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25.17.3 Variable Time Stepping

For VOF calculations (using the Explicit scheme), FLUENT allows you to use variable time stepping in order to automatically change the time-step 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 donor-acceptor scheme as well. Variable time stepping is not available for the implicit scheme of VOF.



The Variable Time Stepping Algorithm


The global time-step $\Delta t_{\rm global}$ is changed in the following manner:


 \Delta t_{\rm global} = \frac{CFL_{\rm global}}{max (\sum \frac{outgoing fluxes}{volume})} (25.17-1)

where the ratio $\sum \frac{outgoing fluxes}{volume}$ 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.

Figure 25.17.6: The Iterate Panel for Implicit Unsteady Calculations and Variable Time Stepping
figure

The variable time step is based on the maximum Courant number near the VOF interface. To calculate that Courant number, FLUENT uses a flux-based 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.


next up previous contents index Previous: 25.17.2 Adaptive Time Stepping
Up: 25.17 Performing Time-Dependent Calculations
Next: 25.17.4 Postprocessing for Time-Dependent
© Fluent Inc. 2006-09-20