Interpolation issue Line2_to_point in FAST

Wind & Water Computer-Aided Engineering Software Tools
1

Hello,

I am trying to replace AeroDyn by my own aerodynamic code in FAST (v8.15).
The coupling between my aero module and FAST is based on what is done for AeroDyn.

Here is my problem.
It seems that there is an issue during the interpolation step, when loads are transferred from the output Line2 mesh of the aero module to the input Point mesh of ElastoDyn. Indeed, some oscillations appear in the input mesh of ED when I plot forces along the span, while they were not present in the output mesh of the aero module (cf. attached graph below, where spanwise forces per unit length in z direction on blade 1, at a given time of the simulation, are plotted on the output mesh of the aero module and the input mesh of ED, with different number of nodes).
z-force-spanwise.png
Here I run a very simple simulation, only ED and my aero module are enabled (the wind is set by my aero module as well).
I have a mesh with variable spacing between the nodes for the output mesh of the aero module and constant spacing for ED, but I have the same problem with constant spacing for the aero output mesh. I have also similar oscillations when I use AeroDyn instead of my aero code.

Have you noticed this issue before ? Are these oscillations expected when using the “transfer_line2_to_point” routine ?
If yes, could they have consequences on the deformation of the blades or on the global behavior of the wind turbine ?

Thank you. Best regards,
Bastien Duboc

2

Hello again,

I found out that there is no oscillation problem with the “transfer_line2_to_line2” function, because the lumped loads are transferred from the source mesh to the destination mesh using a weighted sum (Eq. 54 and 55 in the following document nrel.gov/docs/fy16osti/63203.pdf).
However, in the “transfer_point_to_point” function, lumped loads on destination mesh are just computed with a sum (no weight) of the lumped loads on the source mesh (Eq. 47 and 48 in the same document). It seems that it is what causes my oscillations.
Is there any reason for not using a similar weighted sum here ?

Best regards,

Bastien Duboc

3

Dear Bastien,

It turns out there was a bug in the Line2-to-Point mapping search for loads since the algorithm was first implemented. While the Line2-to-Point mesh-mapping algorithm for loads documented in our AIAA paper is correct: nrel.gov/docs/fy16osti/63203.pdf, there was a bug in the implementation. Basically, while the total integrated loads were correct, the local loads distributed along the Point elements were incorrect when the number of points elements was greater than the number of Line2 nodes.

But Bonnie Jonkman of Envision Energy has fixed the bug! See the commit here: github.com/NWTC/NWTC_Library/co … e5b2b7de80. Hopefully your results will look more like you’d expect after applying this fix.

Best regards,