Hi,

I am simulating the NREL 5MW turbine. I have two questions concerning the calculation of the rotor thrust force in FAST with AeroDyn.

I ran a YawDyn simulation of the turbine with a rigid rotor and compared the rotor thrust calculated with that of a simplified FAST simulation (with only the rotor rotational degree of freedom with the calculation of aerodynamic forces switched on). I used a non-turbulent wind file, in which the wind speed is increased by 1ms at intervals of 40s (the wind speed is constant throughout each 40s interval). I expected the results to be identical, or at least very close. However, I discovered that there was a constant offset throughout the simulation, which is independent of wind speed. I then ran a second FAST simulation without the aerodynamic force calculation and saw that the calculated rotor thrust was constant and equal to the offset which I had observed before. After further investigation it seems that this offset is related to the rotor tilt angle, as setting rotor tilt to zero causes this “offset” in rotor thrust to dissappear i.e. the rotor thrust calculated with the aerodynamic force calculation switched off is equal to zero. I have searched the AeroDyn and FAST source codes for the calculation of this structural dependent rotor thrust, but have not been able to locate it.

Question 1: It would be interesting to know how it is calculated, can you please inform me?

Also, in Martin Hansens “Aerodynamics of Wind Turbines”, he mentions that the rotor cone angle contributes to the rotor thrust by causing a centrifugal force which is given by:

omega^2.*m.*sin(cone angle)

where omega is the rotor speed, and m is the blade element mass.

Question 2: Is this contribution taken into account in AeroDyn or FAST?

Thank you.

Barry.