Dear Spencer,
Yes, I’m familiar with the problem, as explained in the report you mentioned. I have talked to other modelers in Europe and heard that this problem has been seen in many different turbines and with different aero-elastic software. I’ve also met a PhD student at DTU in Denmark that researched the causes/solutions of this problem. My understanding is that the current belief is that the instability would likely not occur in the physical world and that the aero-elastic software only predict a problem due to simplifications in how the software treat the dynamics of deep stall. My understanding is that the industry’s current approach to dealing with this problem is to either (1) bypass it by choosing yaw errors that don’t result in the instability or (2) increase the structural damping in the blade edge / tower side-to-side mode until the instability goes away. For (1), instead of limiting the range of yaw errors, one may simply ignore the case that causes the instability (in your case this could mean dropping the 35-degree case).
Best regards,