Upwind vs downwind blade–tower clearance at high wind speeds

Wind & Water Computer-Aided Engineering Software Tools
1

Hi all,

We are comparing equivalent upwind (UW) and downwind (DW) configurations of the same model in OpenFAST.

For the DW case, we mirrored the rotor geometry by changing the signs of OverHang, ShftTilt, PreCone and NacCMxn (+ for DW and - for UW), while keeping the same blade structural and aerodynamic definitions.

Regarding aerodynamics, we use:

  • TwrPotent = 0 in both UW and DW
  • TwrShadow = 1 enabled in DW

At low and moderate wind speeds, the DW configuration behaves as expected and gives higher blade–tower clearance. However, above rated, the trend reverses: the DW minimum TipClrnc1 decreases with wind speed, while the UW clearance increases. At high wind speeds, the DW clearance even becomes lower than the UW one.

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This seems strongly correlated with OoPDefl1: above rated, OoPDefl1 decreases in both cases, and in DW it eventually becomes negative. That would mean the blade is deflecting upwind, i.e. towards the tower in the DW configuration.

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While doing some research about it, I found this previous forum discussion, which seems related: Extreme events - #17 by Jason.Jonkman

I would like to understand why this reversal in blade–tower clearance occurs at high wind speeds and whether the negative OoPDefl behavior discussed in the previous thread is related to it.

I have also checked the tower aerodynamic influence, and disabling TwrShadow in DW does not remove the trend.

Any guidance would be greatly appreciated.

Kind regards,

Kepa

2

Dear @Kepa.Cueto,

I agree that the forum topic you linked to explains what you are seeing, especially my posted dated Feb 5, 2010. I would expect the UW rotor to also experience negative OoPDefl1 if you increased the wind speed further.

Best regards,

3

Thanks Jason, that makes sense.

Just to confirm our interpretation: at sufficiently high wind speeds, would it therefore be physically expected for the blade to come closer to the tower in the downwind configuration than in the equivalent upwind configuration, as the OoPDefl1 becomes negative?

I also tested the UW configuration at higher wind speeds, and I can confirm that OoPDefl1 eventually becomes negative there as well.

Best regards,

Kepa

4

Dear @Kepa.Cueto,

Yes, I fully agree.

Best regards,