Stress Analysis of Blades and a Tower

Dear Pravin,

If the structural-twist is zero, than the local blade coordinate system at the root (xLb/yLb/zLb) is identical to the blade coordinate system (xb/yb/zb), with no transformations needed. However, at other stations along the blade, the deflection of the blade can cause a small difference between the local blade coordinate system (body-fixed to the deflected blade at each cross section) and the blade coordinate system (which is constant along the blade, independent of deflection). So, while you no longer need to correct for the structural twist, you may still have to consider the deflection, or accept an error, which may be negligible if the two stations are nearby each other or the deflection is small.

I’m not sure I understand your second question (“b”).

Best regards,

Hi Jason

Thanks for the clarification on twist angle and transformation. My point “b” is that referring to airfoil below, the forces (lift, drag and their resultants in normal and tangential directions) are acting on aerodynamic center O as per image below. But there is no material at this point as we have shear webs passing through the hollow of the blade. So in FEA tool, there will not be any node at aerodynamic center. So to represent these forces should we distribute them evenly on the airfoil boundary nodes?

loadpoint.png

-Thanks and Best Regards
Pravin

Hi Jason

Do you have any feedback on my below question about where can I apply the applied forces in FEA as there is no material at the aerodynamic center ?

-Regards
Pravin

Dear Pravin,

I’ve not had personal experience of taking the lumped load output from a beam-type model and applying that to an 3D FEA model to find stress/strain. I would guess that applying the lumped loads directly would result in unrealistic stress concentrations. I’ve read the following paper where they recommend to linearly distribute the load along the chord, but I cannot say whether that is a typical approach. Hopefully someone else with more experience in this area can comment.

Berg, J. C.; Paquette, J. A.; and Resor, B. R. “Mapping of 1D Beam Loads to the 3D Wind Blade for Buckling Analysis.” 52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference. 4-7 April 2011, Denver, Colorado. AIAA 2011-1880.

Best regards,

Thanks a lot Jason for great reply and really Thankful to your ever helpful ways !!

-Regards
Pravin

@Jason.Jonkman
Dear Jason,

Excuse me, I have a question to ask you, please help me, thank you~

For the Blade 1 Local Span Loads of NREL 5MW, which and where is the acting location of the outputs of ElastoDyn, such as SpniMLzb1, SpniFLxb1, SpniFLyb1, SpniFLzb1 for the blade segments i ? In my understanding, the location is located at corresponding aerodynamic blade analysis node, the center of the blade segment i, along the local blade-pitch axis. Is my understanding correct? However, I find it is strange that the blade-pitch axis passes through airfoil section at 0.25 chord. That is, at root, both AeroCent and blade-pitch axis are 0.25 chord rather than 0.5 chord.

Thank you for your time and every assistance.

Baoxuan

Regards,

Dear @Baoxuan.Wang,

The local blade span outputs from ElastoDyn are the reaction loads at the corresponding structural output node dictated by ElastoDyn inputs NBlGages and BldGagNd. In the NREL 5-MW baseline models provided by NREL, NBlGages = 1 and BldGagNd = 9, so Spn1 corresponds to the 9th structural analysis node. The locations of the blade structural analysis nodes are written to the ElastoDyn summary file and node 9 is at 50% span.

The structural analysis nodes in ElastoDyn lie along the pitch axis. But the pitch axis does not need to be at 25% chord. The aerodynamic analysis nodes can be offset from the pitch axis. As of FAST v8 and OpenFAST, AeroCent is no longer an input in ElastoDyn. When AeroDyn v15 is enabled (CompAero = 2), the aerodynamic analysis node locations are specified in AeroDyn and can be offset from the pitch axis.

Best regards,

Dear @Jason.Jonkman
Thank you for your reply. In fact, I want to know the location (in each airfoil) of the structural analysis nodes about the local blade span outputs from ElastoDyn. It seems that the structural analysis nodes in ElastoDyn lie along the pitch axis, at 25% chord of corresponding airfoil?
Regards,

Dear @Baoxuan.Wang,

No, 25% chord is not assumed. Other than when aeroacoustics or AeroDyn surface visualization is enabled, the location of the leading and trailing edge of an airfoil is not information that is needed by OpenFAST. The structural analysis nodes lie along the pitch axis in ElastoDyn. And the aerodynamic analysis nodes (origin of the lift/drag/pitching aerodynamic loads) are specified in AeroDyn v15 as offsets from the pitch axis. The leading and trailing edge are only defined when specifying the airfoil geometry in the airfoil data file, only used for aerodynamic visualization and aeroacoustics.

Best regards,

Okay, this is very helpful, thank you very much :+1:

Dear @Jason.Jonkman
I’m still confused about the position of the pitch axis of NREL 5MW baseline WTB.
I want to map the pitching moment MZ of each blade analysis node (FAST ElastoDyn outputs: Spn1MLxb1,Spn2MLxb1,…Spn9MLxb1 ) at the pitch axis to the 3D FEM. Thus, I think I need to know which point in the airfoil is the pitching moment calculated based on?

Dear @Baoxuan.Wang,

See my post dated Jan 28, 2011 in the following forum topic: NREL 5MW Rotor Geometry.

Best regards,

Dear @Jason.Jonkman
Thank you~
sincerely

Dear @Jason.Jonkman
In my understanding,
PitchAxis = Fraction of chord from leading edge to pitch axis;
and the local blade span outputs from ElastoDyn (SpniMLzb1, SpniFLxb1, SpniFLyb1, SpniFLzb1; i=1-17) lie along the pitch axis ;
that is to say: the local blade span outputs from ElastoDyn for structure analysis nodes (1-17) lie on the range from 0.253chord to 0.375chord, at the blade span from 1.809m to 59.691m, right?


But in your post dated Jan 28, 2011 in the following forum topic: NREL 5MW Rotor Geometry, I find the given values of PitchAxis are from 0.5chord to 0.375chord.
image

image

Dear @Baoxuan.Wang,

The local span outputs at the pitch axis for the NREL 5-MW baseline wind turbine blade lie between 0.5chord and 0.375chord.

In the ElastoDyn blade input file, there is only a single input PitchAxis, and AeroRef is not entered. Instead, AeroRef is defaulted to 0.25 internally within ElastoDyn, thus: AeroCent = 0.5 - PitchAxis. So the PitchAxis input to ElastoDyn should be interpreted a bit differently than the PitchAxis column explained in NREL 5MW Rotor Geometry.

But please note that the PitchAxis input to ElastoDyn is only used by the older AeroDyn v14, which will soon be depracated. When that happens, the PitchAxis input will be removed from ElastoDyn altogether as it will not longer be needed. When AeroDyn v15 is used, the aerodynamic center is specified relative to the pitch axis via AeroDyn v15 inputs BlCrvAC and BlSwpAC.

I hope that helps.

Best regards,

Dear @Jason.Jonkman
Thank you for your reply.
As for the local blade span outputs from ElastoDyn (SpniMLzb1, SpniFLxb1, SpniFLyb1, SpniFLzb1; i=1-17) lie along the pitch axis, I seem to find their span-position are located at the distributed blade aerodynamic Node as follows:

rather than

I am not sure.
Best regards,

Dear @Baoxuan.Wang,

NBlInpSt in ElastoDyn is the number of blade input stations for specifying the distributed mass and stiffness sectional data. Internally within ElastoDyn, these data are interpolated to the blade structural analysis nodes. When AeroDyn v14 is enabled, the blade structural analysis nodes in ElastoDyn and the blade aerodynamic analysis nodes in AeroDyn v14 are aligned and defined by the distributed blade aerodynamic data table in AeroDyn v14 (need not be evenly distributed). When AeroDyn v15 is enabled, the blade structural analysis nodes in ElastoDyn and the blade aerodynamic analysis nodes in AeroDyn v15 are independent, with the structural analysis nodes dictated by ElastoDyn input BldNodes (evenly distributed) and the aerodynamic analysis nodes dictated by AeroDyn v15 input NumBlNds and the distributed blade aerodynamic data table (need not be evenly distributed).

Best regards,

Dear @Jason.Jonkman
Thank you for your time and constant assistence, this is very helpful. :grinning:
By the way, about the structural damping, If I want to get the consistency between the FAST results and the three-dimensional finite element model results, should the modal damping be taken as 0?


Best regards,

Dear @Baoxuan.Wang,

Only if the structural damping in your 3D FEA model is zero.

Best regards,

Dear @Baoxuan.Wang,

Can you clarify what loads you are transferring from FAST/OpenFAST and how you are applying them to your 3D blade FEM model?

Best regards,