Pre-Bent Blades

As they have become longer and more flexible, many commercial blades have been designed pre-bent (in the flap-wise direction) in order to avoid tower strikes.

I have been trying to find a relevant posting on this forum. If you know of one, can you post a link here?

I suspect that the modifications to FAST would be relatively straightforward since blade coning is already accommodated. The mode shapes and frequencies of a pre-bent blade are likely to be similar to a straight blade for small amounts of pre-bend, so it could simply be a matter of correctly superimposing the scaled modal degree-of-freedom on the correct initial shape of the blade.

If you know of somebody who has modified FAST to incorporate pre-bent blades, please can you post details here?

I think it’s probably somewhere in SUBROUTINE RtHS but I haven’t been able to find it yet. Perhaps you know where this calculation is done. Please could you post a refence (subroutine name, line number, sample of code) here showing where this is calculated in FAST?

I believe this is part of the plan for modifying FAST prior to release of FAST 8. How far has this advanced, please?

Kind Regards,

Mark

Dear Mark,

The approach you propose to model blades with pre-bend may be fairly straightforward, but I question the accuracy you’d obtain from the result. Here are a couple things to consider:

*Built-in curvature and sweep in the blade will lead to coupling between flapwise and edgewise bending and torsion which would not be captured by the approach you propose.
*The FAST model has no rigid-body blade-pitch DOF. When the blade-pitch angles change because of commands from the pitch controller, because there is no blade-pitch DOF, there are no associated pitch rates or accelerations computed by FAST and the inertia, Coriolis, and other potential loads from the pitch motion will not be accounted for. The magnitude of these ignored terms will depend on how fast the blades pitch and on the degree to which the blades are precurved and preswept and on how much they deflect.

Issues such as these are what has kept us at NREL from adding a simple model for blades with pre-bend. The dynamics of a composite spinning blade with bending, torsion, pre-bend, and large deflection is quite compilicated, which is why it has taken a while for NREL to add these to FAST. However, NREL is working on introducing these features, as described in my Nov 26, 2012 post in the forum topic found here: http://forums.nrel.gov/t/coupled-blade-modes-in-fast/314/1.

Best regards,