The effect of Tilt Angle


I am wondering the effect of tilt angle error for aerodynamic performance of wind turbine by FAST code. As I know, the effect of tilt angle is the same with yaw angle. It means that the aerodynamic power and thrust tend to decrease due to the effect of yaw/tilt error. When I used FAST code to simulate onshore 5MW wind turbine, I realized that the aerodynamic thrust with titlt angle condition is higher than case without titl angle condition. Can anyone give me advise?

Belows are some input for FAST simulation:

  1. All structure DOFs was diabled.
  2. Pitch, Yaw, ang Generator mode were also diabled.
  3. All freestream velocity and rotating speed were applied from Dr.Jonkman’s report (J. Jonkman, et al., “Definition of a 5MW Reference Wind Turbine for Offshore System Development”, Technical Report NREL/TP-500-38060, 2009.)
  4. Dynamic Stall option was enabled.

Because I can not attach figure, I post those aerodynamic thrust and power magnitude, here.

  1. Aerodynamic Thrust (kN)—> Unlike my expectation
    Pitch(deg) rpm Velocity FAST_BEM FAST_GDW FAST_BEM FAST_GDW
    (No Tilt) (No Tilt) (Tilt) (Tilt)
    0.0000 9.1600 8.0000 473.8000 484.9000 381.5000 391.9000
    0.0000 11.8900 11.0000 786.2000 803.9000 695.7000 712.0000
    10.4500 12.1000 15.0000 514.0000 508.1000 424.5000 418.2000
    17.4700 12.1000 20.0000 413.7000 402.9000 326.1000 315.0000
    23.4700 12.1000 25.0000 364.6000 343.6000 278.4000 259.4000

  2. Aerodynamic Power (kW) —> Like my expectation
    Pitch(deg) rpm Velocity FAST_BEM FAST_GDW FAST_BEM FAST_GDW
    (No Tilt) (No Tilt) (Tilt) (Tilt)
    0.0000 9.1600 8.0000 1879.0000 1980.0000 1898.0000 1990.0000
    0.0000 11.8900 11.0000 4855.0000 5079.0000 4907.0000 5109.0000
    10.4500 12.1000 15.0000 5286.0000 5182.0000 5362.0000 5251.0000
    17.4700 12.1000 20.0000 5276.0000 5041.0000 5406.0000 5165.0000
    23.4700 12.1000 25.0000 5259.0000 4725.0000 5432.0000 4947.0000


ToanThanh, Tran

Dear ToanThan Tran,

I’m assuming that your column labels are switched. My guess is that the columns 4-5 are with tilt and columns 6-7 are without tilt. With this labeling, your tables show that tilt increases thrust and reduces power, whereas you’d expect both to reduce with tilt.

Power is reducing with tilt as you’d expect, but thrust is not. I’m not sure how you derived what you label as “aerodynamic thrust”, but my guess is that you are using FAST output RotThrust (or equivalently, LSShftFxa). However, FAST output RotThrust not only includes the applied aerodynamic thrust, but also the gravity and inertial loads of the rotor. You’ve disabled all structural DOFs, so, the inertial loads are zero. However, the gravity load is still nonzero. For a rigid structure, the gravity load contribution to RotThrust is -RotMassGravitySIN(ShftTilt). To increase tower clearance, ShftTilt is negative in FAST for an upwind turbine, so, the gravity load adds to the aerodynamic thrust for an upwind turbine. To increase tower clearance, ShftTilt is positive in FAST for a downwind turbine, so, the gravity load subtracts from aerodynamic thrust for a downwind turbine.

I hope that helps.

Best regards,

Dear Jason,

Thank you for your response.
Absolutely, your guess is right. I made mistake when I posted tables.
You are right because I am using FAST output RotThrust. Therefor, it includes gravity and inertial loads of the rotor as you mentioned above. I understood that my thrust values include gravity load contribution (-RotMassGravitySIN(ShftTilt)) for tilt angle case. That’ why thrust value with tilt angle case is larger than case without tilt angle.
Thank you so much.

Best regards,

ToanThanh, Tran

Dear Jason,

I plan to compare the aerodynamic thrust and power of floating offshore wind turbine. As you mentioned before, the RotThrust output of FAST code is included the gravitational effect of rotor mass due to the tilt angle (5 degree). I am wondering that how we can extract a pure aerodynamic thrust in case of floating offshore wind turbine simulation. As we know, the gravitational effect of rotor mass must be changed due to the rotational modes of platform motion. Therefore, I think that the RotThrust output of FAST code should be subtracted the gravitational effect of rotor mass considering the rotational motion of platform (e.g., pitch, roll, and yaw). Moreover, the structural force due to a platform acceleration should be included. Could you give me your comment on this problem.

I really appreciate your help.
I looking forward to hearing from you soon.

Dear ToanTranh,

I agree with your comments. The tilt angle used when subtracting the rotor gravity term from the rotor thrust should include the effects of tilt due to deflection of the support structure.

To eliminate the rotor inertial contribution to the thrust, you could average the result over some time scale much larger than the natural periods of structural oscillation.

We are expecting to release an update to FAST v8 soon that includes the long-awaited AeroDyn overhaul (AeroDyn v15) (among other changes). You’ll be happy to know that we’ve added a pure aerodynamic rotor thrust output (plus many other outputs) to this version of AeroDyn.

Best regards,

Dear Jason,

Thank you for your quickly response.
I am happy to hear that the next release of FAST code will be added more output functions and others. I am looking forward to downloading it.

Again, thank you for your comments.