Dear Jason,
I am working on the blade converter script from GHBladed to openfast format and have got some doubts about the definition of some aero shape and structural properties in AeroDyn and BeamDyn input files.
1. The definition of BlCrvAC/BlSwpAC/BlCrvAng in AeroDyn input file
In the documencation, the three variables represent the location offset of aerodynamic center. But in this topic, the neutral center of blade section is recommended to use for AeroDyn v15 which the figure in the topic comes from GHbladed. Definition of parameters - #9 by Jason.Jonkman
Which point should I use, the aerodynamic center(typically 25% of chord from the leading edge) or the neutral point(the elastic center)?
Another question is while BlCrvAv can describe the prebend shape, why BlCrvAng is introduced again in AeroDyn? Can I set BlCrvAng=0?
2. The definition of stiffness and mass matrix in BeamDyn input file
From my understanding, the shear stiffness, axial stiffness, bending stiffness and torsional stiffness could be diagonalized with respect to different principle axes respectively, the principle elastic axes for axial force and bending moments while the principle shear axes for twisting moment and shear-forces. So what kind of principle axes should I use for the local blade reference axis when defining the 6X6 stiffness and mass matrix? If I take the principle neutral axes as the local blade reference coordinate, is there necessary to keep consistence for all of the blade shape and structural properties, such as BlCrvAC/BlSwpAC in AeroDyn input, kp_xr/kp_yr in BeamDyn primary input and the 6X6 matrix in BeamDyn input?
Best regards!
Dear @Zhizhao.Yin,
OpenFAST allows the aerodynamic discretization and reference axis to be different from the structural discretization and reference axis. In AeroDyn, BlCrvAC, BlSwpAC, and BlCrvAng should be based on the reference point for the aerodynamic pitching moment (which we call the aerodynamic center in AeroDyn), which is likely quite independent from any center (tension center, shear center, mass center) in the structural model. BlCrvAng represents the out-of-plane angle of the airfoil due to curvature, as explained in the online AeroDyn documentation on readthedocs: 4.2.3. Input Files — OpenFAST v4.1.1 documentation. It is also possible to derive BlCrvAng from BlCrvAC, as discussed in the following question on OpenFAST GitHub issues: Obtaining Blade geometry inputs BldSwpAC, BldCrvAC · Issue #451 · OpenFAST/openfast · GitHub.
In BeamDyn, I would choose a reference axis (kp_xr, kp_yr, kp_zr) that follows the natural geometry of the curved blade (if the blade is straight, I would choose the pitch axis as the reference axis; but for curved or swept blades, I would start the reference axis at the root at the pitch axis and choose a curve that naturally follows the curve and sweep) and define the sectional mass and stiffness matrices with respect to that. For an isotropic material, the shear and torsion stiffness terms are coupled independently from the axial and bending stiffness terms, but for an anisotropic composite material, the sectional stiffness matrix can be fully populated. The document (BeamDynInput_05_2020.pdf) attached in my post dated Jul 20, 2020 in the following forum topic explains that well: Simulations using BeamDyn.
Best regards,
Dear Jason,
Thank you very much for the explanation. It is helpful.
With regards!