Instability simulating with Windpact 1.5MW - FAST 7

Dear Jason and Dear Marshall,

first of all I would like to thank you all the help and time you’ve spent in this forum.
With your guides and softwares we were able to make great progress in wind-research.

I searched unsuccessful in this forum and NWTC-Guides about the following problem and couldn’t find a answer.

When I simulate with FAST 7 the Turbine Windpact 1.5 MW sometimes I get the warning: “Small angle assumption violated in SUBROUTINE SmllRotTRans() due to a large blade deflection”. This warning always comes in the middle or at the end of simulation.

When I look the outputs after this warning, it seems to me that the blade finds itself at some kind of " resonance case" or the Blade-damping suddenly turns near zero (See the attachment).

I did not make any alteration in the programm, I used exactly the version with original input-configuration from the NREL-Website (Only change is the simulation time, from 40 to 700 s).
The Wind-Field was created with TurbSim (also without any program modifications).

I tried unsuccessful following procedures to fix this problem:

  • I Reduced the DT (Integration time), after it didn’t work I tried to increase as well.
  • I changed the initial pitch angle (I didn’t expect any change, since the problem appears in the middle or end of Simulation)
  • I used another values for the Damping-Factor from Blades

The only thing that could resolve the problem was when I used another blade-profile (2MW Wind Turbine from GH Bladed) or when I changed the first random seed from Turbsim.

The Turbsim Full-Field that followed to this problem, were simulated with the following configurations:

-7360133 Random seed #1
2 - ALL IEC turbulence models scaled to exact specified standard deviation
A IEC turbulence characteristic
8.000 Reference wind speed [m/s]

The others input-configurations from Turbsim weren’t changed.

With some Random Seeds it works perfectly but with others comes always this Problem. If you change the wind speed, turbulence or random seed the problem disappears.
I sometimes have the feeling that when the turbulence is too high, it is easier to appears this problem (But I´m not sure about that).
Could this be a problem with the Wind-Field or Blade-Profile from the Windpact 1.5 MW?

Sorry for the long text but I wanted to explain as best as possible.
Thanks in advance for any help.

Kind Regards,

Peter Collmann

Dear Peter,

I’ve seen similar problems before in simulations that experience a numerical instability in the generalized-dynamic wake (GDW) (DYNIN) option of AeroDyn. Do you have DYNIN enabled in AeroDyn? The DYNIN model in the current version of AeroDyn is numerically unstable for heavily-loaded rotors (i.e., low wind speeds). So, only the EQUIL model can be applied at low wind speeds. In fact, when using full-field turbulent winds, the current version of AeroDyn will automatically switch the option from DYNIN to EQUIL when the mean wind speed is less than 8 m/s (the mean wind speed of the entire full-field file is used in this calculation). I see that your mean wind speed is 8 m/s, which is at the threshold; it is possible that the WindPACT 1.5-MW model needs a bit higher threshold.

Best regards,

Dear Jason,

thank you for your fast reply. You were right, the problem was with the GDW-Model. After I changed to BEM (EQUIL), all simulations were successful.
Thank you for your help.

Best regards,