Structural validation of custom blade design

Wind & Water Structural Analysis
1

Dear all,

I’ve been working on a project designing wind turbine blades using an in-house simulation software, and have been running my designs through openFAST for validation. I’m running into a strange issue resulting in a mismatch in blade edgewise displacement between the two codes - and while I appreciate the results from the in-house software won’t be possible for people to recreate, I’m hoping to get some suggestions on the underlying causes to my errors.

Despite outputting very similar aerodynamic forces and modal frequencies, and having identical cross-sectional properties, I’m unable to get a match in steady-state in-plane deflection. To use a familiar reference point I’ll illustrate the error using the NREL 5MW rotor rather than any of my designs. The attached images show results comparisons from the NREL 5MW RWT in a uniform windfield of 12m/s with fixed pitch and RPM. The first showsime-averaged deflection as a function of spanwise location on the left, and the time series for the tip node on the right. The second shows the time-averaged in-plane and out of plane forces and aerodynamic coefficients.

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I’ve taken the following lines of investigation to try and pinpoint my issue, and in all cases the discrepancy remained:

  • Looking at the in-plane displacement with no rotor movement to observe deflection under self-weight
  • Removing edgewise damping from the openFAST input files, testing if it was hindering the in-plane deflection in any way
  • Fixing my pitch-axis input as per this forum post
  • Comparing modal frequencies and mode shapes using ACDC as recommended in this forum post
  • Tinkering with the aerodyn settings - strangely turning off the “UseBlCm” is what gives me this very close match in flapwise displacement. As a result I looked into the airfoil data files I was using and they were identical in moment coefficient for both programs.

I would greatly appreciate any pointers or suggestions as to what could be causing such an issue under my specific circumstances, and I’m happy to share any further details that would be useful in narrowing down the problem.

Best wishes,
Abdirahman

2

Dear @Abdi.SheikHassan,

To debug what seems to be a difference in the structural beam formulation, I would suggest isolating the blade statics and dynamics, so, the first bullet in your list of investigations is what I would focus one; perhaps bullet four as well. What was the result of this test (or these tests) without aerodynamic loads? Have you tried comparing the blade deflections with known applied point loads? (In OpenFAST, you can apply point loads on the blade through the Structural Control (StC) submodel of ServoDyn.)

Also, which beam model in OpenFAST are you using: ElastoDyn or BeamDyn? Perhaps try comparing to both?

If you have concerns with the aerodynamic applied loads, I would first suggest comparing these loads in the absence of blade deflection (you model a rigid rotor in OpenFAST by disabling BeamDyn, as well as the blade degrees of freedom in ElastoDyn).

Best regards,