# FAST Output: RotCt is not constant for the same TSR

Hi,

I got puzzeled by the last very simple simulation I did with fast - maybe you encountered the same issue…
The output for RotCt (rotor thrust coefficient ) created by a fast simulation with a deterministic slope change in wind speed does not make sense. RotCt after the transition time ist lower than the original value although my slope change in wind speed is not high enough to put me to nominal operation.
I would have expected RotCt to have the same value for both stationary conditions 1. before the transition and 2. after setteling of the transient dynamics. The TSR value behaves correctly and relaxes to the initial optimal value after the transition.

Does anyone have a clue how RotCt ist computed internally or why the value does not return to its initial value although TSR remaines the same?
Computing the ratio of Yaw Bearing Axial Force and reference Force, does lead to a more reasonable result.

Ursula

Hi Ursula,

In FAST, RotCt is computed from the axial force in the low-speed shaft, LSShftFxa (RotThrust), normalized by 0.5AirDensSweptArea*WindVxi^2. The axial force in the low-speed shaft includes contributions from aerodynamic, gravity, and inertia forces. From your e-mail I can see that you are seeking the pure contribution from aerodynamics. But, I suspect, the gravity and inertia contributions are influencing RotCt in your case.

The magnitude of the inertia term depends on the hub and blade masses and the degree to which the tower and blades are vibrating. This term can be can be eliminated from RotCt by averaging RotCt over time scales larger than the natural periods of the tower and blade vibrations.

The magnitude of the gravity term depends on the rotor mass, shaft tilt, and tower deflection. This term can be eliminated by subtracting off RotMassGravitysin(ShftTilt+YawBrRDyt) from RotThrust and recomputing RotCt in a post process.

The reason we chose to implement the thrust calculations in this way is because if one was to measure the parameters in an experiment on a real machine, they would have to put a load cell in the shaft and the measured loads would include the additional influences of rotor gravity and inertia. (We have done this, for example, to measure the rotor thrust on small, furling wind turbines.) However, this has the undesirable consequence of RotCt possibly exceeding the value of 2 and not being what an aerodynamicist expects it to be.

I hope that helps.

Best regards,