X Foil e Ncr.

[Ehstìkatzi]

Che significato ha il valore di Ncr impostabile in X Foil come descritto qui sotto ?

-- Transition criterion --

Transition in an XFOIL solution is triggered by one of two ways:

free transition: e^n criterion is met
forced transition: a trip or the trailing edge is encountered

The e^n method is always active, and free transition can occur
upstream of the trip. The e^n method has the user-specified
parameter "Ncrit", which is the log of the amplification factor
of the most-amplified frequency which triggers transition.
A suitable value of this parameter depends on the ambient
disturbance level in which the airfoil operates, and mimics
the effect of such disturbances on transition. Below are typical
values of Ncrit for various situations.

situation Ncrit
----------------- -----
sailplane 12-14
motorglider 11-13
clean wind tunnel 10-12
average wind tunnel 9 <= standard "e^9 method"
dirty wind tunnel 4-8