Interface Sharpening through Slip Velocity Modeling

Adding the slip velocity to the VOF method improves the modeling assumptions to allow a more physical behavior of the multiphase system in scenarios when the interface between phases is not resolved. In such cases, the VOF model behaves like the Mixture Multiphase (MMP) model, which can improve the modeling significantly and restore a sharp interface.

The VOF multiphase model operates on the assumption that the interfaces between phases are resolved. In Simcenter STAR-CCM+, in the proximity of an interface, there are three cells: one cell has a volume fraction of a particular phase equal to 1, one cell where the volume fraction of the same phase is 0, and one cell between the other two cells where the volume fraction of the phase has a value between 0 and 1. The two phases that are in contact at the interface move with the same velocity, so their motion is described with the single velocity field and the modeling assumptions of the VOF model are fulfilled.

In some scenarios, the interface is not resolved and the modeling assumptions of the VOF model are not satisfied. These scenarios can be due to inappropriate initial or boundary conditions which do not produce a resolved (sharp) interface. Alternatively, the selected time and space discretizations may not be sufficient to resolve the interface and its motion (for example, a droplet has to be resolved with at least 3x3x3 cells, and the interface can not cross more than one cell per time step). When the interface is not resolved, the VOF model behaves like a homogeneous mixture with a smeared interface between the phases where each phase moves at the same velocity.

However, by including the slip between phases, Simcenter STAR-CCM+ models the effects resulting from the phases moving at different velocities. Modeling of the slip velocity is an integral part of the Mixture Multiphase (MMP) model. Including the slip velocity as part of the VOF method leads to an improvement of the modeling assumptions resulting in a recovery of a sharp interface.

Occasionally, due to the strong non-linearities associated with the drag-based slip velocity, the simulation can become unstable or even diverge. In these situations, to obtain stable results, you are advised to use one of the slip velocity limiting options available in Simcenter STAR-CCM+.

For more information on slip velocity modeling, see Phase Slip.