time step for VOF equations

ehsan1363 · January 5, 2020, 08:08

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?

Gert-Jan · January 5, 2020, 09:04

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.

ehsan1363 · January 6, 2020, 03:45

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.

Gert-Jan · January 6, 2020, 03:56

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.

ehsan1363 · January 6, 2020, 05:02

thank you so much for your point of view.

January 6, 2020, 03:56		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27	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. Ashkan Kashani likes this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
bash script for pseudo-parallel usage of reconstructPar	kwardle	OpenFOAM Post-Processing	41	August 23, 2023 02:48
Transient simulation not converging	skabilan	OpenFOAM Running, Solving & CFD	14	December 16, 2019 23:12
time step continuity problem in VAWT simulation	lpz_michele	OpenFOAM Running, Solving & CFD	5	February 22, 2018 19:50
Floating point exception error	lpz_michele	OpenFOAM Running, Solving & CFD	53	October 19, 2015 02:50
plot over time	fferroni	OpenFOAM Post-Processing	7	June 8, 2012 07:56

January 5, 2020, 09:04		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27	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.

January 6, 2020, 03:45		#3
ehsan1363 New Member Join Date: Sep 2015 Posts: 4 Rep Power: 10	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.

January 6, 2020, 05:02		#5
ehsan1363 New Member Join Date: Sep 2015 Posts: 4 Rep Power: 10	thank you so much for your point of view.