# Modeling of the Tower head mass according to NREL conference paper

Hi Dr. Jonkman

currently I am trying to create a model of the 5MW wind turbine in ANSYS for validation purposes and I came across a conference paper of NREL about “Modal dynamics of large wind turbines with different support structures”. This paper proposed the describtion of the tower head mass consisting of the rotor-nacelle-assembly and it also presented the modal frequency of the land based 5MW wind turbine with and without head mass:

I managed to model the tower without the head mass pretty precisely based on the modal frequencies but as soon as I added the head mass my models natural frequencies begin to offset from those in the table. I assume that I did not model the tower head mass correctly. Since the location of the center of tower head mass is given in table 1 (1.97m offset from tower top and 0.41m in upwind direction), I modeled a point mass with the given information at that point and made a connection between the node at tower top and the location of tower head center of mass with two rigid beams which have a perpendicular constellation like below

Since the resulting modal frequencies are all shifted about 0.4Hz, I assume my model of this connection is not very accurate. Do you have an idee how to better model the connection between the tower top and the tower head center of mass?

Best regards

PS: This is the conference paper what I am talking about:

Dear @Zong.Linyang,

I’m not sure I understand why you need two rigid beams. Couldn’t you rigidly connect the tower-top center of mass with the tower-top through a single rigid link?

Regardless, does your tower-top point mass include rotational inertias in addition to the mass? These inertias will have a large impact on the tower natural frequencies.

Best regards,

Hi Dr. Jonkman,

yes, the point mass include all rotational inertias. I used two rigid beams because the tower-top center of mass is offseted to the tower top in two directions. The tower has a height of 87.6m but the tower-top center of mass is at 1.97 above the tower top and also the tower-top mass has an offset in the upwind direction.

Best regards

Dear @Zong.Linyang,

If you are able to match the tower-top mass, center of mass, and rotational inertias, I’m not sure what else could be the problem.

Best regards,