
For VOF calculations (using the Explicit scheme), FLUENT allows you to use variable time stepping in order to automatically change the timestep when an interface is moving through dense cells or if the interface velocity is high.
Variable time stepping is available for all the explicit schemes of VOF, which includes the donoracceptor scheme as well. Variable time stepping is not available for the implicit scheme of VOF.
The Variable Time Stepping Algorithm
The global timestep is changed in the following manner:
(25.171) 
where the ratio is calculated for each cell. FLUENT takes the maximum of this ratio to calculate the global time step.
Specifying Parameters for Variable Time Stepping
For transient VOF calculations, when the Variable option is chosen in the Iterate panel (Figure 25.17.6), FLUENT presents additional parameters. With the exception of the Global Courant Number field, all parameters are the same as for adaptive time stepping (see Section 25.17.2). The default value for the Global Courant Number is 2.
The variable time step is based on the maximum Courant number near the VOF interface. To calculate that Courant number, FLUENT uses a fluxbased definition where, in the region near the fluid interface, FLUENT divides the volume of each cell by the sum of the outgoing fluxes. The resulting time represents the time it would take for the fluid to empty out of the cell. The smallest such time is used as the characteristic time of transit for a fluid element across a control volume.