# Relative wind speed

Hi all,

if we have a offshore platform, due to the platform movement the speed which is sensed by the turbine (relative speed between turbine and wind speed) is different from wind speed. Does FAST consider wind speed or relative speed in calculation of torque, …?

Thank you

Dear Omid,

FAST will use the relative wind speed in this case.

I general, FAST takes the structural position/orientation/velocity of each aerodynamic analysis node (based on all structural motions, including rigid-body motions and beam deflections) and sends the values to AeroDyn. AeroDyn uses these values along with the local wind-inflow conditions to calculate the aerodynamic loads.

Best regards,

Hi Jason,
Thank you so much for the quick reply.
Sincerely,
Omid

Hi, Jason.
Nice to saw you again in Tokyo.
Because of the wind shear and platform’s motion, especially the pitch motion, the relative wind velocities of each blade element are different, even the elements are in the same position of different blades.
Does AeroDyn calculate their aerodynamics separately, considering the rotor’s azimuth angle, wind shear and platform’s pitch motion from FAST?
For example, node 1 of blade 1, node 1 of blade 2 and node 1 of blade 3?
If so, does it fulfill the momentum theory?

Best wishes,
Lei DUAN

Dear Lei,

Yes, AeroDyn will use the local relative wind speed to calculate the aerodynamic load separately for each node (different for every node of every blade). While this approach may violate assumptions in the original momentum theory, it has been shown to give reasonable results when compared to test data (and better results that would be obtained if the local wind speeds were not used).

It was good to meet you again in Tokyo too.

Best regards,

Hi, Jason.
Thank you very much for the reply.

Best wishes,
Lei DUAN

Hi, Jason.
Following your suggestion, I used the local wind-inflow conditions to calculate the aerodynamic loads, similar to AeroDyn.
However, I was puzzled by the skewed inflow correction.

The local wind inflow is always in three directions (n-direction, t-direction and the direction along the blade) because of rotor’s rotation, even only horizontal wind, tilt angle and platform’s pitch angle are considered.
Also, the local wind-inflow speed is different from point to point.
The equation provided by AeroDyn Manual seems not for the local wind-inflow calculation.

Thus, could you tell me how does AeroDyn apply the skewed inflow correction into local wind-inflow calculation?

Thanks a lot.

Best wishes,
Lei DUAN

Dear Lei,

The current version of AeroDyn applies the skewed-wake correction based on Eq. (17) in the AeroDyn Theory Manual: wind.nrel.gov/designcodes/simula … Theory.pdf. In this equation, the local induction factor, “a”, is used based on the wind speed/structural motions local to each node, but the skew angle, “chi”, is based on the disk-averaged wind velocity and the disk-averaged axial induction normal to the rotor plane.

I hope that helps.

Best regards,

Dear Jason,
Thank a lot for your answer. It helps me much.

Best wishes,
Lei DUAN