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Courant number

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Old   October 4, 2010, 07:02
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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!
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Old   October 4, 2010, 08:55
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Quote:
Originally Posted by fireman View Post
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".
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.
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Old   October 4, 2010, 09:12
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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.
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Old   October 4, 2010, 12:49
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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?
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Old   October 4, 2010, 12:54
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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.
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