FAST: Rotor torque during increase of wind speed

Wind & Water Computer-Aided Engineering Software Tools
1

Hello everyone,

I am having trouble making sense of some simulation results with FAST:

I am using the Simulink interface to do some very simple simulations of a wind turbine. All control inputs are set to fixed values, i.e., generator torque, power, pitch angles and yaw command. The wind input file is a simple hub-height wind with a constant value. After some time, the wind signal jumps to a higher value. Thus, since all control inputs are kept constant, the wind turbine accelerates to a higher rotational speed. So far, everything works out fine.

However, I have a question regarding the simulation results. I would expect the rotor torque (RotTorq or LSShftMxa) to jump to a higher value the instant the wind signal suddenly increases, since aerodynamic torque is suddenly increasing. In my simulation, with my turbine model, this is not the case. The increase in rotor torque is only miniscule. If this is the case, what causes the wind turbine to accelerate as it does?

I checked the scenario using the NREL 5MW reference turbine model. There, the increase in rotor torque is more apparent. I think there is one important difference between the two turbines: The NREL reference turbine has a gearbox with i=97, whereas the model I am working on has no gearbox, i.e., i=1. Could this be an issue?

I am using FAST 8.10, but I also checked with FAST 7, there is no change.

Am I looking at the wrong output signal? Is there another way to obtain the aerodynamic torque on the rotor?

Thanks,
Peter

2

Dear Peter,

RotTorq is the same as LSShftTq and is discussed at length here. Basically, rotor torque is the difference between the applied aerodynamic torque and the inertia due to rotor acceleration/deceleration. So, even though the aerodynamic torque may jump, the delta may be offset by the inertia due to rotor acceleration/deceleration. When the generator torque is held constant, the steady state RotTorq will equal this generator torque (multiplied by the gearbox ratio). The rotor acceleration/deceleration will be impacted by the rotor flexibility, which is why I suspect you are seeing differences between the NREL 5-MW model and your other model.

Until recently, the applied aerodynamic torque was not an output directly available from FAST. But we’ve recently added it as an output (named RtAeroMxh) in AeroDyn v15 within FAST v8.12.

Best regards,

3

Dear Jason,

thanks for your help.
I noticed that AeroDyn 15 supports the output of the aerodynamic torque, but I was wondering if there was a way around restructuring my model for AeroDyn 15 so I can take a look at the signal.

However, this seems to be impossible (according to the other threads), if I want to have all DOFs enabled and calculate the aerodynamic torque. So I guess I will take a closer look at AeroDyn 15.

Thanks again for your help!

Best regards,
Peter