15MW Reference Turbine - ElastoDyn + BeamDyn Blades

[Sam.Ramsahoye]

Hi,

For such a large turbine with composite blades and bending/twist coupling, when can one get away with just using ElastoDyn for their simulation? If one is ultimately interested in foundation loads, are differences between the two ultimately smoothed out in macroscopic characteristics of the foundation signals, like standard deviation and damage?

Best,
Sam

[Jason.Jonkman]

Hi Sam,

In general, the ElastoDyn blade model does account for many nonlinear terms–e.g., radial shortening (important for centrifugal stiffening), centrifugal, Coriolis, and gyroscopic terms–but the model does have several important simplifying assumptions:

• Straight when unloaded (but structural pretwist is considered to include coupling between flap and edge)
• Isotropic material with no offsets in mass or stiffness from the pitch axis
• Bending only
• Moderate deflections, not permitting slopes more than about 15 degrees.

BeamDyn is more appropriate when these assumptions are invalid, i.e. for blades with:

• Build in curvature (when unloaded)
• Composite-material-induced couplings or offsets of the mass or stiffness form the pitch axis
• Torsion, shear, and stretching (in addition to bending).
• Large deflections.

I have not been directly involved in the development of the IEA Wind 15-MW reference turbine , but my understanding is that NREL has focused on the ElastoDyn modeling for now, despite the importance of some of these items.

Best regards,

[Lixian.Zhang]

Dear all

Thank you very much for reading my email.
I’m simulating the dynamic response of 15 MW monopile offshore wind turbine. And I faced some problems at below rated wind speed.
When I simulated the below rated condition with constant wind speed(5 m/s), the rotor thrust force and generator power is larger than the reference report of 15 MW wind turbine as shown below. The controller is ROSCO controller provided in GitHub.

The reports show that the generator power and rotor thrust of 15 MW should be around 2 MW and 1000 kN.

Also, the command window shows the wind speed is 7.3 m/s not 5 m/s.

And I didnt change anything of the 15 MW example file except the wind speed.

I would be very appericate if you can help me!

Kind regards
Lixian Zhang

[Jason.Jonkman]

Dear Lixian.Zhang,

It looks like the values of power and thrust that you are getting are expected based on the wind speed of 7.3 m/s estimated from the ROSCO controller. Are you simulating with wind shear? If so, this could make the effective disk-averaged wind speed greater than the wind speed you’ve specified.

Best regards,

[Lixian.Zhang]

Dear Dr.Jason

Thanks very much for your reply! It seems that I’m using the wrong version of Openfast. The results of constant wind speed now looks good to me both for the rotor thrust and blade pitch when I use openfast v2.4. However, the simulation results of turbulent wind for below ratted wind speed still not so good as shown below.

And the command window shows that mach exceeds 0.3, then the wind speed became very large.

The simulation result are quite reasonable for over ratted wind speed(18 m/s) as shown below.

I would be very appericate if you can help me in resolving this issue?

Thank you very much!

Best regards
Lixian Zhang

[Jason.Jonkman]

Dear Lixian,

I’m not sure I understand your comment about “using the wrong version of OpenFAST.” I would generally not expect different versions of OpenFAST to give very different answers for the same input settings.

Regarding your new results, it looks like there is a start-up transient that could likely be reduced through the use of better initial conditions. We generally recommend that you initialize the model with an initial rotor speed and initial blade-pitch angles set to their expected (mean) value for a given mean hub-height wind speed.

Otherwise, I’m not sure I know enough about this model/controller to know why the power keeps dropping to zero for your low wind-speed case.

Best regards,