# Simulation of tower top vibration induced by blade rotation

hello,

is there anyway to simulate the vibration in the tower, which are induced due to the blade rotation?
I tried to simulate this in OPENfast on the onshore 5MW wind turbine with only CompElast turned on, and with setting the blades Dof and the tower fore aft bending Dof to true, I can read out the frequencies in the PSD. But I can’t find the blade rotation frequency in the PSD. Is this because the tower eigenfrequency does not correlate with the blade rotation? Since the blade eigenfrequency do correlate with the blade rotation, how can I find out the tower vibration which are coming from the blade flap and edgewise vibration?

Regards

Dear @Zong.Linyang,

OpenFAST will certainly simulate the vibration of the tower induced by blade rotation. For a 3-bladed rotor without imbalances operating in turbulent inflow , I would expect the blades to oscillate at natural frequencies and at the excitation frequencies of 0P, 1P, 2P, 3P, etc. and I would expect the tower to oscillate at natural frequencies and at the excitation frequencies of 0P, 3P, 6P, etc. I would only expect the 1P frequency to show up in the tower if the rotor has an imbalance, e.g., mass imbalance.

Best regards,

Dear Jason

I managed to find the 3P frequency in the tower fore-aft vibration now. But the 3P frequency component seems very insignificant. I tuned the turbulent inflowwind to 25m/s at a reference height of 90m and the blade rotation is set to 10rpm. And only with all three blade Dof set to true, the 3P frequency (0.5 Hz) has a little peak. (see picture below)

Is this result representable for the excitation effect induced by blade rotation? I thought it would have a greater impact, but it seems the tower vibration caused by blade rotation is not relevant.
I also consider to apply a cyclic wind load with only time variation but no turbulence, I came across the uniform wind file, but I cannot find any reference how to create such file. I really appreciate any help.

Best regards,
Zonglin

Dear @Zong.Linyang,

Can you clarify what turbulent wind conditions you are simulating?

It sounds like you are simulating the NREL 5-MW baseline wind turbine at mean wind speed of 25 m/s, but you mention a rotor speed of 10 rpm; I would expect at this wind speed the rotor would spin at a mean of 12.1 rpm. Did you disable control and force a prescribe speed of 10 rpm instead?

Best regards,

Dear Jason,

Thank you for providing the Inflowwind manual, it helped a lot!
I created the same inflowwind file as 90m_12mps_twr.bts, also with the kaimal spectrum and the normal turbulence model. Only I changed RefHt to 90m and URef to 25m/s. And indeed I turned off ServoDyn and set the blade rotation to 10rpm, which means this frequency and its harmonics should be included in the frequency plot, am I right? I am simulating the 5MW_Land_DLL_WTurb.

Furthermore, I did a simulation to analyse cyclic loads due to gravity acting on rotating components. Therefor I turned off everything(CompInflow, CompAero, CompServo) and only left CompElast on. Now the wind turbine must be idling in still air at the prescribed 10rpm (0.166Hz) and I should be able to identify the 3P frequency (0.5Hz) in the tower top fore aft frequency domain. But I am not sure if the marked 0.5466 Hz in the picture below is the 3P frequency. I also uploaded a picture of my ElastoDyn parameters.

Best regards
Zonglin

Dear @Zong.Linyang,

I would not expect that you’d see the 3P frequency show up if you disable aerodynamic loads because the 3P excitation will come from rotational sampling of the turbulent wind field.

I would guess the 0.55-Hz frequency you are seeing in the structural-only simulation is the 1st flapwise regressive mode.

Best regards,