Is a 20% forced transition etc. in xfoil similar to adding a vg?
I understand a 20% forced transition in xfoil, is tripping the boundary layer from laminar to turbulent. Isn't that what a VG does?
A classic shaped naca foil like 2415, has a max cl of 1.5. I understand that vg's usually add a cl of about 0.5-0.7.
So... if I force a transition to turbulent at about 15-20% of chord in xfoil, shouldn't the max cl of a classic foil go up from 1.5 to about 2?
It doesn't....?!?
No
It is more like putting a turbulator (such as a trip wire) there. Vortex generators will create a small vortex (hence the name) which puts much more energy into the boundary layer. XFOIL has no mechanism for simulating vortex generators.
Adding a forced turbulent transition will weaken the boundary layer downstream of its natural transition point, so it will tolerate less pressure rise. I assume you put them on the upper side: Now the airfoil will stall a bit earlier (at a lower angle of attack and lift coefficient).
15-20% is an unusual location for a turbulator, especially on the upper side. Their best location depends on the Reynolds number and the pressure distribution. On gliders, they make most sense on the lower side at around 80% of chord. On the upper side they might help a bit in part of the polar when placed at around 50%.
