Learning Forum

[Losiola]

Hello evryone
Hope this messages finds you all in good health

i am solving a 2D airfoil simulation (transient simulation using fluent ) with KWSST turbulence model and for the Pressure-velocity coupling i chose COUPLED ,but under Controls > solution Controls there is Flow Courant number and which is by default 200 which confused me because :

From my previous experience  From the Articles i ve read on the internet they recommend CFL<=1 , but when i look on the fluent user's guide i found that Flow Courant Number has to be specified when using Coupled algorithm but it should not be less than 10 !!! Can anyone please help me and explain to me Why is this difference ? And How much Value should i Set for CFL?

Many Thanks

[singh33]

Dear Losiola,

As far as I have the knowledge of the courant number, There is conditon you mentioned for CFl <1 this depends upon the numerical stability and surely be less then but not strictly for implicit solver you can set it to 20 and for explicit solver you can have up to 5 only it os for Numerical or the solution stability nothing to do with flow physics fo problem.

 

Now coming to flow physics take an example of the flowpast a cylinder there is time period associated with it that has to to resolved with suitable number of cycles to accurately predict the solution from there you get a time step size, That time step then can be used for calculating a courant number which may or may not be similar to the courant no. set for numerical stability criteria.

 

our task is to match the two and I think the court no. mentioned above is the one obtain from flow physics as the numerical stability one is set by mentioning the time step size.

 

Please correct if I am wrong

[Amine Ben Hadj Ali]

That courant number is just a way to make the matrix more diagonal dominant and it has nothing to do with the time step size required for temporal discretization. It just adds a sort of cell based Under Relaxation: Large Value no relaxation small values large relaxations.