questions about reference control input

This forum if for discussing controls. Questions about how to implement controls in FAST are more appropriate to the CAE Tools forum.

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Cheng.Zhang
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Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Thu Jul 12, 2018 3:24 am

Dear Jason,

I want to use mlife to calculate the Damage Equivalent Load (DEL) as a performance index for my controller, and I read the topic concerning the Mlife and the Mlife_user, but I still don't understand how to calculate it.

For example.
If I want to calculate the DEL of blade flap-wise bending (RootMxb1) and tower fore-aft bending (TwrBsMyt).
How can I choose the parameters of NSlopes, SNslopeLst, BinFlag, BinWidth/Number, TypeLMF, LUlt.

I read a paper that you are a co-author https://arc.aiaa.org/doi/abs/10.2514/6.2008-1307. In this paper, you also use DEL as a performance index (e.g. 'Flap DEQL', 'Tower FA DEQL', 'Tower SS DEQL', 'LSS DEQL'). Could you tell me how to calculate the DEL of those item?

Thank you!

Best regards,
Cheng

Jason.Jonkman
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Location: Boulder, CO
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Re: questions about reference control input

Postby Jason.Jonkman » Fri Jul 13, 2018 12:11 am

Dear Cheng,

SNslopeLst is the slope of the SN curve. If not known for the specific components you are analyzing, it is often appropriate to assume the slope is 3, 4, or 5 for steel structures or 8, 10, or 12 for composite structures (you can choose one or use all three to assess the sensitivity). You can check the sensitivity of the binning based on your own results. TypeLMF and LUlt are discussed in our FAQ (https://nwtc.nrel.gov/FAQ#TypeLMF), but are unused if you are not using the Goodman correction to account for the influence of the load means on the DELs.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Kamineni.Akhil
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Location: Germany

Re: questions about reference control input

Postby Kamineni.Akhil » Thu Jan 31, 2019 4:16 pm

Jason.Jonkman wrote:Dear Cheng,

Yes, the structural DOFs can impact the performance of the controller.

The pitch angle can be negative, depending on definition of aerodynamic twist (the sum of the pitch + twist angles equals the angle between the chord and rotor plane). The twist is often defined such that "optimal" pitch is near zero degrees. It is common for the minimum blade-pitch angle to be saturated at the optimal pitch.
Jason.Jonkman wrote: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.


Best regards,

Dear Jason
Here, "Aero twist = pitch angle + twist angle", I believe twist angle meant structural twist. But is the Pitch angle same as "The blade-pitch angle is the angle (about minus z) from the hub to the blade coordinate system and is constant along the blade."? (Defined by you in topic: Definition of parameters)
By the definition, Pitch angle must be the angle between local elastic axis and the chord. Am I correct?
In the document "Definition of a 5-MW Reference Wind Turbine for Offshore System Development" Aero twist and structural twist are assumed to be identical so that the pitch angle will be zero along the blade. So if I use this data can I ignore pitch angle in my calculations? or Can you please provide different values of Aero and Structural twist?

But Dear Jason in the book "http://www.innwind.eu/-/media/Sites/innwind/Publications/Deliverables/DeliverableD1-21ReferenceWindTurbinereport_INNWIND-EU.ashx?la=da&hash=14D9E77225A5F451D153082579B808E7C9E4C75C" in the section 4.1.3 third paragraph, the definition of structural twist angle is mentioned as the angle between principal elastic axis and chord. I am baffled. Please help me go through this.
Thanks in advance.

Jason.Jonkman
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Location: Boulder, CO
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Re: questions about reference control input

Postby Jason.Jonkman » Thu Jan 31, 2019 4:35 pm

Dear Kamineni,

I've defined the structural twist, aerodynamic twist, and blade-pitch angle in the forum topic you quoted. I'm not sure how to define these any clearer.

I don't agree with your statement "Aero twist = pitch angle + twist angle" or your conclusion that pitch is zero. Instead, I would say that the angle (about minus z) from the rotor plane to the chord equals pitch + aerodynamic twist. Likewise, I would say that the angle (about minus z) from the rotor plane to the principle axis equals pitch + structural twist.

DTU uses the same definition I use, except that DTU defines the twist and pitch angles about positive z (so, you must flip the sign for use in FAST).

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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