DDPM model can only interact with the primary Eulerian phase. The momentum exchange terms from particle tracking are considered only in the primary phase momentum equations. Although the solver setup allows you to enable more than one Eulerian phases with the DDPM model, the solution will not capture the particle phase interactions with the dispersed (other secondary) Eulerian phases. For example, consider a simulation involving dense solid particles in a water continua which also has gas sparged from the bottom; where the phases are setup as below: – Primary phase: Water – Secondary (Eulerian) phase: Gas – Discrete phase (DDPM): Solid-particles If solid-particles and gas bubbles end up in the same computational cell, the effect of particles on bubbles and vice-versa will not be captured.