# Definition of parameters

Dear Mehdi,

1. The PitchAxis input parameter of ElastoDyn is documented in the FAST v8 ReadMe file: nwtc.nrel.gov/system/files/FAST.pdf. Oddly, you need to specify PitchAxis = 0.25 if you want the aerodynamic center to lie on the pitch axis.
2. Strctwst in ElastoDyn is the structural twist, or orientation of the principle axes of bending. AeroTwst in AeroDyn is the aerodynamic twist, or orientation of the chordline.
3. See (1). Also, if you are looking at the source code, please note that the module-level inputs and outputs of AeroDyn v14 don’t fully comply with the FAST modularization framework (e.g. not all inputs are global). You may want to upgrade from using AeroDyn v14 to AeroDyn v15 if you want to review the source code. The module-level inputs and outputs of AeroDyn v15 do comply with the FAST modularization framework, and so, are much easier to understand.

I hope that helps.

Best regards,

Dear Jason,

Sorry for delay. I went to holiday after posting the question and later I forgot to reply to your post.

Best regards,
Mehdi

Dear Jason,

strtwst is the angle made by the principal elastic axis with respect to what?
similarly aerotwst is the angle made by the chord with respect to what?

It is just a single value defined for the blade and not defined for each section of the blade, right ?
Is it the angle between the chord at the root (or tip) made with the hub plane?

I’ve gone through the FAST manual, Posts and Wind Energy Hand book (by Tony Burton) but couldn’t quite reach to a conclusion. If already answered, please direct me to the post. Thanks.

Regards,
Kumara

Dear Kumara,

FAST uses the following definitions:

• The structural twist is the angle (about minus z) from the blade coordinate system to the local principal elastic axes of bending at a given cross section for the undeflected blade.
• The aerodynamic twist is the angle (about minus z) from the blade coordinate system to the chord at a given cross section for the undeflected blade.
• The blade-pitch angle is the angle (about minus z) from the hub to the blade coordinate system and is constant along the blade.

So, the sum of pitch plus twist is the angle (about minus z) from the rotor plane to the appropriate local axis (principle or chord) for the undeflected blade.

I hope that helps.

Dear Jason,

Since the aero-twst and structural twst are defined (as per your reply) relative to blade axis system, I referred to the FAST Manual (page10) for the definition of blade coordinate system. The definition of y-axis of the blade coordinate system is given in the manual as below.

“yb,i axis Pointing towards the trailing edge of blade i and parallel with the chord line at the zero-twist blade station. (i = 1, 2,or 3 for blades 1, 2, or 3, respectively)”. Which twist is the definition of above referring to? I guess it is aero-twst.

I find these definitions circular. Kindly let me know if i’m missing something.

Regards,
Kumara

Dear Kumara,

“Zero twist” can refer to either structural or aerodynamic. This just means that the blade system and local principle/aerodynamic exes are coincident when the twist is zero.

Best regards,

Dear Jason,

1. can you explain how the parameters BlCrvAC, BlSwpAC, BlCrvAng and BlTwist of AeroDyn v15 with ElastoDyn in FAST v8.16 are found in the attached coordinate system of Bladed v4.4 for the NREL 5-MW Baseline Turbine?
Till now I assumed that x=0, y=0, x’=0, y’ = 100(0.25+AeroCent) and Aerodynamic Twist = AeroTwst.
(y’ and x’ are in % of Chord)

2. Also is it correct, that in regard to AeroCent in FAST v7.02
BlCrvAC=sin(StrucTwst)AeroCentChord
and
BlSwpAC=cos(StrucTwst)AeroCentChord
?

Best regards,

Simon

Dear Simon,

A similar question was discussed in the following forum topic: Geometric Properties of the NREL 5-MW baseline blade - #3 by Paul.Schunemann.

Relating the AeroDyn v15 variables to your figure:
BlCrvAC = X
BlSwpAC = Y
BlCrvAng = not shown in your figure; see Figure 3 in the draft AeroDyn User’s Guide and Theory Manual for more information: nwtc.nrel.gov/system/files/Aero … 15.04a.pdf.
BlTwist = Aerodynamic Twist

BlCrvAC and BlSwpAC are related to AeroCent and AeroTwist from FAST v7.02 as follows:
BlCrvAC = -(0.25-AeroCent)ChordSIN(AeroTwst)
BlSwpAC = -(0.25-AeroCent)ChordCOS(AeroTwst)

I hope that helps,

Best regards,

Dear Jason,
I am trying to simulate a 5 MW model offshore with fast 7.0 (for compatibility with previous projects) but I have a problem with the value of AEROCENT.
FAST User’s Guide indicates that AEROCENT is limited to values between 0 and 1. Moreover, it states that if the pitch axis in the turbine blade does not actually pass through the airfoil section at 25% chord we have to use the following equation:
AEROCENT=0.25-[(fraction of chord from leading edge to actual pitch axis)-(fraction of chord from leading edge to actual aerodynmic center)]
The problem I have is that in some airfoils the value of AEROCENT is negative. For example in some airfoils the pitch axis lies at 75% chord and the aerodynamic center lies at 25% chord, so the value of AEROCENT is -0.25.
What is the solution for this problem?
Best regards,

Dear Ignacio,

You can just go into the FAST v7 source code, comment out the call to check the value of AeroCent and trigger an error, and recompile FAST. That check was added to prevent the modeling of unphysical blades, but the check is not perfect–and is unneeded in your case.

Best regards,

Dear Jason,
Going into the FAST v7 source code and triggering an error works for me.
Thank you so much for your response.

Dear Jason,

I am trying to adapt the OpenFAST model of a V27 turbine to the new turbine object in Orcaflex. I am facing some difficulties however to fullfil the blade geometry data since two of the inputs required are the offset of the neutral axis and the aerodynamic centre from the leading edge of the aerofoil (x and y components given with respect to the geometry frame placed at the leading edge, as a percentage of the chord) and I am not sure of how to calculate these from the BlCrvAC, BlSwpAC and BlTwist parameters in FAST.
¿Could you help me with that?

Thank you very much in advance.
Best regards,
Rocío

Dear Rocío,

BlCrvAC and BlSwpAC input parameters in AeroDyn define the aerodynamic center of each airfoil cross section with respect to the blade coordinate system, as distances from the pitch axis. The actual location of the leading edge is not defined or used by AeroDyn (but often the aerodynamic center is assumed to lie at 1/4 chord). The neutral axis in ElastoDyn is assumed to lie on the pitch axis.

Best regards,

Hi Jason,

Thank you for your answer. I was hoping I could derive those inputs somehow from FASTs inputs, but I see it is not that easy.
I’ll take 25% of the chord for the aerodynamic centre for now exept for the root (50%) and, if I understood correctly, can I make neutral axis= PitchAxis parameter in ElastoDyn_Blade.dat?

I also need the CoG coordinates as percentage of the chord from the leading edge…Some idea where can I get that data from?

Have a nice weekend.
Best regards,
Rocío

Hi Rocío,

What I said is that ElastoDyn assumes the neutral axis and pitch axis are coincident. ElastoDyn also assumes the center of gravity of each cross section is coincident with the pitch axis.

Obviously, the blade structural model of ElastoDyn is quite limited (bending only, no offsets of the mass or elastic axis from the pitch axis, etc.). We use the BeamDyn module of FAST / OpenFAST for modeling the blade structural dynamics with higher fidelity.

Best regards,

Hi again Jason,

I wrote some time ago because I was trying to adapt an V27 OpenFAST model to the new turbine Orcaflex feature and I had some doubts about the definition of certain parameters you very kindly clarified.
In that time you told me the neutral axis in ElastoDyn is assumed to lie on the pitch axis. As I had the corresponding “…_ElastoDyn_Blade.dat” file containing the pitch axis list I took that data. However, even then the distribution of the pitch axis there specified seemed a bit weird to me, since it placed the pitch axis for the root cylinder and thicker, near to root profiles, already at ~26% of the chord from the leading edge instead of 50% for the cylinder and near to 50% values for the inmmediate following sections, which is what would have seem reasonable to me.
After that, checking on the Orcaflex adaptation of the 5 MW NREl tubine Orcina did, I realized they had taken Pitch/netral axis values from the list you provided in a post of the following thread: NREL 5MW Rotor Geometry (Fri Jan 28), but these values are different from those other specified in the NRELOffshrBsline5MW_Blade.dat file , downloaded with the F8/OpenFAST archive. The values on the list you provide in the forum post make sense to me, while similarly to the case of the V27 FAST model I am trying to adapt, the list on the “…blade.dat” file don’t.
I was wondering if there is some transformation here needed that I am missing…Why are the values for the pitch axis distribution different on the list you provided on the post compared to those in the 5MW baseline files?
I am attaching the files I make reference to.

Best regards,
Rocío Torres

Dear Rocio,

Yes, indeed this is confusing. Unfortunately, the two definitions of PitchAxis are not the same. The forum post you reference used the following definition (see NREL 5MW Rotor Geometry for details):

PitchAxis = 0.25 - AeroCent - AeroRef

But in ElastoDyn within FAST v8, PitchAxis is defined as (from the FAST v8 ReadMe file: wind.nrel.gov/nwtc/docs/README_FAST8.pdf):

PitchAxis = 0.5 – AeroCent

Regardless, the PitchAxis variable in the ElastoDyn module of FAST v8 is not used when AeroDyn v15 is enabled. This input in ElastoDyn is used to define the aerodynamic center offset from the pitch axis for AeroDyn v14. But when AeroDyn v15 is enabled, the aerodynamic center is defined within AeroDyn v15 instead of ElastoDyn. Once AeroDyn v14 is removed from OpenFAST (AeroDyn v15 is meant to replace AeroDyn v14), the PitchAxis input from ElastoDyn will be eliminated as well.

Best regards,

Hi Jason,

Thank you. But I am afraid I am still confused…Then the “PitchAxis” variable in the ElastoDyn module of FAST v8 is NOT the fraction of the chord from the leading edge to the actual pitch axis, is it?

Is there any way I can calculate the actual pitch axis from the information contained in the FAST v8 model files I have for the V27? I guess the only way would be to have the “Aeroref” values and use the first equation, but I don’t have those for the first blade span sections…

Thank you!
Best regards,
Rocío

Dear Rocio,

Correct. PitchAxis in ElastoDyn is not necessarily the fraction of the chord from the leading edge to the actual pitch axis.

The PitchAxis input to ElastoDyn of FAST v8 / OpenFAST, as well as the old AeroCent input in the structural model of FAST v7, only determine where the aerodynamic center is relative to the pitch axis. The actual leading and trailing edge locations are not needed by the software, and so, are not derivable from the data specified in the FAST input file(s).

Best regards,

Hi Jason,

Crystal clear now. Thank you very much.

Best regards,
Rocío