I added seismic input in the soildyn module. During testing, I created two load cases with seismic and different wind speed in parked condition:
(case a): the steady wind speed 24m/s and pitch =90° in parked condition, and add a earthquake in same direction
(case b): still air and pitch = 0° in parked condition. only a earthquake
I found that the cases a 's response was smaller than the cases b’s about 1%(base tower moment and displacement). This is opposite to my expectation.
Because in case a, the aerodynamic damping should be 0, and the wind and seismic inputs were in the same direction, so the combined forces of wind and seismic loads should be larger than just the seismic force alone. However, the results were opposite. I noticed in case a that the pitch angle remained 90° , but due to the vibration of the upper structure caused by the seismic input, did this lead to a situation where although the output pitch angle was 90°, the actual aerodynamic damping was not 0? I quiet confused.
I set the wind density to 0 for comparison and verification. I found that this phenomenon does exist:
In the load case with both wind and seismic in idling condition,firstly, the vibration of the upper structure caused by the earthquake leads to slight changes in aerodynamic damping, and secondly, even in idling condition, the blades still have areas after pitching to full feather, so there is still wind loading. The aerodynamic damping is close to 0 but not exactly 0;
The wind loading has no effect on the tower (I use the DTU 10MW and AD14 ,CalcTwrAero =false).
The vibration caused by the seismic shut down without wind leads to larger amplitude because there is no wind loading or aerodynamic damping.
In conclusion, the seismic input may change the aerodynamic force distribution under the rated wind speed condition.
Thanks a lot for your attention.
Yes, I answered my own questions myself, but probably wrong. I am a PhD Student at Zhejiang University, and after discussion with my own supervisor, we think that the difference between case A and case B is so small that it can be ignored, and that the aerodynamic resistance is negligible under both conditions.