I am George, a student who have research on wind turbine. Thank you for providing us this forum to discuss problems concerning academic and technique problems on wind turbine.
I am now trying to use OpenFast to simulate the dynamic response of a 5MW onshore wind turbine (as specified in Test18 Baseline) under different wind speed so as to determine the fatigue damage at tower base under each case. The wind are set to be turbulent with different average velocity for each case as generated from Turbsim.
However, when I compare the “stress-time history series” at the tower base (TwrBsMyt) under 4m/s and 6m/s, the tower seems to vibrate much more severely under 4m/s than under 6m/s, leading to a greater fatigue damage under lower wind speed (as shown in the following two pictures).
All the settings in AeroDyn, ElastoDyn, ServoDyn and primary files are same as default except for the Wind Inflow file are set to read Turbsim file of different wind speed.
My questions is that:
* Is it reasonable for a tower to vibrate severer under lower wind speed? (I guess when the rotor is under a stochastic wind speed around cut-in, the rotor control system will switch back and forth between on and off, thus increasing the tower vibration due to unsteady control command, but it just my personal guess).
* Or is there anything wrong with my operation of the software, or are there more possible reasons for this scenario that I encountered?