the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
An experimental study on the aerodynamic loads of a floating offshore wind turbine under imposed motions
Felipe Novais
Axelle Viré
Abstract. The rotor of a floating wind turbine is subject to complex aerodynamics due to changes in relative wind speeds at the blades and potential local interactions between blade sections and the rotor near-wake. These complex interactions are not yet fully understood. Lab-scale experiments are highly relevant for capturing these phenomena and provide means for the validation of numerical design tools. This paper presents a new wind tunnel experimental setup able to study the aerodynamic response of a wind turbine rotor when subjected to prescribed motions. The present study uses a 1:148 scale model of the DTU 10 MW reference wind turbine mounted on top of a 6 degrees of freedom parallel kinematics robotic platform. Firstly, the thrust variation of the turbine is investigated when single degrees of freedom harmonic motions are imposed by the platform, with surge, pitch and yaw being considered in this study. For reduced frequencies greater than 1.2, it is found that the thrust variation is amplified by up to 150 % compared to the quasi-steady value when the turbine is subject to pitch and surge motions, regardless of the amplitude of motion. A similar behaviour is also noticed under yaw motions. Secondly, realistic 6 degrees of freedom motions are imposed by the platform. The motions are derived from FAST simulations performed on the full-scale turbine coupled with the TripleSpar floater and the tests aim at exploring the thrust force dynamics for different sea states and wind conditions, obtaining reasonable agreement with the simulations. Finally, the work shows the capabilities and limitations of an off-the-shelf hexapod to perform hybrid testing of floating offshore wind turbines in wind tunnels.
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Federico Taruffi et al.
Status: open (until 12 Oct 2023)
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RC1: 'Comment on wes-2023-86', Anonymous Referee #1, 13 Sep 2023
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The paper describes nicely experiments focusing on thrust measurements on a model turbine mimicking floating motions in different degrees of freedom. The paper is well written and the topic is addressing a highly interesting and important aspect of floating wind turbines. Nevertheless, the authors could improve the paper by adding more details and getting a little more in the discussion.
1) In section 2.1 „Wind turbine model“ information of the turbine itself like the diameter, hub height etc. are missing. They are later in table 1 and 2 but the reference to these tables are missing.
2) From figure 1 it looks like the experiments are performed in a closed test section. Here the total blockage of the setup in the test section should be added. The presents of the turbine model could impact the velocity measurements based on the Pitot tube that was placed in the test section. What was the distance between the Pitot tube and the turbine?
3) The authors say that the turbulence intensity in the wind tunnel is below 0.5% up to 1 m behind the nozzle and lower than 2% 6m behind the nozzle. Where was the turbine placed during the experiment and what is the turbulence intensity at the distance? Also, turbulence intensity might not be enough to really identify any structures in the flow especially when operating the wind tunnel at very low velocities. Did the authors measure the incoming wind without the turbine present? Did they determine the spectrum and scales in the flow? An increase from 0.5% TI to about 2% over a distance of 5m ist quite significant.
4) The authors say that since the turbine has a fixed pitch there are no problems with any misalignment between the blades — who did they measure it? Even by mounting three blades in a fixed position there can be some differences in the pitch angle.
5) It is not quite clear how the turbine is operated. Is the turbine actively driven bei the motor or is there a control system that keeps the rotational frequency constant? The authors should explain that a little more. Could the constant speed of the turbine effect the comparability of the results to a real turbine which has variable speed control?
6) In figure 3, what is the dotted line?
7) In section 4.1 in the discussion of figure 4 the authors discuss torque instead of power. They should make clear which relation they use in order to be able to do so. Also, they should mention the uncertainty in the torque measurement.
8) The mentioned uncertainty in the thrust measurements is about 10%. I can imagine that the signals are also suffering from periodic fluctuations due to small imbalances in the turbine. Is that the case and how did the authors deal with that to come up with the 10% uncertainty ? Also, periodic vibrations in the turbine could couple with frequencies from the hexapod. Did the authors make sure that their result of larger variation in the thrust is caused by such interaction?
9) The authors should add error bars in the deltaT plots.
10) The discussion of figure 11 is very brief. By looking at the presented signals I was wondering why for the lower frequency the measured forces for the condition with and without wind are not in phase while they are in phase for the higher frequency. I did not expect that. Can you comment on that?
11) In the same figure the force measurement at time 0s for the case with wind is negative. Could the authors explain that.
12) I do not really see the benefit of adding the pitch motion, yaw motion and wave load case to the paper. All these sections are very short and the conclusions are not quite clear to me. Especially the results shown in figure 13 should be shown with error bars since the variation between the single frequencies are very small.
13) The frequency in figure 14 is not clear to me. The real turbine case should move at very slow frequencies compared to the model turbine. From the description in the text it is not clear why they are in the same range. The authors should also discuss the sharp peak int he experimental data. Since the frequency is not the same for the two cases I wonder what the origin is.
Citation: https://doi.org/10.5194/wes-2023-86-RC1
Federico Taruffi et al.
Federico Taruffi et al.
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