Xfoil Ncrit

[Alvarogy]

Hi, I am new to boatdesign.net and to XFoil.

I am using XFoil to calculate the lift to drag coefficients of a NACA 4412 for a DSS system on a 50 footer.
I've read that the Ncrit to be used for this cases ranges between 3 to 5, but the problem is that I don't know how to change the Ncrit in XFoil, and the default one is 9.
Anybody knows the command to do this?

Thank you very much,
Alvaro

 

[Dolfiman]

"Drunken-Sheep" answer in an other forum here below :

"Load your airfoil in XFOIL. Type OPER. TYPE VPAR. TYPE N <insert the value for N>.
N must be a number larger than 0 and can't be too close to 0 either or XFOIL will not converge. Small values for N mean very turbulent flow. Large numbers mean very laminar flow."

aerofoil problem -- CFD Online Discussion Forums https://www.cfd-online.com/Forums/main/120467-aerofoil-problem.html

 

[Dolfiman]

You have also this interface which can give you Cl, Cd, ... for Ncrit 5 and 9. :
NACA 4412 (naca4412-il) http://airfoiltools.com/airfoil/details?airfoil=naca4412-il#polars

 

[tspeer]

Hi, I am new to boatdesign.net and to XFoil.

I am using XFoil to calculate the lift to drag coefficients of a NACA 4412 for a DSS system on a 50 footer.
I've read that the Ncrit to be used for this cases ranges between 3 to 5, but the problem is that I don't know how to change the Ncrit in XFoil, and the default one is 9.
Anybody knows the command to do this?

Thank you very much,
Alvaro

Dolfiman has the commands correct. I would use even lower values of Ncrit, say 1<Ncrit<3. I also run cases with the boundary layer fully turbulent, by tripping the flow at 2.5% chord. This is done with the OPER->NPAR->X command. If you use a negative value (say, -0.025) transition will be referenced from the stagnation point rather than the leading edge.