Coupled blade modes in FAST

Dear @Cao.Yuming,

I agree that torsion can be important for modern wind turbine blades, but adding a blade torsion DOF to ElastoDyn is much more difficult than you’d expect. Some have attempted to capture blade torsion through modification of ElastoDyn or similar software such as Flex, and were not successful at obtaining an accurate responses. Simply adding a torsion mode on top of the existing flapwise and edgewise modes would not produce good results in most cases. To capture blade torsion properly, you must capture:

  • Torsion-shear coupling, including offsets of the shear center from the pitch axis
  • Possible couplings to other modes due to composite effects
  • Nonlinear dynamics, including centrifugal stiffening of the torsion mode

NREL has not yet been funded to work on such improvements, but rather, has focused on introducing BeamDyn and resolving its known issues.

We have thought about adding a simple quasi-static torsion correction to ElastoDyn, e.g., by adding an elastic twist angle proportional to the pitch moment. We are still looking at ways to fund such an effort. While not highly accurate, this would be relatively simple to implement and may provide decent first-order effects of torsion.

Best regards,