time step for VOF equations
 

Hi everyone.
According to section "7.18.5.5.Timestep" in ''ANSYS-CFX Solver Modeling Guide'' it is often helpful to reduce the timestep for the volume fraction equations by an order of magnitude below that of the other equations. As CFX has an implicit solver, I need to know how I can use a different time step for VOF equations?:confused::confused:

In a fully converged solution (residuals, imbalances, values in monitoring points, .....), the timestep has no effect on the solution.
The way to get to this converged solution is just easier by using a small timestep for volume fraction. A reduction by a factor 10 is quite blunt. A factor 2, 3 or 4 might work as well, and will be faster. Just give it a try.

Hi Gert-Jan,
I need to know how we can reduce the time step only for VOF equations. As CFX has an implicit solver we can not change the time step only for VOF equations whereas the CFX user manual says it is possible!! I want to know the way. I don't know if I should use CCL or not.:confused:

You can use a different (false) timesteps in Steady state calculations. That is what the help is about. You cannot do this in transient calculations.

For steady state: In CFX-Pre go to Solver Control > Equation Class Settings. There select the equation for the Mass fractions and choose a timescale factor (or Physical timestep) which is substantial smaller than the time setting in the Basic-Settings-tab.

thank you so much for your point of view.