Dear OpenFAST community,
- I am using the 5MW_TLP_DLL_WTurb_WavesIrr_WavesMulti file for simulation in OpenFAST.
- I am trying to comparison of motion RAO (Figure 26-RAO WAMIT 27m) calculated by Denis Matha in “Model Development and Loads Analyis of an Offshore Wind Turbine on a Tension Leg Platform, with a Comparison to other Floating Concepts”.
- The way I do it is calculating motion RAO using white noise spectrum, with reference to the paper("Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines) and forum posts
[Method]
I used 8000 seconds out of a 10000second simulation.
wave time series → Sxx (spectral density ) using Blackman-Tukey
ouput response(surge, pitch) → Sxy (spectral density) using Blackman-Tukey
motion RAO = Sxy/Sxx
- For surge motion RAO, I think the peak values match and y(RAO)-values for each frequency appear to be influenced by smoothing.
However, For pitch motion RAO, the magnitude of the peak values varies significantly.
Is there something I am forgetting or doing wrong?
Please give suggestions on how to analyze.
Thank you for your help in advance.
Dear @Wonjun.Choi,
It looks like your natural frequencies are lining up well between your results and Matha’s results for both the surge and pitch modes, but the level of damping is quite different, with your results showing significantly less damping. This was done many years ago now, and I don’t recall the details, but I see in section 3.5 of Matha’s thesis-turned NREL report that it says his results include aerodynamics enabled with the rotor spinning at a constant 12.1 rpm at 11 m/s. I don’t see that your results include aerodynamics, but the lack of aerodynamic damping in your model could explain the difference.
Best regards,
Hello, dear Dr.Jonkman
First of all, I appreciate your compassionate response.
- Modified content: Aerodynamic effects were considered when the rotor rotates at a constant speed of 12.1 rpm at 11 m/s.
- Result: Aerodynamic damping effects have been confirmed. It was effective in Pitch RAO, but the peak value(y) decreased in Surge RAO.
I was comparing the pitch WAMIT 27m in Figure 26 with WJ(OpenFAST).
I think there could be a difference between the value transformed from time domain to frequency domain and the WAMIT(spoke 27m).
I am reviewing whether there are any errors in value settings and trying various approaches.
Best regards
WonjunCHOI
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