# natural frequency of tower and blades

I am using FAST to simulate wind turbine loads. My question is regarding to the natural frequencies of the flexible parts such as tower and blades. By using finite elements software like ANSYS, the calculation of natural frequency results in different amount compare to the what Modes and BModes does. In addition, the Blade manufacturer has stated natural frequencies in blade catalog which doesn’t match with Modes and BModes results.
In my opinion, there is two different approaches:
First, accepting what FAST simulation does, although there is differences between the real and simplified model.
Second, tuning the natural frequencies by varying factors to adjusting mass, flap or edge stiffness to the real model.
Are these factors usable for tuning frequencies? or not?
By setting these factors, do they affect the whole simulation results?

Dear Feizbakhshi,

You haven’t provided much details on how you modeled the solution in ANSYS, why you expect differences compared to Modes/BModes, or how different the results are, but one possibility is that Modes/BModes are specifically tailored to predicting the mode shapes of centrifugally stiffened rotor blades (the natural frequencies of blades are impacted by the rotational speed, among other things).

A recent forum topic has briefly discussed the use of blade mass and stiffness adjustment factors for tuning to match known data: http://forums.nrel.gov/t/the-differfence-between-flsttunr-and-adjedst/990/1. Yes, the adjustment factors affect the “whole simulation”.

Best regards,

Thank you so much for your response

My mistake was about blade frequencies. Because in catalog the stationary frequencies have been stated. In Modes software, I change the rotational speed to zero and got a good approximation of natural frequencies(with ±5% deviation) without changing the adjustment factors.
In case of Tower, ANSYS modeling would consider the effect of flanges or tower door which is different geometry compare to tabular tower in Modes or BMode. However, we didn’t do ANSYS modelling for new tower and blade geometry and I couldn’t provide numbers to compare them.

Regards