[Nek5000-users] Courant number

[nek5000-users at lists.mcs.anl.gov]

Hi to all,
from theory i know that courant number must be always less than 1. Now, 
in many examples, i saw that Courant number is fixed and sometimes the 
value is less than 1 and sometime greater than 1, for example in 
turbchannel, it is fixed to 2. So my question is, why? With a Courant 
number greater than 1, i should have instability?! How nek5000 estimate 
the Courant number? Nek in fuction of change the value?

Thanks