“This post is about the issue I have faced with huge blade and tower load oscillations”
I have a load case where the turbine is rigid, the drive train rotates freely without load as the generator is disabled and the gravitation is switched off. The pitch is fixed (zero) and the yaw is fixed 10 degrees. I have represented the yaw error as inflow wind with direction change of 10 degrees. The initial speed of the rotor is 8 rpm. The wind speed is variable from 0 to 10 m/s in 100 seconds. I have used the equilibrium inflow model and the tip and hub losses are switched on. The simulation results are quite strange as there are huge oscillations in the blade root forces and bending momets, particularly in blade Root_Mx. There are also similar oscillations in tower loads. I have also tried increaing the time step of the simulation time, but it did not work. What could be the reasons for it?
I have attached an image of the blade Root_Mx. The simulation time step is 0.02 and the simulation time is 300 seconds. As I have used the filtered data to plot the graph, the oscillations does seem to be periodic, but they are periodic in the actual case.
You have somewhat of an odd simulation set up and it would help to see other outputs to understand what is going on (like the rotor speed). My guess is that the rotor would accelerate up until a speed where the mean aerodynamic torque is zero. Once the rotor speed reaches a quasi-equilibrium, I would expect the mean blade-root Mx to be zero (like the mean torque). I would expect the blade-root bending moments to oscillate with the rotor frequency (and potentially its harmonics) because in skewed (yawed) flow, the blades move into and out of the flow. This is basically what you are seeing.
Tanks for your response. The information you have provided is very much useful to me. However, I could not figure it out the reason for such high amplitude of the oscillations. Moreover, The oscillations did not converge to zero after the rotor speed has reached quasi equilibrium state as you have said. I have attached the plot of the rotor speed.
Your plot of rotor speed confirms what I was saying–the rotor accelerates up until a speed where the mean aerodynamic torque is zero. Once the rotor speed is fixed, the mean aerodynamic torque and blade-root Mx are zero, but Mx oscillates because in skewed flow, the blade moves into and out of the flow.