RMS Courant Number vs MAX Courant Number
 

Hi all,

sorry for confronting you guys with such a stale topic.

I thought when everybody is talking about the Courant Number, such as CFL has to be less than 1 etc. ...
it is addressed to the max Courant number.
Now I've read the following sentence in Ansys CFX, 12.1 Help:
"For accuracy, the average Courant (or CFL) number should be in the range of 0.5-1. "
What does it mean? Do I have to consider the RMS Courant number.

Help will be appraciated

Thanks in advance.
zzz

This is related also to mesh size.

Since the courant number is calculated using dx (distance between cell centre or interface depending on solver) what this means is that in regions where the flow is fast you need smaller cells to achieve the courant number value of between 0.5 and 1.

Hi
 

IN my transient simulation of homogeneous multiphase problem involving free surface also, i have RMS courant Number = 0 , Max Courant nUmber = 0 :eek:, is it OK??? If not what could be the problem??

Thanks

Accuracy and stability. If you use an explicit scheme and violate the CFL condition then stability should suffer. However the Courant number should be as close to unity as possible for accuracy. When it comes to accuracy then I guess that you could use an average value for the entire mesh.