# Courant number

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 October 4, 2010, 07:02 Courant number #1 New Member   Join Date: Mar 2009 Posts: 19 Rep Power: 17 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！

October 4, 2010, 08:55
#2
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Quote:
 Originally Posted by fireman 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.

 October 4, 2010, 09:12 #3 Member     Pranab N Jha Join Date: Nov 2009 Location: Houston, TX Posts: 86 Rep Power: 16 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. jyothsna k likes this.

 October 4, 2010, 12:49 #4 New Member   Join Date: Mar 2009 Posts: 19 Rep Power: 17 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? DungPham likes this.

 October 4, 2010, 12:54 #5 Member   anonym Join Date: Mar 2009 Posts: 65 Rep Power: 17 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. DungPham and MAK_JUST_University like this.

 July 28, 2016, 12:55 #6 Senior Member   Thomas MADELEINE Join Date: Oct 2014 Posts: 126 Rep Power: 11 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

 June 24, 2018, 11:59 Courant number sudden increase #7 New Member   Chayanit Nigaltia Join Date: Jan 2018 Posts: 29 Rep Power: 8 I started my simulation for dynamic mesh.I set maxCo=0.9. Initially everything went very good. But after that there is a sudden jump in Co from 0.9 to 6.0 How to resolve this issue?

 September 11, 2021, 11:33 what is sound speed #8 Member   Join Date: Aug 2018 Posts: 89 Rep Power: 7 hi, I want to calculate hydro-acoustics. Can anyone let me know what is sound speed in water I should choose?Thank u!