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-   -   Courant number (http://www.cfd-online.com/Forums/fluent/80692-courant-number.html)

fireman October 4, 2010 07:02

Courant number
 
Hi, everyone,

I would like to know what the Courant number is used for? I can understand that CFL number is used to control the time step in couple explicit solver with explicit unsteady formulation. But in couple implicit or explicit solver with implicit unsteady formulation, the time step will be manually specified in the iterate panel, It may imply that the courant number can not control the time step anymore, so why should I need a courant number in these case? Thank you!

Laci October 4, 2010 08:55

Quote:

Originally Posted by fireman (Post 277653)
Hi, everyone,

I would like to know what the Courant number is used for? I can understand that CFL number is used to control the time step in couple explicit solver with explicit unsteady formulation. But in couple implicit or explicit solver with implicit unsteady formulation, the time step will be manually specified in the iterate panel, It may imply that the courant number can not control the time step anymore, so why should I need a courant number in these case? Thank you!

Because you "cannot predict the future". :D
The Courant number helps to find the appropriate time step for a given mesh You will find the equation how to calculate the time step according to your elements size.

pranab_jha October 4, 2010 09:12

Exactly as Laci said. Here is some information: http://en.wikipedia.org/wiki/Courant...Lewy_condition

In Fluent, if while running a transient simulation, the Courant # goes above 250 (default, you can change that too), your computation stops there and then. It means your solution is diverging. The flow speed is too fast to be calculated at that time step size or at that grid size.

fireman October 4, 2010 12:49

Thank you very much, Laci and pranab_jha. Since the Courant number helps to find the appropriate time step, why do i need to specify a time step in the iterate panel for simulation? and what's the relationship between the time step found by Courant number and the time step manually specified in the iterate panel?

Laci October 4, 2010 12:54

As I remember, you can set the time step manually (when you have to take care about the Courant number) or there is an automatic option. With this option Fluent will find the "appropriate" time step for your problem, with variable time steps. Be careful if you choose this second option, sometimes it makes errors which lead to high Courant number.
This feature (automatic) gives quite good guess, how much should be the first time point.

Thomas MADELEINE July 28, 2016 12:55

Hi all,

first sorry for resurrecting this topic, I don't know if i should open a new one or not...

I have some issues with my simulation on FLUENT on my question is quite simple :

can we have a too low courant number ? or a timestep too small ?


I explain myself :

I run a transient simulation of a tank where I print the forces on the walls (created by the fluid).
After a few step (10), I stop my simulation and want to know some instant reactions of the flow (the aim is that the solution will not change here between two timestep).

for that i reduce drasticaly my time step, and pursue the simulation for two timestep (and compare the two data files).

I have tried with four kind of time-step, and all the case has converged (residuals below 1e-7, quite a simple case though)

But when I compare the wall-forces, the smaller timestep cases have gone totally wrong...

Has somebody already seen this kind of problem ?

Sorry for my bad english, I hope I have made myself clear.

Thanks

Thomas


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