Hello all,

Once again, thank you for your continuous support through this forum, its more than amazing how much it helps both sides improve their knowledge.

To try and make my post as short as possible, I will summarize what I want in the following lines:

I’m trying to create a 2D model for an Onshore Wind Energy Conversion Device Tower.

What is my model supposed to do?

Given the elasticity and section characteristics as well as forces and moments acting on the top of the tower, IT SHALL calculate the acceleration of the tower top.

What is my procedure?

I want to validate my model by the following steps:

- I take the Onshore 5MW turbine (provided within wind.nrel.gov/public/jjonkman/NR … Bsline5MW/ )
- I run FAST, obtain information related to forces and accelerations at Yaw Bearing.
- I apply the obtained forces from FAST onto my model, and let my model calculate the acceleration
- I compare the acceleration obtained by my model to the acceleration obtained by fast: if the match: perfect. if Not: debug and try again.

PS: I set the structural damping in the tower file for all modes as 0.

What is my problem:

My model fails to get something similar to Fast’s acceleration.

Question:

Is the approach of considering the tower as a cantilever beam with a lumped mass on top (for Hub-nacelle assembly) correct? I used a beam element stiffness matrix with 3 DOFs per node, namely: Axial deformation, Transverse deformation and Rotation.

For the Mass matrix I utilized a consistent mass matrix.

Of course, I divided the tower into (N) beams and assembled the Global stiffness matrix, as the tower has a tappered section. It is worthy to say that the number of divisions performed on the tower is the same as those provided in the FAST .fst file

For the solution of the equations of motions: I formulated the well-known dynamic differential equation that relates Mass, acceleration, Damping, velocity, stiffness and displacement to the forces:

Ma + Cv + Kx = F(t)

for the special case of 0 damping, one gets:

Ma + Kx = F(t)

to solve for the acceleration, I used the Newmark Constant Acceleration Scheme.

I’m absolutely sure that there is no mistake in the stiffness, mass matrices as well as the solution scheme, as I tested it on a generic cantilever problem with F(t) and it yielded exact results.

Am I missing something? Any help is highly appreciated.

Best Regards and Uppermost Respects,

MJ