Forces and stresses in the bulk material will be calculated statistically based on the void fraction and the pacing in one cell. The collision or contact itself will not be modelled like in a DEM simulation where a fairly complex contact equation is solved, which represents friction, rolling friction, adhesion, cohesion, mass and inertia of the particle and so on.
In this simulation, the lack of a good contact model can be seen in the surface contact of wall and particle. The particles decrease in velocity very quickly on the wall. This leads to a patch building of the particles.
From my experience this looks very unrealistic from all the real life application in bulk material flow and all simulations I have seen so far. I would not use this method for this application. However, the particle models in openFoam can be useful if you can be sure, that the particle – particle contact will not be important in your simulation. This could be a simulation of dust in air or in water, where the particles collisions will not have an impact on the general behaviour of the flow media or the particle movements itself.
In the last years it came very obvious that the DEM is a very useful and physical correct method for solving the bulk material behaviour. Because of this the DEM should always be used if particles must be considered in a simulation and the number of particles would be defined as bulk material. In science various numbers exist, to define when to use DEM in CFD, but these are in my opinion to inflexible. It always depends on what you want to solve. A general number like the volume fraction of particles to fluid is not useful.
Some explanations about the solver and the theory behind this can be found in the release-Notes of OpenFoam 2.3.0 in the DPM section.
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