Definition of parameters

[Mehdi.Garmabi]

Dear Jason,

for my verification purposes, I am trying to model the DTU 10 MW onshore model using the latest version of the FAST (v8.15).
I have some question regarding the definition of new parameters specially parameters of ElastoDyn.
Is there a documentation, where these parameters are defined? If no, could please help me with following questions.
1) How is the PitchAxis defined? Is it the relative distance from leading edge to pitch axis? Are aerodynamic centers determined based on this PitchAxis? When yes How?
2) What is difference between the StrcTwst and AeroTwst?
3) Right now I am using the modules: ElastoDyn, InflowWind and AeroDyn v14 to model the WT. I have put the PreCone(1:3)=0, BlPitch(1:3)=0, UndSling=0, ShftTilt=0 and PitchAxis of the root station with cylindrical shape equals to 0.5. Afterwards I am checking the position and orientation of AeroDyn's nodes. I wondering, why even at time=0 the y-coordinate of the first node of the AeroDyn's model is not zero?

Thank you allot and best regards,
Mehdi

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M.Sc. Seyedmehdi Garmabi
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Leibniz Universität Hannover
Institut für Statik und Dynamik (Institute of Structural Analysis)
Appelstraße 9A
30167 Hannover (Germany)

Tel.: +49 (0)511 762-4703
Fax: +49 (0)511 762-2236
Mail: m.garmabi@isd.uni-hannover.de
Web: http://www.isd.uni-hannover.de
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[Jason.Jonkman]

Dear Mehdi,

Here are my answers to your questions:
1) The PitchAxis input parameter of ElastoDyn is documented in the FAST v8 ReadMe file: https://wind.nrel.gov/nwtc/docs/README_FAST8.pdf, based on the definition of AeroCent from the old FAST manual: https://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,

[Mehdi.Garmabi]

Dear Jason,

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

Best regards,
Mehdi

[KumaraRaja.Eedara]

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?

Also, could you please help me in understanding blade pitch angle.

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

[Jason.Jonkman]

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.

[KumaraRaja.Eedara]

Dear Jason,

Thanks for your quick response.
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

[Jason.Jonkman]

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,

[Simon.Wiedemann]

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)*AeroCent*Chord
and
BlSwpAC=cos(StrucTwst)*AeroCent*Chord
?

Best regards,

Simon

[Jason.Jonkman]

Dear Simon,

A similar question was discussed in the following forum topic: viewtopic.php?f=13&t=1699&p=8026.

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: https://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)*Chord*SIN(AeroTwst)
BlSwpAC = -(0.25-AeroCent)*Chord*COS(AeroTwst)

I hope that helps,

Best regards,

[Ignacio.Castello]

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,

[Jason.Jonkman]

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,

[Kacper.Wonidos]

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.

[Rocio.Torres]

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

[Jason.Jonkman]

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,

[Rocio.Torres]

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