Calculation of Rotor Thrust LSShftFxa

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.

Dear Barry,

Regarding question (1), I’ve answered a similar question in my August 24, 2011 post in the forum topic found here: http://forums.nrel.gov/t/aerodyn-outputs/392/1. The main reason for the offset of rotor thrust is that FAST includes all loads transmitted across the shaft between the rotor and gearbox in the value of LSShftFxa (or RotThrust), including the applied aerodynamic loads, gravitational loads (from the rotor mass and shaft tilt/tower deflection/platform pitch), and inertial loads (from the rotor mass/inertia and acceleration/deceleration). The gravitational and inertial loads cause the FAST rotor thrust output to deviate from the pure aerodynamic thrust.

Regarding question (2), the answer is “yes.” FAST calculates the centrifugal forces and these terms would be included in the inertial loads transmitted between the rotor and gearbox, so, these forces would be included in the rotor thrust calculation of FAST as well.

I hope that helps.

Best regards,

Jason,

Yes, that helps me a great deal. Thank you very much for your reply.

Regards,

Barry.

hi
Which output of FAST software gives us centrifugal force?
If an output from FAST does not give us this force directly, how can it be calculated?

Dear @Ali.Rouhbakhsh,

The centrifugal force is computed by OpenFAST, but the force itself is not output directly, at least isolated from other forces. The axial load at the blade root is likely dominated by gravity and centrifugal loading.

Best regards,