15.11.2 Use and Limitations

The collision model assumes that the frequency of collisions is much less than the particle time step. If the particle time step is too large, then the results may be time-step-dependent. You should adjust the particle length scale accordingly. Additionally, the model is most applicable for low-Weber-number collisions where collisions result in bouncing and coalescence. Above a Weber number of about 100, the outcome of collision could be shattering.

Sometimes the collision model can cause mesh-dependent artifacts to appear in the spray. This is a result of the assumption that droplets can collide only within the same cell. These tend to be visible when the source of injection is at a mesh vertex. The coalescence of droplets tends to cause the spray to pull away from cell boundaries. In two dimensions, a finer mesh and more computational droplets can be used to reduce these effects. In three dimensions, best results are achieved when the spray is modeled using a polar mesh with the spray at the center.

If the collision model is used in a transient simulation, multiple DPM iterations per time step cannot be specified in the Number of Continuous Phase Iterations per DPM Iteration field in the Discrete Phase Model dialog box. In such cases, only one DPM iteration per time step will be calculated.