Volume of fluid (VOF) techniques offer a means of simulating a fluid flow with one or several free surfaces, and are popular for injection and filling-type problems. There are probably as many techniques as codes implemented, differing in the way that the "moving front" is handled and advanced. What is common among VOF techniques is the use of a time-dependent approach, which is applied on a fixed mesh; that is, the kinematic condition is not tracked and the simulation domain is not remeshed. Theoretically, a drawback of such techniques is that the location where the zero force condition is applied does not correspond to a region on which natural boundary conditions apply. Consequently, additional approximations are required, beyond those typically employed as part of the finite element method.
Because of the explicit nature of VOF algorithms, a so-called Courant type of limitation always occurs: the time step is limited to a fraction of the time it takes to transport information through an individual element (or cell). It is common to require hundreds of time steps to compute an entire simulation. Because each of these time steps are performed on a fixed domain with an inexpensive numerical technique, a VOF model can still require less CPU time than a moving mesh simulation, such as the Arbitrary Lagrangian-Eulerian (ALE) approach (described in Free Surfaces). The VOF model implemented in Ansys Polyflow is intrinsically more robust than the ALE approach when modeling free surfaces that merge, separate, or are convoluted, and allows you to simulate problems that a remeshing-based technique simply could not attack (for example, a complex cavity filling).
It should be noted that the VOF model is not a replacement for ALE methods in all cases. Because the VOF technique relies on partly filled cells (which means that the location of the free surface does not correspond to an element boundary), it is intrinsically less accurate then the ALE technique for flows with well defined interfaces, such as extrusion or multiple layer problems. The ALE method may be more appropriate when searching for a well defined steady-state free surface through a steady-state (or an evolution on a free surface) algorithm.
Important: The VOF model does not account for the effect of surface tension on the free surface.
Important: The VOF model has not been tested in combination with other Ansys Polyflow models, such as the mesh superposition technique (MST), internal radiation (discrete ordinates model), etc. Due to the nonlinearities and dependencies involved in the various models, it is not possible to guarantee the convergence and accuracy of problems that combine VOF with other models. It is recommended that you attempt such combinations with caution.