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Structural Analysis - Variation between Mean and Max Forces

Posted: Wed Nov 06, 2019 11:24 am
by David.Huitema
Hello everyone,
I have a question about forces/moments at high wind speeds. We are looking at FAST predictions for verifying our structural design, using the forces output by FAST as input to an FE model of the blades and tower. In particular, we are using the IEC 61400-2 standard for our small wind turbine, and we are seeing large variations in the forces for the normal turbulence model.

Our turbine is a small wind turbine, with 1 meter long composite blades, a permanent magnet generator, and tail furling. All degrees of freedom have been enabled, except for the tower degrees of freedom (i.e. assuming a rigid tower). The turbulent wind time step is 0.01 s.

An example output is included below for the output wind speed of 25 m/s using a 10-minute turbulent inflow file generated by TurbSim using for the IEC Kaimal spectral model. One figure is included as a jpeg, the other figures are in the pdf file. Running MCrunch we get the following output (table and graph). Are the values for the thrust, root bending moment, etc. valid at the maximum condition? At these high winds speed, induction has been disabled, and the initial conditions for the turbine are a rotational speed of 100 rpm and an initial nacelle-yaw angle of 85 degrees. I am asking about using the maximum forces at this design load case (as output by MCrunch) as input to our structural finite element model, as the variation between the mean and the maximum is so significant.

Re: Structural Analysis - Variation between Mean and Max Forces

Posted: Thu Nov 07, 2019 10:32 am
by Jason.Jonkman
Dear David,

I would certainly question the accuracy of this simulation. It has a very large range of wind speeds (14 to 37 m/s), rotor speeds (-11 to 2241 rpm), tail-furl angles (-91 to 6 deg) and yaw angles (-5 to 99 deg). Without knowing anything about your turbine or simulation set up, I'm not sure I can comment on much. But I would suggest starting with simpler cases to see if the response you are getting aligns with your expectations.

Best regards,

Re: Structural Analysis - Variation between Mean and Max Forces

Posted: Mon Nov 11, 2019 7:04 am
by David.Huitema
Dear Dr. Jonkman,

Thank you for your reply. The range of wind speeds is due to using a normal turbulence wind model with a mean wind speed of 25 m/s. The wind file was generated by TurbSim. The results for the non-turbulent wind files (produced by IECWind) are much more in line with expectations. I will take a look at simpler cases, thank you.

Thank you,

David Huitema