5MW -WT, FAST8, adjustment

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Sir,

I used FAST7 as the turbine model for my control design verifications.
Since I need to use LIDAR measurements in my controller, I decided to use FAST8.
I could set-up the test18 in FAST8 to work with the baseline controller (exactly the same controller that I used with FAST7).
I ran simulations with EOG wind of 20m/s. Altough the simulation results are genrally fine but there are few issues:

1- there is a small oscillation on the steady state values of the outputs. (please check the attached MATLAB figure -

5MW-results.jpg
5MW-results.jpg1920×1083 136 KB
)

2- Once the simulation starts, I get below messages in MATLAB command window: (which are repeated for all the 3 blades)

Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. BladeNode = 1, Blade = 1
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. BladeNode = 2, Blade = 1
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. BladeNode = 3, Blade = 1
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. BladeNode = 4, Blade = 1

3 -
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 5, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 6, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 7, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 8, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 9, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 10, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 11, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 12, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 13, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 14, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 15, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 16, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 17, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 18, Blade = 1
Warning: Turning off Unsteady Aerodynamics due to high angle-of-attack. BladeNode = 19, Blade = 1

Could you please let me know how to solve these issues?
Thanks
Elham

2

Dear Ehlham,

Here are my answers to your questions:

  1. I would guess the oscillations you are seeing result from the solution not being in equilibrium. I wouldn’t expect this, but are you saying that FAST v7 did not have oscillations, but FAST v8 does?

  2. The unsteady aerodynamics (UA) routines of AeroDyn v15 within FAST v8 are overhauled from what existed in FAST v7; UA now issues a warning when conditions are such that UA should be disabled. For cylindrical (or nearly cylindrical) sections near the blade root, where the lift (or normal-force coefficient) is zero, UA is automatically disabled within AeroDyn v15.

  3. UA also gets disabled if the angle of attack is too high (defined by AeroDyn input UACutOut). My guess is this happens right away in your simulation, because–from your plot of BldPitch1–it doesn’t appear that you’ve initialized the blade-pitch angle to be appropriate to the mean wind speed that you are simulating, which is likely leading to a large angle of attack at time zero. By setting the initial blade-pitch angles to be consistent with the mean wind speed (say 17.5 deg at 20 m/s) my guess is that UA will remain enabled.

Best regards,