the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
On the development of a hardware-in-the-loop wind tunnel setup to study the aerodynamic response of floating offshore wind turbines
Abstract. In floating wind turbines, the met-ocean conditions lead to motions of the floater affecting the rotor aerodynamic loads, which in return influence the motion of the floater, in a highly coupled way. Numerical design tools have proven to fail to predict some aerodynamic phenomena, such as the increase in thrust variation caused by unsteady effects. Thus, experimental testing is essential for tuning and validating these codes. Hybrid testing in wind tunnels, by reproducing numerically and actuating the floater motions while measuring aerodynamic loads on a physical scale turbine model, overcomes the scaling issues of traditional wave basin tests allowing a higher fidelity in the reproduction of the aerodynamics. This work presents the development of a hybrid hardware-in-the-loop setup designed to study the aerodynamic response of floating wind turbines in wind tunnels. A scale model of a multi-megawatt floating wind turbine is mounted on top of a six degrees-of-freedom hexapod robot. The full coupling of aerodynamic and floater dynamics is obtained with a hardware-in-the-loop approach with force-feedback-motion-actuation architecture. The rotor loads measured on the physical rotor are fed into a floater dynamic numerical simulator which calculates the motion in real-time and actuates it through a moving platform called hexapod. Key outcomes include the development of a hardware-in-the-loop numerical model with a force correction method to cope with scaling effects and an assessment procedure to verify the simulator, correction model, and measurement-actuation chain. The aerodynamic effects on the motion response are preliminarily investigated on a 10 MW floating concept, with direct estimation of the rotor aerodynamic damping showing a 210 % increase of damping in pitch with the turbine in operation. The capability of testing combined wind and wave cases is also demonstrated, setting the framework for future studies.
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Status: final response (author comments only)
- RC1: 'Comment on wes-2025-100', Anonymous Referee #1, 11 Jul 2025
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RC2: 'Comment on wes-2025-100', Anonymous Referee #2, 21 Jul 2025
Dear Authors,
It was very interesting to review this paper. Empirical data plays a key role in increasing our understanding of the physics of such complex systems, and for the development of numerical tools.
The paper is well written and well structured, and there is no doubt that a considerable amount of work has been put in the development of a HIL system for testing of offshore wind turbines in a wind tunnel.
The introduction states that this work primarily focuses on the verification and validation of the setup. This is an important step in the development of the setup, but one would expect more results and a more thorough analysis of the results to be really convinced about the verification and validation of the setup.
Also, the introduction is interesting but lacks clarity regarding the advantages and the limitations of model testing in a wave basin compared to a wind tunnel. In the abstract , it is stated that the wind tunnel HIL tests overcome the scaling issues of traditional wave basin tests. I would think that this is false, and that the only advantage of wind tunnel tests compared to wave basin tests is in the quality of the wind. Also, additional challenges arise for the wind tunnel tests due to the non-froude scaling and the very limited mass of the RNA.
The paper in its currents state needs more work but will be worth publishing once the two above comments have been addressed. The specific comments below are meant to help you to improve the paper regarding my two comments above, but additional results and discussions are needed regarding the validation and verification.
Specific comments:
L38-40: One of the main advantages of laboratory testing compared to field testing is that the environment is controlled in a laboratory. I think that this should be stated as one of the reasons why laboratory testing is often preferred.
L51-55: It is important to give a clear review of basin tests with performance scaling.
- Why do you state that an arbitrary velocity scaling is used while my impression is that Froude scaling is often used. See for example Bredmose (2017). By using Froude scaling in basin tests, the problem about mismatch in the scaling of the gravity is for example avoided.
- How do you define low quality wind flow. The wind quality is the main difference between basin tests and wind tunnel tests and deserves therefore more than one sentence.
- The mass of the RNA is difficult to match, but is it really not achievable? Already in 2017, Bredmose was only 12% above the specification. But even with a RNA that is a bit heavier than desired, it is still possible to achieve the correct centre of gravity, which is correct for rigid models.
L95: The hybrid setup: Can you add more details about the quality of the flow in the wind tunnel (spatial and temporal variation, capabilities, …), since this is supposed to be better for wind tunnels compared to basins. And this is one of the main “selling” arguments for HIL wind tunnel tests.
L117: The hexapod: Since the main objective of the paper is to verify and validate the setup, one expects more information about the tracking errors of the hexapod. “… without tracking errors”: Does this mean 0 errors, or negligible? A bode plot with amplitude and phase would be very valuable here.
L143: I do not agree with the statement that the main driver for hybrid tests in wind tunnels is to overcome the Fn-Rn scale conflict. The same issue is present in HIL wind tunnel tests and therefore, performance scaling is also used in wind tunnels. The main driver for HIL wind tunnel tests is in the quality of the wind. It is also important to highlight the challenge that arises in HIL wind tunnel tests, due to the non-Froude scaling. While this is not a problem in basin since Fn scaling is used there in combination with performance scaling.
L189: Please explain how you arrive at the mismatch in ratio of 150. The mismatch in ratio between the aero and the inertial forces is the same as the Renolds mismatch (given in table 2) times the mass mismatch (10). The mismatch in ration between aero and gravitational is the same as the mismatch in Rn*Fn*mass mismatch (10).
L221: Floater Dynamics. Since the objective of the paper is verification and validation of the HIL setup, this section about the numerical model should be exhaustive. The information is scarce, and it is difficult to know what is exactly included in the numerical model.
- Equation 4: Added mass on the LHS is a part of the F_Hydro. Same for F_rad in L454.
- L246: Is it only radiation damping or should it be radiation loads?
- L247: Please clarify what is meant by wave diffraction since Faltinsen and Newman have different interpretation for this term.
- What about second order wave loads?
- L456: What is F_diff,2 and why is it not included in A7.
L266: Floater simulator: If the objective is to verify and validate, then one should compare FAST and the Simulink with the same parameters and verify for a good agreement. By tuning the parameters for agreement, you are hiding possible errors in the numerical model.
Figure5: The agreement in pitch is not so good while pitch is one of the key DOF for Floating wind turbines. Would it be possible to use the angular pitch acceleration derived from the accelerometers at tower base and top?
L309: What do you do with the estimated latency? Can you give additional explanations on why you believe that you can split the delay evenly between motion and communication. Other laboratories have tried to compensate the delay, see for example [2] Have you tried similar?
Technical Corrections:
L34: Can you add a reference to the statement about premature maintenance.
L49: Rn does not compromise the hydrodynamic loads (viscous effects are correctly scaled), but it compromises the gravity related hydrodynamic loads.
L51: Is it correct to state that an arbitrary velocity scaling factor is used in performance scaled tests? My impression is that in basin tests, performance scaling is used in combination with Fn scaling. See for example [1].
Figure2: Did you have to adjust the blade pitch angle to achieve the desired thrust?
L146: Even with non-Froude, inertial phenomena can be correctly reproduced. The problem is that the balance between inertial and gravitational loads is not correctly reproduced.
Table 3: Please add Froude mismatch
L184: The same challenge would arise also for custom made components. The challenge is mainly due to the complexity of RNA and the need for sensors and actuators.
References
[1] H. Bredmose et al., “The Triple Spar campaign: Model tests of a 10MW floating wind turbine with waves, wind and pitch control,” Energy Procedia, vol. 137, pp. 58–76, Oct. 2017, doi: 10.1016/j.egypro.2017.10.334.
[2] Zhihao Jiang, Binrong Wen, Gang Chen, Xinliang Tian, Jun Li, Danxue Ouyang, Zhike Peng, Yehong Dong, Guiyong Zhou, Real-time hybrid test method for floating wind turbines: Focusing on the aerodynamic load identification, Journal of Ocean Engineering and Science, 2024,
Citation: https://doi.org/10.5194/wes-2025-100-RC2 - AC1: 'Comment on wes-2025-100', Federico Taruffi, 29 Aug 2025
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- 1
Dear Authors,
I was invited by the Associate Editor to review your article and was pleased to accept, as the topic is of significant interest to me. I dedicated a considerable amount of time to its review. As you state, there are currently very few setups capable of conducting hybrid wind tunnel experiments for floating wind turbines and the development of such systems could be important for advancing floating wind technology. Therefore, I believe the topic can be of interest to the research community and the readers of this journal.
However, the manuscript requires substantial revisions to be technically sound, reproducible, and impactful.
As it stands, the article primarily describes the development of a HIL setup as a preparatory step for your future experiments, with limited contribution to the understanding of coupled dynamics in floating wind turbines. While I fully understand the need to establish a foundation for future work, a publication must provide clear value and insights for the reader.
General comments
I recommend that you clearly state, in the introduction, the relevance of your work, its main objectives, and its novelty compared to previous studies. In other words, the readers should understand what they can gain from the article. To support this, I suggest expanding the literature review so that gaps in the current state of the art become more evident, and your contribution can be better contextualized.
A second general point concerns the technical accuracy in some parts of the manuscript. I believe responding to the Specific Comments can help improve this aspect, but I also encourage greater care in ensuring accuracy and clarity in the article.
Specific comments
Technical corrections
References
[1] Papi, F., Jonkman, J., Robertson, A., and Bianchini, A.: Going beyond BEM with BEM: an insight into dynamic inflow effects on floating wind turbines, Wind Energ. Sci., 9, 1069–1088, https://doi.org/10.5194/wes-9-1069-2024, 2024.
[2] Schulz, C. W., Netzband, S., Özinan, U., Cheng, P. W., and Abdel-Maksoud, M.: Wind turbine rotors in surge motion: new insights into unsteady aerodynamics of floating offshore wind turbines (FOWTs) from experiments and simulations, Wind Energ. Sci., 9, 665–695, https://doi.org/10.5194/wes-9-665-2024, 2024.
[3] Fontanella, A., Facchinetti, A., Daka, E., and Belloli, M.: Modeling the coupled aero-hydro-servo-dynamic response of 15 MW floating wind turbines with wind tunnel hardware in the loop. Renewable Energy, Volume 219, Part 1. https://doi.org/10.1016/j.renene.2023.119442, 2023.
[4] Jiang, Z., Wen, B., Chen, G., Tian, X., Li, J., Ouyang, D., Peng, Z., Dong, Y., and Zhou, G.: Real-time hybrid test method for floating wind turbines: Focusing on the aerodynamic load identification. Journal of Ocean Engineering and Science. https://doi.org/10.1016/j.joes.2024.06.002, 2024.