Pitch Misalignment Query

[PaulKSmith]

Hello,

I am trying to understand which twist I should use to represent pitch misalignment of a single blade? Is it ElastoDyn_blades > "StrcTwst" or AeroDyn15_blade > "BlTwist" ? From my understanding, changing "BlTwist" by constant angle throughout the span will represent the same aerodynamic load as for the pitch misalignment. Please correct me if I am wrong. I am confused what "BlTwist" does then? Does it account for the change in structural properties due to misalignment in overall structural dynamics? I would appreciate if anyone could help me understand this.

[jjonkman]

Dear @PaulKSmith,

Regarding the difference between BlTwist in AeroDyn and StrcTwist in ElastoDyn, see a similar question asked and answered in the following OpenFAST issue: #2044.

To define a pitch offset between blades in OpenFAST, you could either add an offset to BlTwistand StrcTwist for a given blade, or you could introduce a pitch offset in the pitch controller.

Best regards,

[PaulKSmith]

Hello @jjonkman ,

Thank you for your reply. So, that means I need to manually give twist to both StrcTwist and BlTwist. Thanks for the confirmation.

I remember I read in some post that flapwise and edgewise bending are coupled using shape functions ? Is that correct? I could not find that in ElastoDyn manual though. It would be helpful if you could share some insights on this.

Regards,

[jjonkman]

Dear @PaulKSmith,

Yes, I agree that ElastoDyn will couple the flapwise and edgewise motions based on the structural pretwist. See the following topic on our forum for more information: https://forums.nrel.gov/t/coupled-blade-modes-in-fast/314.

Best regards,

[PaulKSmith]

Hello @jjonkman

Thanks for sharing the post link. It helped clearing doubts.
I have another doubt and could not find any reference in any other posts. I would be grateful if you could help. I am trying to calculate loads on blade individual nodes using B1N1Fx and
B1N1Fy. The outputs are in N/m. Does it mean I need to multiply effective length to get in N at a specific node? Also, can I use effective length as 50% of the node-to-node distance on both side ?

Regards,

[jjonkman]

Dear @PaulKSmith,

Indeed, AeroDyn's aerodynamic force outputs are expressed as distributed forces per unit length (in N/m). If you want to lump the loads as point loads at nodes (in N), you could multiply by an appropriate length, e.g, half of the distance to the previous inboard node plus half the distance to the next outboard node.

Best regards,

[PaulKSmith]

Hello @jjonkman ,

Thank you so much for your time and effort.

Regards,