## Fluids

#### Acoustic Courant Number in Ansys CFX

• Christijaaan
Subscriber
Hi everybody,ndoes anyone know the equation used in CFX to calculate the acoustic mach number? I know this one here:nCFL = a * delta_t / delat_x ... with a = speed of sound, delta_x = related grit distance, delta_t = simulation time stepnThe problem is... I calculate a simualtion time step be means of the above equation as a function of a CFL number of for example 1. But later on the results show that the courant number calculated by ansys is 3. What does it mean? nMany thanks in advance for any kind of helpnChristian Lehrn
• DrAmine
Ansys Employee
It is based on the speed of sound.nCheck the documentation: 1.1.3.15. Courant Numbernn
• Christijaaan
Subscriber
thanks for your fast reply. Yes, that's what I already wrote. Maybe again in other words... I was wondering why there is a factor 3 between the Ansys Courant number (simulation result) and the manually determined courant number (determined by means of the simulated speed of sound and the simuation time step). Is it because it is 3D? Thats currently our only hypothesis.... Thank you!n
• DrAmine
Ansys Employee
Probably the length scale used for calculation of Courant Number.n
• DrAmine
Ansys Employee
In general Courant Number for a multi purpose CFD code one generally uses fluxes over faces of a cell to get the the velocity scale right.n