## Rotor-disk-averaged relative wind velocity in the hub coordinate system

Discuss the theory and modeling of rotor aerodynamics.

Moderator: Bonnie.Jonkman

Borja.Sopelana
Posts: 3
Joined: Mon Feb 25, 2019 4:01 am
Organization: UPV-EHU
Location: Spain

### Rotor-disk-averaged relative wind velocity in the hub coordinate system

Dear all,

I understand that output RtVAvgxh is computed as the disk-averaged relative wind speed (wind speed minus structural speed).
But, What factors are taken into account to calculate the structural speed?

Is there any configuration (DOFs enabled/disabled) with which RtVAvgxh only depends on the average of the wind speed?

Best regards,

Jason.Jonkman
Posts: 5738
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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### Re: Rotor-disk-averaged relative wind velocity in the hub coordinate system

Dear Borja,

The rotor-disk-averaged relative wind speed in AeroDyn is calculated by taking the relative wind speed at each aerodynamic analysis node along each blade and averaging across all nodes and all blades. So, all structural DOFs that impact blade motion (blade, drivetrain, nacelle, tower, platform/substructure) are included in the calculation.

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

Borja.Sopelana
Posts: 3
Joined: Mon Feb 25, 2019 4:01 am
Organization: UPV-EHU
Location: Spain

### Re: Rotor-disk-averaged relative wind velocity in the hub coordinate system

Dear Jason,

Just one doubt:
I'm using the v8 version of FAST and AeroDyn15. In the effective wind (RtVAvgxh) and in the aerodynamic torque (RtAeroMxh) that I get, a kind of noise appears. Looking at the frequency response, I don't think it's due to the vibrations of the tower or the blades.

Then, Can it be the aerodynamic noise? And, Is it possible to disable/activate this flag in the latest version?

Best regards,

Jason.Jonkman
Posts: 5738
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Rotor-disk-averaged relative wind velocity in the hub coordinate system

Dear Borja,

The CompNoise flag in FAST v7 was used to enable an aeroacoustic module (which is no longer available in FAST v8 or OpenFAST). I think what you are referring to is numerical "noise".

I'm not sure I can comment without seeing the "noise" you are referring too, but I would suggest taking a PSD of the RtAvgxh and/or RtAeroMxh to identify the frequency content and source of the "noise". I can foresee these aerodynamic outputs to be noisy because they are outputs associated with the aerodynamic applied loads, not filtered by the inertia of the rotor.

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

Borja.Sopelana
Posts: 3
Joined: Mon Feb 25, 2019 4:01 am
Organization: UPV-EHU
Location: Spain

### Re: Rotor-disk-averaged relative wind velocity in the hub coordinate system

Dear Jason,

Studying the frequencies, we see that it is noise. Then, it will be that they are not filtered by the inertia of the rotor.

I'm working with a simplified model, introducing the aerodynamic torque. My model does not filter these frequencies. In FAST, the aerodynamic torque is filtered before it is introduced into the model? Or does the model filter it intrinsically?

Best regards,

Jason.Jonkman
Posts: 5738
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Rotor-disk-averaged relative wind velocity in the hub coordinate system

Dear Borja,

No, FAST does not filter the aerodynamic loads before they are applied to the structure. But the inertia of the structure may act as a filter when examining the reaction loads.

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