Hi all,
I currently have troubles with creating a correct BeamDyn Blade model.
I run a comparison between OpenFAST (Ver. 2.2.0) and GH Bladed 4.8 (using multi-part blade). My baseline model is the Bladed model which has 5 blade parts. I disabled all flexibility besides blades and have fixed pitch angle and rotor speed as well as steady wind conditions to reduce model differences. As a model check I also run FAST with ElastoDyn blade model (All other modules are identical).
The rotor torque as well as the bending moments on the blade root fit well between Bladed and FAST with ElastoDyn while FAST with BeamDyn produces only a fraction of rotor torque.
The plots above are normalized to the mean values of the Bladed runs.
My BeamDyn model uses the trapezoidal quadrature with an order of interpolation of 5. rhoinf is set to 0. All other settings are on default.
I already tried several modification/simplifications without significant influence on the given sensors. Has anyone experienced similar differences and might give me an hint.
Best Regards,
Marvin
Hi Marvin,
I’m not sure what would cause these differences. Have you verified that the basic properties of the blade, e.g., total integrated mass, center of mass, and first bending in flapwise and edgewise directions are similar between the models? Does the blade have specific complications, e.g., offsets of mass or stiffness centers from the pitch axis, built-in precurve or presweep, or composite material couplings?
Best regards,
Hi Jason,
Thanks for the quick response.
I double checked the blade mass and moments of mass. The difference between BeamDyn and Bladed is below 0.2 %. ElastoDyn has differences in mass and center of gravity up to 2 % (do to interpolated values), mass moments fit well. So this shouldn’t be a problem.
The original Bladed model included an complex structure including precurve and presweep and different definitions for mass and structural axis. As I wrote above, I already tried many simplifications. These include the the reduction of complexity by redefining the axis (mostly with 0).
Both mass and stiffness matrix are in the presented plot defined without values on the off-diagonal.
Currently I use the same values for the stiffness in all three models. Is this correct?
Do you have any other ideas?
Best Regards,
Marvin
Dear Marvin,
When you say that you “redifin[e] the axis (mostly with 0)”, do you mean that the reference axis is straight (i.e., kp_xr, kp_yr are zeros)? If not, is the reference axis defined at least in terms of a smooth curve, e.g. by fitting a low-order polynomial?
When you say that are “us[ing] the same values for the stiffness in all three models”, that makes sense if each model is using the same reference curve (which would have to be straight and lie on the pitch axis (kp_xr = kp_yr = 0) due to simplifications in ElastoDyn); is that the case?
Best regards,
Hi Jason,
The model for the presented comparison, still includes pre-bending (kp_xr). But the pre-bend is a smoothed curve.
I actually use the same values in the models even so the axis in BeamDyn and Bladed is curved. Could you give me a hint on how to convert the stiffnesses correctly for the curved blade.
I also run simulations without pre-bend, where the structural and aerodynamic axis is on the pitch axis. This doesn’t had a mayor influence on the loads level, but the simulation showed additional excitations with BeamDyn (see plot).
(axis are normalized by same values as before)
Thanks a lot for your support.
Best Regards,
Marvin
Hi Marvin,
I’ve attached a document written by Emmanuel Branlard at NREL that describes how to modify the blade structural properties when changing the reference axis. E.g., you could apply the relationships documented here if the original reference axis is not smooth, and fitting a smooth curve to the reference axis results in offsets of mass and stiffness from the smoothed reference axis. This document also provides guidance for converting HAWC2 blade models to BeamDyn.
BeamDynInput_05_2020.pdf (252 KB)
Regarding the oscillations introduced by the straight blade, I’m not sure. Which mode(s) of the blade are oscillating? Does adding numerical or structural damping reduce the oscillations over time?
Best regards,
