I am doing some simulations using FAST and I have doubts about some issues related to the thrust produced by the blades. My wind turbine has three blades, it is upwind oriented with free-yaw, no tilt angle and a small cone angle. In one of the simulations I’ve done, I have obtained a high value of blade root thrust (RootFxc1). In that same instant, the blade root thrust on the other two blades is much smaller and if I check the thrust on the shaft (LSShftFxa) I see that the value obtained is much slower than the sum of the three thrust values of the blades. So after watching this, what is the difference between these variables? Should not there be a correspondance between them?
Thanks in advance. Best regards,
Your question is similar the question discussed in the forum post found here regarding why the rotor torque does not equal the sum of in-plane moments: http://forums.nrel.gov/t/fast-why-does-rottorq-exceed-the-sum-of-in-plane-moments/91/1.
For the rotor thrust, the thrust is based not only on the sum of the out-of-plane shear forces at the blade root, but also on
(1) the sum of the axial root forces projected on the shaft axis (due to precone),
(2) the weight of the hub projected along the shaft axis (do to shaft tilt or tower deflection), and
(2) the inertia effects of the hub do to e.g. tower deflection and yaw rotation.
Of course, the expression is fairly complicated in the presence of shaft tilt, tower deflection, precone, etc.
I hope that helps.
Thanks for your answer. It has cleared my doubts. I’ve run another case deactivating the tower DoFs and give a value of zero to the mass and inertia of the hub and with the proper coordinate transformation (due to precone) I was able to obtain FAST’s same value for LSShftFxa, LSShftFya and LSShftFza using RootFxc1, RootFyc1…