Learning Forum

[manahara8]

Hi all,

I am trying to use CFL based adaptive time-stepping to run an LES case to a statistically steady-state. I want to run this initial stage of the simulation at a 'high' CFL number of ~5. To do that, I have turned on adaptive time-stepping feature on Fluent and proceeded to run the calculation. In the meantime, I also have a report-definition set-up to print out the maximum Cell Convective Courant number (under Reports/Velocity/Cell-convective Courant no). Interestingly, my Cell-convective courant number reading for the case is around 0.4 with adaptive time stepping on and CFL set to 5.

I also tried rerunning the case with the adaptive time-stepping CFL setting set to 50 and my Cell-convective Courant no increased by factor of 10 to about 4.

So my question is, what is the difference between the Cell-convective courant number and the CFL number that fluent calculates for it's adaptive time-stepping scheme? Are they the same or different? If they are the same, why is there a factor of 10 difference between my control parameter and my calculated parameter?

Any clarification wrt this is highly appreciated. Thanks!

Mana

[Karthik Remella]

Hello:
You increased the CFL number by 10 times (from 5 to 50) and your max Cell Courant number increased by 10 times (from 0.4 to 4). Is my understanding right? If so, this is expected behavior.
Maybe I did not fully understand your question?
Karthik

[manahara8]

Hi, Apologies if my question was not clear. I want fluent to try to reach and maintain a CFL number of around 5 using the adaptive time-stepping method. But, in my simulation the max convective Co number in the fluid zone is around 0.4, but shouldn't Fluent adjust the time step so that the max convective Co number would be around 5 instead? Mana

[Karthik Remella]

Hello:
This is also expected as they are calculated somewhat differently. Please see the links below for a better understanding.
32.13.2. CFL-Based Time Stepping (ansys.com)
37.4. Alphabetical Listing of Field Variables and Their Definitions (ansys.com)
Karthik