Definition of additional AeroDyn blade file inputs for tidal turbines

Wind & Water Tidal Current
1

I am keen to use OpenFAST v5 to create a time-domain simulation of a tidal turbine. Please can I ask for more detail on how the new/additional inputs in the AeroDyn blade file are defined. Some details are provided in 4.3.3. Input Files — OpenFAST v5.0.0 documentation but I would be very grateful for some clarification on the following inputs:

a) BlCpn: Why is there a single blade normal-to-chord dynamic pressure coefficient (BlCpn) for the whole foil (spanwise section) along the blade? Does this effectively mean the drag coefficient when the angle of attack is 90 degrees?

b) BlCpt: As above, why is there a single blade tangential-to-chord dynamic pressure coefficient (BlCpt) for the whole foil (spanwise section) along the blade? Does this effectively mean the drag coefficient when the angle of attack is 0 degrees?

c) BlCan: How is the blade normal-to-chord added mass coefficient (BlCan) defined? Is it the same as the added mass coefficient (C_A) as defined in DNV-RP-C205 Appendix D (i.e. 1.0 for a cylinder)? Does AroDyn/OpenFAST then calculate the reference area (A_R as per DNV-RP-C205 terminology) and added mass for each node/element along the blade?

d) BlCat: As above, how is the blade tangential-to-chord added mass coefficient (BlCat) defined?

e) BlCam: DNV-RP-C205 specifies how to calculate the added mass moment of inertia for different cross-sectional shapes (e.g. a circle, an ellipse, a flat plate etc.). The parallel axis theorem can then be used to calculate the pitch added mass coefficient for each node along the blade. How is BlCam actually used to calculate the added mass moment of inertia? And is the offset between the node and the blade pitch axis considered through the parallel axis theorem?

Thank you very much for any advice or answers provided!

Tom

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Dear @Tom.Summers,

I believe the AeroDyn implementation plan shared here answers all of your questions about the new dynamic pressure coefficient, and added mass inputs: https://openfast.readthedocs.io/en/main/_downloads/993c27904dec34704905be693a368355/Fluid_Inertia_Added_Mass_Implementation_Plan.docx.

Best regards,

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Dear Jason,

Thank you very much for providing that reference. That answers all of my questions except for the very final one:

The document you provided suggests that the Pitch Added Mass Coefficient (BlCam) is based about the centroid of the cross-sectional foil section. That allows you to calculate the pitching moment due to the inertia of added mass if the centroid aligns with the pitch axis, but you need a parallel axis term if the cross-sectional foil centroid is not aligned with the pitch axis. Approximated as below:

I_added,(pitch axis) = I_added,centroid + m_normal,added * BlSwpAC^2

For highly swept blades where the foil sections are not aligned with the pitch axis (i.e. BlSwpAC is large), is the parallel axis component of inertia of added mass accounted for separately (internally by AeroDyn)? Or does it need to be accounted for in BlCam?

Thanks again!
Tom

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Dear @Tom.Summers,

The added mass is applied at the aerodynamic analysis node, whose location is determined by BlSwpAC, so, the extra term associated with the parallel-axis theorem is implicitly accounted for in the solve. You don’t need to add the term manually.

Best regards,

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