# Structural control (TMD) effect on the tower fore-aft vibration very insignificant

Hey everyone,

I am working on the design of a tmd suppressing the 3P frequency of the 5MW wind turbine. The initial parameter design is based on the fixed point theory, resulting in a Tmd with a eigenfrequency of approximately 0.6 Hz which is the 3P rotational frequency in my simulation. Here is the frequency domain of the tmd x position displacement in operation.

The tmd mass is chosen to be 8638 kg making up around 2% of the 5mw wind turbines total mass. The location is chosen to be at the height of the nacelle. But the results does not make sense. I plotted the tower top fore aft displacement in time and frequency domain with and without the tmd as comparison. In the time domain, the tmd causes even a higher tower top fore aft displacement then without the tmd

In the frequency domain the vibration reduction effect is also not very significant

Clearly i have done something wrong, since the tmd is not supposed to amplify the vibration. Does anyone have an idea what is causing this problem?
Here are additionally the tmd position x displacement and local x direction force in time domain. I appreciate any help

Best regards

Dear @Zong.Linyang,

The displacement of the TMD in your result looks quite small. Perhaps your TMD stiffness is set to high (although the frequency in your first plot looks correct)?

Can you share your ServoDyn and StC input files?

Best regards,

Hey Dr. Jonkman

here are my servodyn and stc inputs

Best regards

Dear @Zong.Linyang,

The forces and displacements you are getting from the TMD appear consistent with the mass and stiffness you specified in the StC input file. It is just that the resulting forces are quite small relative to aerodynamic loads in the NREL 5-MW baseline wind turbine.

Can you clarify what condition you are simulating? Looking at your original plot, I mostly see a mean tower-fore aft displacement, with only a very small oscillation. Given that the oscillation without the TMD is small, it is not surprising that the TMD is not doing much.

Best regards,

Dear Dr. Jonkman

I am trying to simulate the normal wind turbine operation unter the rated wind load. The inflow wind is a steady wind with the rated wind speed of 11.4m/s at a reference height of 87.6m (yaw bearing height). In ElastoDyn I turned the Generator DoF on and set the initial Rotorspeed to 0, then I am simulating the whole start up process until a steady state of power production is reached and i am only analyzing the latter part. I was also wondering since the tower fore-aft oscillation is quite small. Based on your experience, would you say that the fore aft vibration displacement is unusually small under the aforementioned wind load?
Additionally here is the rotor speed, the tower top fore aft displacement and the thrust force at yaw bearing height of the whole simulation

Best regards

Dear @Zong.Linyang,

With steady wind input, I would expect the wind turbine to reach a steady-state condition (or periodic steady state due to rotor rotation), with the TMD not influencing much at all. I would expect the TMD to have much larger influence under stochastic excitation from turbulent inflow.

FYI: To minimize simulation start-up transients and to avoid potential issues with the controller, we typically recommend initializing the rotor speed and blade-pitch angles to their expected (mean) values based on the mean wind speed you are simulating. At rated wind speed for the NREL 5-MW baseline wind turbine, these would be `RotSpeed` = 12.1 rpm and `BlPitch` = 0deg.

Best regards,

Dear @Zong.Linyang,

Based on my little experience, you should not rely too much on literature especially those who did their own MDOF model and they did not compare it with OpenFAST.

Another thing which is very important, a TMD will not reduce vibrations if the frequency at which the TMD is tuned, is not excited. Therefore, you should first determine which is the source of excitation in your system then add a TMD or not.

I think you are working on the 5 MW baseline wind turbine (onshore wind turbine). It seems to me that you did not set up your simulation correctly.

One more thing, the TMD only affects the standard deviation of the response and not the mean value. The mean value is in general affected when you use in a floating wind turbine an active ballast system.

Best Regards,