difference of tower momnet between Bladed and Fast

Dear jonkman,

 I just translated an onshore wind turbine Bladed model to OpenFast model and compared the simulation results between the two models. I found that the moments My and Mz in tower top and base have great difference, especially the Mz value. I builded the Fast model with the exact same parameters value as Bladed and I have checked a several times. I calculated the mode shape by using Bmodes with its parameters value of input files came from Bladed too. Both of the Bladed and Fast model simulated with same aerodynamic conditions:  equilibrium wake, tip and hub loss, B-L stall model. And only used ElastoDyn during Fast simulation. The attachments shown the simulation results within an 6-7 m/s step wind, a 6m/s steady wind at the begining and then instantaneous increases to 7m/s (which is also shown in attachments). And the orange lines refer to Fast, green ones is Bladed.![wind.png|814x480](upload://saCEyGccp6jXKSUemNCDqLRof6M.png)![Mz My tower top.png|963x1163](upload://6ZX7wq4fcJQAd5m7Bj8PFarxJIN.png)
I also found that the blade 1 root moment Mz of Fast is much larger than Bladed. I think the difference maybe that in ElastoDyn the centres of mass and shear of blades are not taken into consideration.![mzb1.png|822x480](upload://vzXN8L1ntFSbA7vtyPyIZvGmZte.png)
Could you tell me what makes the great diifference of My and Mz in tower and blades? And how to deal with it to make the Fast and Bladed results similar? 
Additionally, could you send me Fast theory maunual?  
I am looking forward your feedback, thank you!

Best regards!

Dear Shijing Xu,

I’m not an expert on Bladed, but I would expect you to get similar, but not identical, results between FAST and Bladed. Perhaps the differences you are seeing are the result of small differences in the input file settings between the models are different assumptions made by each software.

The tower-top yaw moment plot you are showing is odd to me because the sign of the mean moments are different; I would not expect this, which suggests some difference in the input file settings.

The difference in tower-top pitch moment could be the result of a different aerodynamic thrust or a different tower-top mass or center-of mass location.

The ElastoDyn module of OpenFAST does not consider mass or elastic offsets from the pitch axis, but the BeamDyn module of OpenFAST does. But it is difficult for me to guess why the pitching moment you are showing differs.

To investigate further, I would probably start by simplifying the model further. Do you get the same static deflections and loads when you disable the aerodynamics and load the wind turbine by gravity only?

Please see the following forum topic for more information on the theory basis of the ElastoDyn module of FAST: http://forums.nrel.gov/t/coupled-blade-modes-in-fast/314/1.

Best regards,

Dear, Jason

Thanks for your reply. I simulated the turbin with disable the aerodynamics and fixed rotor speed in both Bladed and Fast as your suggested, and found that the result still was queit diffrent. The offset and amplitude of fore-aft deflection calculated by Fast both larger than the Bladed results; and the tower top moments My Mz have the same trends. As shown below, the blue line refers to Fast result and orange is Bladed.

I guess there must be something wrong with my Fast model, however I do not know how to find it. could you give me some advises?

Dear Shijing.Xu,

From the tower-top deflection and tower-top pitch moment, it appears to me that the rotor-nacelle assembly mass or center of mass is different between the FAST and Bladed model. The tower-top axial force should help you figure out if it is the mass or center of mass (or both) that are different between the models.

Best regards,