Learning Forum

[Nick Fan]

I used the CFL-Based method for my simulation and set the Courant Number to 100. The other parameters were set as follows. However, when monitoring the results, I noticed that the maximum value was only around 88. What could be the possible reasons for the Courant Number not approaching 100?

[Rob]

How far off the maximum timestep limit are you? It may also be a safety factor to avoid exceeding the value, there's likely some detail in the Theory Guide. 

[Nick Fan]

Thanks for your reply, Rob. I'm not sure that you mean the maximum timestep limit setting is here.

[Rob]

If you're hitting the maximum time step 1s, then Courant Number limit may not be reached. 

[Nick Fan]

Thank you for your reply, Rob. To clarify, should I first calculate the time step size that satisfies my desired Courant number based on my settings?  Is it recommended to use a value larger than this calculated time step size as my "maximum time step size"?

[Rob]

I use adaptive stepping very rarely as you risk fast transients forcing the solver to the minimum value or stability issues if the time step oscillates between two values because of a relatively fast transient. 

[Nick Fan]

Thank you for taking the time to respond, Rob. 
If I want to set the time step size based on the Courant number, is there a way to calculate the desired time step size using Fluent's functionalities?

[Rob]

Not really as you don't know the flow velocity before starting the calculation. 

[Nick Fan]

Thanks for your reply, Rob.
So, when I start a simulation, should I first use the adaptive type method to determine the timestep size that satisfies my Courant number, and then use this timestep size with the fixed type method to complete my subsequent simulation?

[Rob]

I tend to estimate a likely highest speed, work out how long that takes to cross a cell and divide that number by 10. That's my starting time step. I then adjust (carefully) from there. It's  fairly rough rule of thumb we've been using for over two decades and usually works. 

[Nick Fan]

Thank you for your valuable suggestions, Rob.
I will consider your advice in the upcoming simulations.

[Federico]

36.13. Performing Time-Dependent Calculations

[Nick Fan]

Thanks for your valuable information, Federico.