Valid Yaw values for OpenFAST

Hi everyone,

I am studying the behaviour of the OC4 model when wind has high absolute values for PropagationDir, or when wind is aligned with X-axis (hence 0º dir) but the platform is Yawed. In this second case, I have found inestabilities in the simulations (i.e. almost all the variables take absurd values). Moreover, the simulations even abort when checking initial values if PtfmYaw is set to 20 degrees in ElastoDyn (see attached image).

In this conext: could someone confirm me that the theory behind OpenFAST (or FASTv8) lose validity outside the Yaw range of [-15, 15] degrees? I think I have read that in some document, but I cannot find the quote.

Provided that’s true: what would be a work-around to simulate the OC4 with high yaw values? Should I set the PtfmYawDOF to False and carry out different simulations with fixed PtfmYaw?

Thank you in advance.

Dear Pablo,

The ElastoDyn module of FAST / OpenFAST employs small angle assumptions (without a specific sequence), but with a correction on orthogonality of the rotation transformation matrices to ensure that they are orthonormal. This allows any of the platform rotations to become about 15 degrees before the errors start to get large. See the Equations (1) and (2) in my 2009 Wind Energy Paper for more information: Disabling the platform rotation DOFs does not eliminate the error; the solution would still be inaccurate for fixed platform rotations that are large in value.

To model large yaw rotations of the platform, you’d have to redefine the structural and hydrodynamic properties (in ElastoDyn or SubDyn, HydroDyn, and the mooring module) of the floater at different yaw angles.

Best regards,

Dear Jason,

I am grateful for your reply.

I will review the paper you referenced, as you have suggested. I have quite a number of questions in my mind regarding the AeroDyn module and the behaviour of the OC4 model, but first I would like to review the paper.

Thank you again Jason.