Open winder365 opened 3 months ago
Dear @winder365,
Here are my responses:
Best regards,
Dear @jjonkman ,
Thank for your reply.
(1) As your mentioned, the shear center is the point in the cross section where a shear force will not induce torsion. So, if the reaction forces are applied at the shear center, the cross section will not twist. The force you mentioned is in narrow sense or in broad sense? Namely, is it the force like the lift and drag, or it also include the moment? My understand is that, if the force such as lift and drag are applied on the shear center, it will not result in torsion. However, if the moment such as the pitch moment is applied on the shear center, it will result in torsion. Right?
(2) As your mentioned, the offset between the aerodynamic center and shear center can induce torsion. Besides in the post I mentioned above, the offset between the aerodynamic center and pitch axis can also induce torsion. This confused me a lot. As mentioned in the post, the extra moment caused by the aerodynamic force cross with the offset is accounted for within the internal spatial mesh-to-mesh mapping algorithm used by OpenFAST, such as indicate in the following code:
DO j = 1,2 ! number of nodes on dest
jNode = Dest%ElemTable(ELEMENT_LINE2)%Elements(jElem)%ElemNodes(j)
DisplacedPosition = Src%Position(:,i) + SrcDisp%TranslationDisp(:,i) &
- Dest%Position(:,jNode) - DestDisp%TranslationDisp(:,jNode)
torque = Src%Force(:,i)
torque = CROSS_PRODUCT( DisplacedPosition, torque )
Dest%Moment(:,jNode) = Dest%Moment(:,jNode) + torque*MeshMap%MapLoads(i)%shape_fn(j)
END DO
What the var DisplacedPosition
includes? As I understand, it includes the offset between the aerodynamic center and pitch axis (namely the BlSwpAC
), and includes the component due to the blade deformation (especially the edgewise deflection). Right? Besides, what the var torque
includes, does it only include the the aerodynamic force crossed with the moment arm between the AC and pitch axis, as well as the pitching moment (Cm) term.
(3) I am not very clear about the effect of so many offsets on torsion (such as the offset between the aerodynamic center and shear center, the offset between the aerodynamic center and pitch center, and the offset between the shear center). As your mentioned, the offset between the aerodynamic center and shear center can induce torsion. What about the offset between the aerodynamic center and pitch center, and the offset between the shear center? Do these offsets affect torsion and how these offsets affect torsion?
Look forward to your direction.
Dear @winder365,
Here are my responses:
Best regards,
Dear @jjonkman ,
Thank you for your reply. I still have two question:
(1) As we discussed above, shear center is important in blade deformation calculation, but how can I get the position of the shear center along the blade through the 6*6 stiffness matrix? just like this:
(2)How the BeamDyn code consider the effect of the shear center position or shear center offset? Will the BeamDyn code itself calculate the position of the shear center when make simulation?
Dear @winder365,
Regarding (1), the shear center can be inferred from the shear and torsion elements of the 6x6 cross-sectional stiffness matrix. My forum post dated July 20, 2020 contains an attachment, _BeamDynInput_052020.pdf, that explains how the 6x6 stiffness matrix would change for different reference axes. To see how the shear center effects the shear and torsion elements of the stiffness matrix, see Section 2.3.
Regarding (2), the BeamDyn code makes use of the full 6x6 cross-sectional mass and stiffness matrices, and so, does not need to calculate and make use of the shear center directly.
Best regards,
Dear @jjonkman,
I have read a post on the forum about "Aerodynamic center offset effects on blade torsion" (https://forums.nrel.gov/t/aerodynamic-center-offset-effects-on-blade-torsion/4841), and I have known that the offset between the aerodynamic center and pitch axis center will affect the moment arm and the torsional deformation of blade.
However, some questions still confuse me: (1) The pitch axis is actually a virtual axis, why it will have effect on the torsional deformation of the blade? For some special case, such as backward swept blade, the pitch axis is actually far away from the aerodynamic center. (2) What is the role of the shear center (of the blade section) when calculate the blade deflection (especially the torsion deformation). Will the pitch moment (from the aerodynamic force crossed with the moment arm as well as the pitching moment (Cm) term) eventually act on the shear center when calculate the blade torsional deformation in BeamDyn? (3) Does the offset between the aerodynamic center (or pitch axis center) and the shear center affect the calculation of the torsional deformation in BeamDyn?
Thank for your attention