OpenFAST free decay of blade

Dear Jason,
1: I conducted a dynamic analysis of a 5MW fixed bottom wind turbine.I encountered an issue where I calculated free decay cases considering only an initial out-of-plane deflection (OoPDefl) of 1 m and an initial in-plane deflection (IPDefl) of 1 m. I observed that the baseline of the fluctuations in the out-of-plane blade-tip displacement deviates from zero, unlike the in-plane blade-tip displacement. Could you explain the potential causes of this phenomenon?



2: the wave elevation (ζ) is represented as H/2cos(kx-(2pi/T)t),The x-coordinate and y-coordinate of the output point are both 0,Therefore, the output elevation should be H/2cos(-(2*pi/T)*t),However, the output of OpenFast has a phase difference from this expression,I don’t if I should change something else or if I am ignoring something that I should have considered.

I would be appreciated it if you could reply. Thank you :smiley:
Best regards,

Dear @Yingxin.Lv,

I’m not sure I understand your question. What you do mean when you refer to the “baseline of the fluctuations”?

Best regards,

Dear @Jason.Jonkman,
Thank you for your response, and I apologize for any confusion in my earlier explanation. 
In-plane (IP): Symmetric oscillations around zero .
Out-of-plane (OoP): Asymmetric oscillations ( +1.05 m to -0.16 m), yielding a non-zero mean displacement.
Could you advise on the potential causes of this asymmetry? Your expert insights would be greatly appreciated.
Best regards,

Dear @Yingxin.Lv,

You haven’t said if the rotor is spinning, if aerodynamic loads are enabled, or if gravity loads are applied, but I am guessing one or more of these is the source of the mean out-of-plane displacement.

Regarding the regular wave elevation, the SeaState module of OpenFAST will assign a random phase (when WaveMod = 1), unless you’ve directly specified the phase, e.g., (WaveMod = 1P0 for zero phase).

Best regards,