Fluid Film - VOF: Rivulets on an Inclined Plane
Situations occur where a thin film of liquid forms on a solid surface, for example, rainwater on the windshield of a car. This film can flow and accumulate, and gradually grow into a "thick" film, or a pool of liquid that can no longer be described as a fluid film. These situations are multi-scale problems which, if resolved on all scales (using the VOF multiphase model), would require significant computer resources.
Simcenter STAR-CCM+ offers a modeling approach to solve these types of problems in an efficient way. You can combine the VOF model and the thin fluid film model, such that the VOF model is used in areas where the mesh is sufficiently fine to resolve the flow of liquid, and the fluid film model is used in areas where the mesh size is larger than the film thickness.
This tutorial models a thin stream of water flowing down a surface that is inclined at an angle of 45o with the horizontal. The inclined plane is 0.2 meters long and 0.05 meters wide. The computational domain is 0.01 meters high. Water enters the domain through the inlet edge at the top at a rate of 0.005 kg/s and flows down the incline as a thin film or rivulet. The simulation domain is shown below:

The mesh resolution is not uniform. The upper and lower regions of the inclined plane use a coarse mesh, but the central region uses a finer mesh. In this case, the mesh in the central region is sufficiently fine to resolve the flow of liquid, so the VOF model is used. The fluid film model is used in the upper and lower regions.