# What does the NCrit parameter indicate in a CFD analysis?

When analyzing an airfoil in the XFLR5 software I am asked to input some data regarding the free stream. For this, a dialogue box pops up that has the following options:

One field contains the expected Reynolds and Mach numbers for the flow.

The other field says Ncrit. What is this parameter, and what does it have to do with the trip location (top and bottom) which can be inputted together with it?

• I'm voting to close this question as off-topic because it asks about features of a particular software package, rather than about aviation, per se. – Ralph J Jul 3 at 17:25
• I believe the question has some merit since the feature it asks about is not exclusive to this software, but rather an established part of the larger field of computational fluid dynamics. – AEhere Jul 3 at 18:24
• You should make basic research (such as wikipedia) and indicate what is your current level of understanding so that answer can be as precised as possible and adapted to your current knowledge. – Manu H Jul 5 at 11:41

The whole setting group is called transition settings, and refers to the boundary layer transition point (from laminar to turbulent).

The forced transition trip locations are exactly that, expressed as a fraction of the chord, for the top and bottom surfaces. They are akin to placing a trip strip at those locations.

The Ncrit value is a measure of free flow turbulence and is used to simulate the transition location when no forced trip location is given. XFLR5 uses the same method as XFOIL, which is an adaptation of the $$e^N$$ transition theory. Note this linear theory loses its validity at a fluctuation level of 1 to 1.5 percent, which corresponds to an N-factor of 6.9 to 7.3, meaning it is not suitable for highly turbulent flows.

As to what this is important for, the turbulent or laminar nature of the boundary layer affects flow separation and viscous drag. A turbulent layer will generate more drag, but separate at higher airfoil angles of attack.

Where this gets really interesting is at low Reynolds numbers, where a laminar layer will detach, but become reattached if turbulence is introduced into the airflow ahead of the airfoil, such as by a turbulator. It will then remain attached if the turbulator is removed, displaying a hysteresis loop. The same effect will of course occur in the opposite direction, with a barely attached turbulent layer detaching and staying that way if the free flow turbulence decreases enough to cause it to turn laminar.

I must note that this effect is confined to low Reynolds numbers, on the order of $$10^4$$.

• The NCRIT is the same as in XFOIL. – Peter Kämpf Jul 3 at 10:42
• @PeterKämpf thanks, I was looking into that. It seems XFOIL uses a simplified version of the e^N theory, though understanding how exactly it differs would take me a while. – AEhere Jul 3 at 11:05
• Thanks a ton @AEhere !! – surya Jul 5 at 15:12