the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
Experimental investigation of the effects of floating wind turbine motion on a downstream turbine performance and loads
Abstract. This study investigates how the motion of a floating wind turbine affects the aerodynamic performance and dynamic loading of a downstream turbine operating in its wake. Wind tunnel experiments were conducted using a two-turbine setup, where the upstream turbine was subjected to controlled platform motions (both sinusoidal and wave driven) while the downstream turbine remained fixed and was tested in multiple relative positions. Results show that large-amplitude, low-frequency, sinusoidal motions of the upstream turbine, especially in crosswind and yaw directions, can increase the power output of the downstream turbine under low-turbulence conditions and at short turbine spacing (3–5 rotor diameters). The largest relative power gain reached 26 % over the fixed case, although absolute gains remained moderate. The gains obtained under idealized sinusoidal motions were replicated in cases with realistic-wave-driven motions when wind and waves were aligned, but not when wind–wave misalignment introduced crosswind movements of the upstream wind turbine. In parallel, motion of the upstream turbine increased the dynamic loading on the waked turbine. These effects varied with turbine spacing and alignment, and were more pronounced in sinusoidal motion cases. Still, similar mechanisms were observed under wave-induced motion. Overall, these findings underscore that platform-induced wake dynamics are not a secondary effect, but a key driver that must be considered in the design and operation of floating wind farms.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Wind Energy Science.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on wes-2025-106', Anonymous Referee #1, 14 Jul 2025
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RC2: 'Comment on wes-2025-106', Anonymous Referee #2, 11 Aug 2025
The paper presents interesting load measurements for a downwind turbine, in the wake of a dynamically moving turbine (representing offshore floating conditions). The results are important and highly valuable for the field of floating wind energy. The paper reads well, though is not the most concise. If made more to the point and concise it could help future readers. The paper presents results for many different conditions, and hypotheses are proposed for the observed behavior, but are mostly not proven or verified by a deeper analyses of the underlying physics. The reviewer considers the paper acceptable for publication given revisions are made according to the comments below and the comments made by the other reviewer. Most importantly, the paper should describe more clearly which wind measurement data is used, how it is measured, or where the rotor effective wind speed is reconstructed through thrust measurements. More details about how this is calculated is also needed in the paper: formula and used coefficients. I want to put attention to the fact that the rotor reconstructed velocity has its limitations, and if the entire analysis is based on the rotor reconstructed velocity, a second review should be performed to make sure the analysis is correct.- L12 'a key driver' - sentence is missing for what it is a key driver? For wake recovery? unsteady loading ?- L21 'substantial power losses, increased fatigue loads' --> The platform motions could increase or decrease wake losses and fatigue loads, as also shown in this paper. It is more unbiased and correct to word it as 'change', 'affect; or 'impact' those performance characteristics, without suggesting it will increase or decrease.- L59: What is not discussed in the introduction is the different time scales at which platform and wave motions can occur (Strouhal number of motions). A difference should be made between very slow motions with larger amplitude and very fast motions usually with a much smaller amplitude. The impact on the wake can be very different, and could impact the summarized conclusions in this section.- L75: 'more realistic turbulence levels.' --> It is relevant to add for which conditions/ Strouhal numbers and amplitudes these findings were made.- L87: The two key research questions are a bit misleading. We know the answer is yes to both questions. The real question is: how much and for which conditions. I think the real answers from this paper are about the quantification and characterization for specific test conditions.-L131: It would be good to add to this sentence how the authors expect that the results in this paper should be interpreted to make conclusions for wind turbines with larger spacings. Likely the motion signatures in the unsteady loading will reduce for a larger spacing? Can the authors make predictions based on the tests at different locations?- Experimental setup: Is the design of the rotor blades changed for a lower Reynolds number? What is the Reynolds number during the tests? Does the thrust and power coefficient match the full scale value?- L154 'noticeable influence' Can the authors describe what the influence is?- L162: A fixed rpm leads to varying tip speed ratio during pitch or surge motions. Can the authors estimate how much the tips speed ratio varied for the first wind turbine? This can possibly help understand the impacts on unsteady loading.- L163: How much was the thrust (coefficient)? Good to document this for future reference or giving the chance to reproduce or model the results.- L174: Can the authors describe more precisely how this was calculated? The wind turbine thrust coefficient is calibrated with tip speed, or expected to be constant in the operating range?- L176: How is the optimal tip speed ratio determined for WT2? Tested by varying or theoretical?In general it is not clear in the paper where real wind speed measurements are used and where the reconstructed rotor-effective wind speed is used.- Table 3: can the authors add the reduced frequency to the table?- Figure 5: the wake losses seem very high! Usually we expect the power of a downstream aligned turbine to be closer to 40%-50% or even a little higher, while the data shows 20% at a spacing of 5D. Is the thrust coefficient of the turbines very high? Is the incoming turbulence very low? It is important to discuss this in the paper, because it can affect the unsteady loading and its sensitivity to platform motion.- L265: How are the velocities for this formula defined or measured? The paper is missing a description of wind speed measurements. Maybe a more clear reference to previous work would help.- L304: how is this average wake velocity determined? Is this calculated from the measured thrust force?- L306: I can not find the 16% on figure 5- L315: These observations indicate a high thrust coefficient of WT1. Is the magnitude expected to be so high? any references in the literature?- L345: 'delay of ' Assuming the disturbance travels with an average convective velocity in the wake: can the authors use this time difference to estimate the convective velocity and see how it compares to the free stream velocity?Figure6 : How is the thrust and power coefficient determined / how is the velocity measured to determine these coefficients? This is not clear in the paper. Velocity measurements are not really discussed as part of this paper. It looks like CT for WT1 and WT2 are normalized by the same velocity? which is not conventional: usually it is defined wrt the incoming velocity of each turbine, which is lower in the case of WT2.- L374: The higher the reduced frequency, the shorter the streamwise wavelength of the velocity perturbations caused by the motion of the turbine. Based on the incoming wind velocity and the frequency the authors could estimate the wavelength of the velocity fluctuations created by the turbine motion. If this wavelength is of the same size as the integral length scale of the incoming turbulence, or perhaps the rotor diameter, it would make sense that it dissipates or decorrelates much faster. Perhaps the highest motion frequency passes a specific treshold explaining why the turbulent structures dissipate quickly?When the authors say weaker: do they mean smaller in amplitude, shorter in wavelength, or both?- L390: Again not clear how the velocity is measured.Figure 8 a and b: Are these absolute thrust values, or fluctuations of the thrust signal?- L409: It would be good to state that this conclusion is for the specific streamwise spacing. For a larger streamwise spacing the wake spreading will be larger, possibly changing the conclusion.- L511: In this study only the first turbine is moved. By moving it the unsteady loading also increases for WT1. How would the findings in this paper change if WT2 also moves with waves? And what if the motion of WT2 is spatially and temporally correlated to the motion of WT1 such that movements of WT2 could be in sync or anti-sync with changes in windspeed from WT1? It would be helpful for the paper if the authors could make a comment about this for context.- L530: See previous comment. For a bit more completeness, it would be good to add to this summation as well ' spatio-temporal correlation of waves through the wind farm'.
- The discussion section is a little awkward. It doesn't seem to present a new discussion but rather an overview of previous conclusions.Citation: https://doi.org/10.5194/wes-2025-106-RC2 -
AC1: 'Comment on wes-2025-106', Alessandro Fontanella, 12 Aug 2025
Dear Referees,
Thank you for your thorough reviews and constructive comments, which we will address in detail while revising the manuscript. We think your feedback helps to further develop and clarify the results presented in the paper, and we greatly appreciate it.
Both of you raised points regarding the velocity measurements performed in the wake and the estimate of the rotor effective wind speed used to regulate the rotor speed of the downstream wind turbine. We take this opportunity to briefly clarify the distinction here, while a more detailed explanation will be included in the revised manuscript.
The rotor effective wind speed estimate was used solely to set the rotor speed and tip-speed ratio (TSR) of the downstream turbine, ensuring operation close to its nominal performance curve. A dedicated Appendix will be added to explain in detail how this estimator works.
In contrast, the wind speed values discussed in the results were obtained from hot-wire probe measurements. The experimental setup and methodology for these wake measurements are described in detail in [https://doi.org/10.5194/wes-10-1369-2025]. While our original intent was for this work to complement the study presented in [https://doi.org/10.5194/wes-10-1369-2025], we agree with your suggestions that this article would be significantly strengthened by including additional results and details on the upstream turbine wake measurements, and we will incorporate these in the revision.
Citation: https://doi.org/10.5194/wes-2025-106-AC1
Data sets
NETTUNO Experiment 2 – Effects of floating wind turbine motion on a downstream turbine performance and loads Alessandro Fontanella, Stefano Cioni, Francesco Papi, Sara Muggiasca, Alessandro Bianchini, Marco Belloli https://doi.org/10.5281/zenodo.15582186
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