Dear Daniel,
In AeroDyn v15.03, which is what I’m presuming you are using, AeroDyn forced the axial (a) and tangential (a’) inductions to 1 and -1 at the tip node, respectively, when the Prantl tip-loss model was enabled (TipLoss = True), which is why Vrel equals zero at the tip when SkewMod = 1. The same thing should have also happened with SkewMod = 2, but a bug in AeroDyn v15.03 allowed a and a’ to differ from 1 and -1 at the tip.
The limit that a goes to 1 at the tip follows directly from the Prandtl tip-loss model. However, after further consideration, we decided that a’ should not approach -1 at the tip, but should instead by 0 when the Prandtl tip-loss model is enabled. In the newer version of AeroDyn that has not yet been publicly released: github.com/NWTC/AeroDyn (also discussed here: FAST v8 AeroDyn v15 convergence problems for low-speed, high-turbulence - #2 by Jason.Jonkman), among other changes, we fixed the problem with SkewMod = 2 and now force a’ = 0 at the tip when the Prandtl tip-loss model is enabled.
Normally we recommend to use SkewMod = 2 with WakeMod = 1, except for extreme skew angles (much greater than 45 degrees), where SkewMod = 2 is invalid.
Best regards,