Unusual furl behavior happening.

Hello all,
I have a furling and pitching model of what is effectively a burgey type turbine. I have managed to test the model in a variety of states and using real 1 second wind data from site with very good correlation with one exception. When the wind speed drops below the pitching wind speed, after a time, lift on the tail increases, the nacelle yaws, blade pitch increases (to feather) and rotor speed increases. See the attached pdf file of some output plots. The first plot is from a user defined wind input file. The second plot is the rotor vs wind speed plot of the 1 sec data fed into FAST. You will notice the area circled on the knee of the curve where exactly the same thing happens.

I have switched the Furl DOF off and the problem does go away.

Note: the wind input file has no horizontal or vertical components. The wind velocity and time step is specified, all other values are zero: -

eg wind input file…

! Rotor diameter = 5.2 m.
! Wind speed = raw 1 sec data from WW Factory.
! Time Wind Wind Vertical Horiz. Pwr.Law Lin.Vert. Gust
! Speed Dir Speed Shear Vert.Shr Shear Speed
! (sec) (m/s) (Deg) (m/s) (m/s)
1 2.0 0 0 0 0 0 0
400 10.0 0 0 0 0 0 0
700 2.0 0 0 0 0 0 0
1500 2.0 0 0 0 0 0 0

If any body has any experience or thoughts about this I would be very grateful.

Furl1.pdf (376 KB)

I have been trying to make this event happen and I believe that I have got close to the cause.

Looking at the plots I have attached, you can see the difference int he wind input file.

In plot 1 the last ramp down in the wind is slow, from 5.5 m/s to 3.5 m/s over 170 seconds.
In plot 2 the last ramp down in the wind is increased, from 5.5 m/s to 3.5 over 30 seconds.
In plot 3 the last ramp down in the wind is increased again, from 5.5 to 2.5 over 30 seconds.

Notice in the last plot the rotor speed, blade pitch angle and yaw angle. It appears to be caused by the ramp down rate of the wind as no other parameters have been changed between the simulations.

Could this be in part a result of the rotor ‘paddle wheeling’ its way around and then getting stuck at a new yaw angle?

Any thoughts are greatly appreciated.

Furl2.pdf (365 KB)