# Aerodynamic Torque

Hello ,

I wanted to reproduce Aerodynamic Torque from mesures given by FAST :

J_Rotor * RotAccel + RotTorq1000 = T_aero and it didnt give me like T_aero= 0.51.225pi(R^3)RotCqTotWindV .

If you have an idea about that ?

Sincerely .

Dear Younes,

First of all, your equation for T_Aero should involve the wind speed squared.

*The rotor is rigid (so, J_Rotor is constant). That is, FlapDOF1 = FlapDOF2 = EdgeDOF in FAST.
*The support structure is rigid (so, the rotor acceleration is solely related to RotAccel. That is, most structural DOFs in FAST are disabled.
*The aerodynamic loads are computed quasi-steadily. That is, StallMod = STEADY and InfModel = EQUIL in AeroDyn.

Your equation should work with these simplifications.

More information about the drivetrain model in FAST is available in the following forum topic: http://forums.nrel.gov/t/resistant-moment-of-the-rotor-and-of-the-electric-generator/408/1 (in this forum post, RotTorq is identified as LSShftTq; both are equivalent).

Best regards,

What can you advise me to use as Taero in my work : Taero = 0.51.225CqWind^2R^3 or that calculated from FAST Taero = Jr*RotAccel + Shaft_Torque ?

Which expression is translating reality ?

Best regards .

Dear Jason ,

I did what you requested me , and i found that the two torques are equal in steady state , but different in transient regime . So i suppose that maybe if there is no acceleration effect , equations are equals , and this can be traduced as :

• Maybe the problem is in the value of acceleration RotAccel

• Maybe in the inertie Jrotor (i take it from .fsm file )

Sincerely

Dear Younes,

Your equation for Taero is now missing the pi.

Both equations are valid for steady-state. I’m not sure what problem you are referring to in the transient regime and I can’t advise you which to use because I don’t know what you want Taero for.

Best regards,

Dear Jason ,

The problem im facing in the transient regime is that : Taero given by the equation Jr*RotAccel + LSShftTq is higher than Taero given by 0.5RoPi*(R^3)*(V^2)*RotCq when RotAccel is not equal to zero .

But they have the same forme and they are equal in mean value when there is no acceleration .

I have found also that Jr*RotAccel + LSShftTq is the one who is producing rotation of the turbine and not 0.5RoPi*(R^3)*(V^2)*RotCq .

Logicaly , both should be equal in every instant however the conditions …

Sincerely .

Dear Younes,

Ah, now I think I understand the problem. Output RotCq from FAST is only an estimate of the aerodynamic torque coefficient; it is derived from LSShftTq (which includes the inertial effects of rotor acceleration/decceleration), not the pure aerodynamic torque. That is, 0.5rhopi*(R^3)*(V^2)RotCq = LSShftTq. So, in transient conditions, JrRotAccel + LSShftTq is the better estimate of the pure aerodynamic torque. But–from my Apr 07, 2014 post above–this equation still assumes that the rotor and support structure are rigid.

Best regards,

I want also to ask about : what does it mean a negative aerodynamic torque ?

sincerely

Dear Younes,

A negative aerodynamic torque means that the aerodynamic loads want to rotate the shaft opposite to its natural rotation.

What conditions are you getting a negative aerodynamic torque (what DOFs are enabled, what wind conditions are being used, is the rotor parked/idling)?

Best regards,

Dear Jason ;

The aerodynamic torque is normal until a time in which it becomes negative for a duration , then it gets positive

I have this case when i use the folowing conditions :

• All DOFs are enabled (which is the reality )

• Turbulent wind

• Taero = LSShftTq + Jr*RotAccel

sincerely ,

Dear Younes,

From my Apr 07, 2014 post above, your equation for aerodynamic torque (Taero = LSShftTq + Jr*RotAccel) assumes that the rotor and support structure are rigid. But you have not made the rotor and support structure rigid in your simulation so your equation is only an estimate of the true aerodynamic torque. It may be that the aerodynamic torque remains positive even though your estimate is negative. It may also be that something in the turbulent wind field causes the aerodynamic torque to go negative for a brief moment.

Best regards,

Dear Jason , realy thank you for your replies ,

I reconstruct aerodynamic torque from measures (rotor speed) and i found that the Rotor turns not due to pure Taero , but due to Jr*RotAccel+RotTorque .

And so , Jr*RotAccel+RotTorque wan be called the usefull part of the whole pure aerodynamic torque created by the wind .

Are these two conclusions correct ?

Sincerely ,

Dear Eng. Jonkman,

for the NREL 5-MW turbine, to estimate the aerodynamic torque assuming that the rotor and support structure are rigid, I use the equation “Taero =JrRotAccel + HSShftTqGBRatio”.

I have taken “RotAccel” and “HSShftTq” from Fast Output Parameters and GBRatio=97 .

How much is the value for “Jr”?

Can I consider “HSShftTq” and “RotAccel” as measures from sensors?

Thank you.

Best regards

``Gianluca Ippoliti``

Dear Gianluca,

Yes, you can consider RotAccel and HSShftTq from the ElastoDyn output parameters (converted to consistent units).

The rotor inertia, Jr, for the NREL 5-MW turbine is written to the ElastoDyn summary file, Jr = 38677041 kg*m^2.

Please note that if you are using AeroDyn v15, Taero can be directly output via output parameter RtAeroMxh.

Best regards,

Dear Jason,

many thanks

Best regards

Gianluca

Dear Jason,

for the NREL 5-MW turbine, I would like to estimate the aerodynamic torque from a measure of the wind. Has FAST a LIDAR module ?

Thank you.

Best regards

Gianluca

Dear Gianluca,

There is somewhat of an “unofficial” LiDAR module that is part of the Simulink interface, somewhat documented in the following forum topic: http://forums.nrel.gov/t/linking-measured-wind-data-from-lidar-to-turbsim/1043/8.

I know there has been work to incorporate a more extensive and “official” LiDAR module into OpenFAST, based on work developed at the University of Stuttgart, but this effort has not yet been completed.

Of course, you can also output the ambient wind from any location you want via standard outputs from the InflowWind module.

Best regards,

Dear Jason,

I have tried to follow the indications of the document that you gave me. The simulation starts but Simulink doesn’t go over the initialization phase. In other word, Simulink run for a long time the initialization phase but doesn’t go ahead in the simulation. I’m using FAST_v8.16 and Matlab R2015a.

I have set the paramer UseLidar=1, LidRadialV=0, Lidar x measurement coordinate =-100, Lidar y measurement coordinate = 0, Lidar z measurement coordinate = 0.

If I run the program with UseLidar=0 (no lidar), it works correctly.

Is it correct what I have done? May be my FAST have some file corrupted? Is it possible to download from NREL website FAST v8 in order to try with original files?

I would like to measure the wind with the lidar in order to get a low pass filtered measurement of the wind to obtain a smooth aerodynamic torque to be used as a feedback for a data driven control strategy.

Best regards

``Gianluca``

Dear Gianluca,

I don’t see any problems with what you stated. Did you follow all of the steps outlined in the forum post linked above? I’m not sure I can be of much help here because I’ve not used this “unofficial” feature myself before.

The old archive of FAST v8.16 is available on my Google drive: drive.google.com/drive/folders/ … sp=sharing.

Best regards,

Dear Jason,

many thanks. The problem still persists also with the new files. I will read again the link you gave me to try to find a solution
Thank you again

Best regards

``Gianluca``