Tower-top and Platform accelerations with Nacelle yaw

[Abdulelah96]

I am simulating 3 load cases for OC4 5MW semi-sub with 0, 30 and 45 deg wind heading. The Nacelle does rotate with the wind direction, however, my tower top accelerations are measured at the tower base coordinate system (xt, zt and yt). For the case of 0 wind heading, the FA (TTDspFA) displacement and tower-top accelerations (twht3lAxt) are aligned to the tower FA direction. However, when the nacelle rotate in case of 45 deg. The tower top coordinate system (xt) does not rotate with the nacelles, so what I am measuring is a component of the FA vector resolved along xt and yt. Is my understanding correct?

Is there an output parameter that I can export to measure tower-top accelerations that rotates with Nacelle along the FA and SS directions? If not, is it correct if I would rotate the xt, yt measurements to align with nacelle when it yaws? How can I do this rotation or axes transformation?

Lastly, can I also rotate the platform 6 DOF accelerations (PtfmTAxt, PtfmTAyt) that also rotates with nacelle yaw system? I want to align the platform motions to the FA and SS convention.

Thank you in advance,

Abdu

[jjonkman]

Dear @Abdulelah96,

ElastoDyn does not have tower and platform outputs that can be expressed in a coordinate system that yaws with the nacelle, so, you'll have to post-process the outputs ElastoDyn to a different coordinate system yourself.

FYI: The relationship between a vector expressed in the nacelle-coordinate system (xn,yn,zn) and a vector expressed in the tower-top coordinate system (xp,yp,zp) is:

xn = xp * COS(NacYaw) + yp * SIN(NacYaw)
yn = -xp * SIN(NacYaw) + yp * COS(NacYaw)
zn = zp

Of course, the tower-top coordinate system (p) differs a bit from the tower-base coordinate system (t), based on the orientation of the tower as it deflects.

Best regards,