Hello everyone,

I was running some simulations on the IEA 15 MW mounted on UMaine semi-submersible platform. The wind field is stochastic and generated using Turbsim and has a mean value of 10.59 m/s (rated wind speed).

When computing PSD for pitch motion, the natural frequency of pitch is not excited which i found this weird. In fact, Kaimal spectrum (wind spectrum) has high amplitudes at low frequencies which means that the pitch motion should be excited. right ?

Does anyone has encounter this ? or does anyone has an explanation ?

Note: the pitch natural frequency is 0.036 Hz

Best Regards,

Riad

Dear @Riad.Elhamoud,

Perhaps the pitch mode is highly damped through the the aerodynamic loads?

Best regards

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Dear @Jason.Jonkman,

I totally agree with the fact that the aerodynamic damping is the only reason to dampen out the peak at pitch frequency.

For me, the aerodynamic damping is not a constant value. In fact, it depends on wind speed and rotor speed. Right ? If yes, then setting a damping coefficient with constant value to take the aerodynamic damping is not rigorous. Right ?

How the aerodynamic damping is modeled in OpenFAST. Please if you could provide papers. I wanna know how it is modeled and what is the mathematical expression behind.

Best Regards,

Riad

Dear @Riad.Elhamoud,

I agree with your points regarding aerodynamic damping.

Aerodynamic damping is not directly specified in OpenFAST, rather, it is included intrinsically as part of the aero-elastic solution. That is, the aerodynamic force calculated within the AeroDyn module is dependent on the structural velocity from the structural module (ElastoDyn or BeamDyn). As such, there is not a simple mathematical expression that can be referenced, rather the damping arises from the coupling between the aerodynamic calculations and structural equations of motion.

Best regards,

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Dear @Jason.Jonkman ,

Sorry for my late reply. I just want make sure that i understand well your answer. So, there is no mathematical expression used in OpenFAST that takes into account the aerodynamic damping.

The fact that taking the structural velocity when computing aerodynamic loads (aero-elastic calculation) creates the damping effect. Right ?

I have also some questions:

Concerning Morison equation (strip theory), If i understand well what you have written in your Ph.D. thesis, the drag force in heave direction equals zero. That statement is valid for bottom fixed or even floating wind turbine ? I am a little bit confused. I was thinking if this term is omitted, how heave motion will be damped

Is there any dynamic analysis or definition for the 5 MW barge wind turbine ?

Best Regards,

Riad

Dear @Riad.Elhamoud,

I agree with your points about aerodynamic damping.

Regarding strip theory, my PhD thesis turned NREL report was accurate at the time of writing, but since then, the HydroDyn module of OpenFAST has been greatly enhanced, including support for multiple viscous drag members and end effects.

My PhD thesis turned NREL report contains the definition and dynamic analysis of the NREL 5-MW baseline wind turbine atop the ITI Energy barge.

Best regards,

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