Karthik R
Administrator

Hello Joseph,


CFL condition, in CFD, addresses the stability of your numerical simulation in a finite volume approach. Put simply, this condition must be satisfied for a transient simulation to be stable, especially when using an explicit time-integration scheme. Depending on the minimum size of your grid, the CFL condition controls the time step you can use to march forward in time. If your time step is greater than the 'minimum' time step size specified by the CFL condition, your solution with diverge.


It is defined as del_t <= (C * del_x / v_fluid) where,  del_t - time step size, del_x - minimum grid size, v_fluid - fluid velocity scale, C - Courant number


To learn more on how CFL condition is derived, please refer to this lecture notes from MIT opencourseware. 


https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-90-computational-methods-in-aerospace-engineering-spring-2014/numerical-methods-for-partial-differential-equations/upwinding-and-the-cfl-condition/1690r-the-cfl-condition/


Please also search for 'Courant Number' in ANSYS Fluent Users and Theory guides to read more about how Fluent handles Courant number for various models (especially check out the explicit VOF model).


Hopefully this equips you with a little background on Courant number.


Thanks.


Best Regards,


Karthik