Out-of-plane tip displacements

Hi Jason/NREL,

I have a quick question regarding out-of-plane tip displacements in OpenFAST.

Consider a blade with initial precone and no wind load. The turbine is then subjected to an extreme wind load with the rotor parked giving a significant deflection of the blade. In terms of OpenFAST/ElastoDyn output, is the out-of-plane tip displacement (perpendicular to rotor / parallel to wind) measured from the initial position (option B) or from the rotor plane (option A). See sketch below (please forgive my poor drawing ability!).

I imagine the answer is A but I just wanted to confirm with you.

A related question. If the deflection was smaller (e.g. 50% of that shown in sketch), would the out-of-plane tip displacement assume a negative value?

Kind regards,
Aengus.

Dear @Aengus.Connolly,

In ElastoDyn, the undeflected blade is straight and aligned with the pitch axis and the out-of-plane blade deflection is measured relative from the pitch axis and can be both positive and negative. (Your illustration appears to show some upwind precurve/prebend, which is not possible in ElastoDyn, although other OpenFAST modules such as AeroDyn and BeamDyn support precurve/prebend).

Best regards,

Hi Jason @Jason.Jonkman,

Thank you for the clarification regarding the tip displacement output, this makes sense now.

At the risk of complicating matters further, if ElastoDyn does not support initial precone/prebend condition, does this not introduce some inconsistency between the structural (ElastoDyn) and aerodynamic (AeroDyn) model? Consider for example, a very low wind speed which does not introduce much blade deflection. Aerodynamic forces and moments will be computed on the basis of the precone/prebend shape of the blade, but these forces will be applied in a structural model whose initial/undisturbed position involves a straight line blade i.e. the positions of the blade nodes/stations, and orientation of blade elements, are different in both modules.

Note that this follow-up question is not urgent from my perspective, I am just curious. I have one or two other questions also but I will raise these in separate threads for clarity.

Kind regards,
Aengus.

Dear @Aengus.Connolly,

To clarify, ElastoDyn does consider blade precone, but it does not consider blade curvature such as prebend or precurve outboard of the pitch axis. If you want to model blades with built-in curvature, switching from modeling the blade structural dynamics in ElastoDyn to BeamDyn is preferred. BeamDyn can consider built-in blade curvature, as well as AeroDyn.

Best regards,