Query on MHK RM1 ElastoDyn Inputs

Wind & Water Tidal Current
1

Hi all,

I have some doubts about how ElastoDyn blade inputs are set up in the MHK RM1 Floating Reference Model in GitHub. If I understand correctly, the blade structural parameters were derived from Structural Design of a Horizontal-Axis Tidal Current Turbine Composite Blade (Bir et al., 2011). I’m hoping someone could enlighten me why the following mass and stiffness adjustment factors were applied in the ElastoDyn_Blade input file?

    2.5   AdjBlMs     - Factor to adjust blade mass density (-)
1.0E-05   AdjFlSt     - Factor to adjust blade flap stiffness (-)
1.0E-05   AdjEdSt     - Factor to adjust blade edge stiffness (-)

I further noticed that the unit mass of the blades are not consistent with the values in Table 4 of the conference paper. Does the input file assume that the blade is filled with certain material?

When running the analysis with stiffness adjustment of 1.0, I stumbled upon this error which suggests that the blades are undergoing large deflection and there is a tower strike.

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I find this peculiar because, if I understand correctly, changing the blade stiffness adjustment factor from 1.0E-05 to 1.0 would make the blade stiffer and it would deflect less.

Regards,
Andhi

2

Hi Andhi,

That 2011 paper is the correct reference for the blade structure. When testing with ElastoDyn we found that the blades were very stiff, requiring a very small time step to resolve their natural frequency and avoid numerical instability. I believe the tower strike you are seeing is from numerical instability and is nonphysical. We have mostly worked with these blades as rigid, turning off the blade degrees of freedom in ElastoDyn, but wanted the degrees of freedom active for the regression test. In order to be able to use a more reasonable time step for the test we increased the mass and decreased the stiffness.

Thanks,

Will Wiley

3

Hi Will Wiley,

Thank you for the insightful reply.

Could you further clarify if the blade mode shape coefficients in the ElastoDyn_Blade input corresponds to the modified blade properties with increased mass and decreased stiffness?

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Thanks & regards,
Andhi

4

Hi Andhi,

I believe those mode shapes were generated for the original properties, but since the change in stiffness and mass is uniform along the blade, the mode shapes should still be valid. The frequencies corresponding to the modes will change, but the shapes should not.

Thanks