# why the rotor rotate when wind speed is 0.01m/s?

Dear all,

I meet some questions when I simulated the Test01 (FAST V7).

I use the OpenLoop.mdl to simulate.
In the primary input file, I disable the yaw control and pitch control, and set VSCotrl to 0, GenModel to 1.
Then I set the wind speed to 0.01 m/s, the other wind parameters to 0 in file Shr12_30.wnd
I found that even the wind speed is 0.01 m/s, the turbine is still rotating. How did this happen?

Is there something wrong with my simulation?

Dear Cheng Zhang,

You’ve selected the simple-induction generator model, which–because of the simple torque-speed curve–will use the generator as a motor to start the turbine when the rotor speed is zero. If you wish to have the generator have no torque, set GenTiStr to True and TimGenOn > TMax.

Best regards,

Dear Jason,
yes, you are right!
Thank you for your answering all my questions, it’s so nice of you! And thanks for this forum, that help me to learn the FAST, and solve all my problems concerning to the FAST.

best regard,
ZHANG Cheng

Dear Jason,

I’m calculated the tower top translational YawBrTDxt,YawBrTDyt,YawBrTDzt and rotation YawBrRDxt,YawBrRDyt,YawBrRDzt. Are there some connection between the translational and rotation ?(e.g how can I calculate YawBrTDxt by using the tow-top rotation, or some other methods.)

I know it is a simple question, but I’ve been calculating for a long time, did not get the right result.
Could you please give me the computing methods of tower-top motion?

Best regards,
ZHANG Cheng

Dear Cheng ZHANG,

If only one of the two tower mode shapes are enabled in ElastoDyn of FAST, then the tower-top rotation would be proportional to the tower-top displacement based on the slope of the tower mode shape at the tower top. However, if both tower mode shapes are enabled, than you’d have to know the relative contribution of each mode to the total displacement to relate the rotation and displacement via the slopes.

Best regards,

Dear Jason,

Yes, When I enable the TwFADOF1 and disable TwFADOF2, TwSSDOF1,TwSSDOF2. I got the result as the figure below.
If I understand correctly. the YawBrTDxt = tan(YawBrRDyt) * (TowerHt - YawBrTDzt ), but the result does not coincide with the figure.
The model I used is 5MW, the TowerHt=87.6
So, do I misunderstand something or my calculate was wrong?

best regards,
ZHANG Cheng

Dear Cheng,

Your calculation assumes that the first tower mode shape is a straight line such that the rotation is uniform across the tower elevation. In reality, the mode shape is specified by the user in the tower input file of the ElastoDyn module of FAST. For a tower with a single DOF, the solution would be:

dphi(h)/dh

solved at h = TowerHt, where:

q_TwFADOF1 = first tower fore-aft DOF
phi(h) = first tower fore-aft mode shape at elevation h
dphi(h)/dh = slope of the first tower fore-aft mode shape at elevation h

I hope that helps.

Best regards,

Dear Jason,
Sorry, I can’t understand it, in my understanding, tower top motion is the motion of Yaw Bearing
C.M.
, it should be the motion of a point with respect to the no deflection point. So why it relate to the mode shape?

It seems that I fell into a wrong thinking.
Also, I can’t understand
q_TwFADOF1 = first tower fore-aft DOF
phi(h) = first tower fore-aft mode shape at elevation h
dphi(h)/dh = slope of the first tower fore-aft mode shape at elevation h

Could you please give me some more detailed equations?

best regards,
ZHANG Cheng

Dear Cheng Zhang,

Yes, the tower-top deflection is relative to the undisplaced position. The deflection of the tower is governed by the specified tower mode shape(s), based on the specified polynomial coefficients i.e.:

phi(h) = C_2*(h/H)^2 + C_3*(h/H)^3 + C_4*(h/H)^4+C_5*(h/H)^5+C_6*(h/H)^6
dphi(h)/dh = (2C_2(h/H) + 3C_3(h/H)^2 + 4C_4(h/H)^3+5C_5(h/H)^4+6C_6(h/H)^5)/H

where:

H = TowerHt
C_i = the polynomial coefficients i.e. TwFAM1Sh(i) for i=2,3,4,5,6

I hope that helps.

Best regards,

Dear Jason,

I think that I didn’t explain my problem clearly. so I make a picture…
That is what my understanding of tower top motion, Could you tell me where I am wrong?

You wrote h = TowerHt and H = TowerHt too ? I’m confused, and q_TwFADOF1 = ??

Just as important, Are there have some documents including the computational formulas of tower motion (e.g.YawBrTDxt, YawBrRDxt), blade motion and platform motion?
Many thanks.
Best regards,
ZHANG Cheng

Dear Cheng ZHANG,

Yes, h is the height along the tower; at the tower-top, h = TowerHt, so, h/H = 1. Likewise, phi(h = TowerHt) = 1 (because SUM(C_i,i=2,3,4,5,6) = 1). q_TwFADOF1 is the displacement of the first tower-bending fore-aft mode DOF.

Best regards,

Dear Jason,
I used the equations you posted, the YawBrTDxt and YawBrRDyt are coincident with the FAST. Thank you for your help！
Now I find another question, when I use the floating platform（NRELOffshrBsline5MW_Platform_ITIBarge), and set WaveMod = 0, the PtfmTDxt seems unnormal, the value is too large.Could you please tell me where the problem is?
And does which part of motion has the greatest impact on generation power?

Best regards,
ZHANG Cheng

Dear Cheng ZHANG,

The mean offset of the platform surge, pitch, etc. is driven more by the wind than the waves. It sounds like you’ve only disabled the incident waves.

Best regards,

Dear Jason,

1. Yes, I only disabled the wave, and I know the platform surge is driven more by the wind than the waves, what I can not understand is, why the mean offset of the platform surge can be large as nearly 40 meters? Is this normal?
And, what affects the platform surge displacement except for the wind speed? (In this case, wind speed is 14m/s.)

2. I mean, in a floating wind turbine system (Suppose turbine face wind), the deflection of the system(e.g. platform deflection, tower deflection, blade deflection…) will affect the generation power, So which kind of defection affects the power more.

Best regards,
ZHANG Cheng

Dear Cheng ZHANG,

The more slack the mooring system is, the more the platform will surge as a result of aerodynamic thrust. The platform will continue to surge until the line tension(s) counteract the thrust.

The mean platform surge can also be effected by second-order mean-drift hydrodynamic excitation.

The deflection most important to power generation is the deflection that most causes the blades to misalign from the wind. Which component (platform, tower, blade) causes the most misalignment will depend on the system you are analyzing.

Best regards,

Dear Jason,