the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jun 2025
Wake control through tip-speed ratio adaptation
Abstract. Offshore wind farms can generate wake losses between 10 and 20 % even after layout optimization. By altering the turbine operational parameters, it is possible to reduce internal wake effects. A wake-control approach by modifying the operational tip-speed ratio is presented here using LES high-fidelity modelling with DTU 10 MW reference wind turbines. Single- and two-turbine simulations for tip-speed ratios ranging from 5 to 12 are assessed for wake losses and improved power production. Simulations were conducted with non-turbulent inflow, as subtle rotor- and wake aerodynamic effects are difficult to identify in turbulent flow.
Single-turbine simulation results show that the wake development is strongly influenced by the operational tip-speed ratio. At a tip-speed ratio of 8, a stronger wake with increased turbulence and a relatively short recovery distance is observed. Tip-speed ratios greater than 8 create a more turbulent near wake, increased mixing, and the shortest recovery distance.
The tip-speed ratio influences not only the magnitude of turbulence in the wake, but also the axial position where the wake becomes fully turbulent. With increasing tip-speed ratio, the point of a fully turbulent wake state moves upwind towards the rotor, enhancing turbulent mixing and reducing wake recovery distance. At high tip-speed ratios wake turbulence dissipates faster, and downwind turbines are not exposed to increased turbulence loads. At a tip-speed ratio of 10, the minimum wind speed at 6 rotor diameters downwind is enhanced by 50 % compared to the optimal operational tip-speed ratio of 8.
An increase in net power production is observed by operating the upstream turbine at a higher tip-speed ratios compared to the downwind turbine operating at the tip-speed ratio of 8. The net power production increases up to 10 %.
These results demonstrate the potential of varying tip-speed ratio to control wake development to maximize net power production of turbine arrays. Furthermore, turbulence-induced loads can be modified with this control strategy. These proof-of-concept simulations show the interesting potential of tuning the operational tip-speed ratio for wake control.
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RC1: 'Comment on wes-2025-86', Anonymous Referee #1, 23 Jul 2025
The manuscript entitled “Wake control through tip-speed ratio adaptation” deals with the characterisation of the wake flow for a wind turbine that does not operate at its design Tip-Speed-Ratio (TSR), and the consequences for a downstream wind turbine. The tools of investigation are first the multi-physics simulation tool Qblade for the rotor aerodynamics analysis, then the CFD, in particular Large-Eddy-Simulations (LES), coupled with the multi-physics simulation tool OpenFAST that models a DTU 10MW wind turbine for the wake flow field analysis and the wind turbine interactions.
Despite the fact that the investigated topic is of interest for the wind energy scientific community, the present manuscript and its content present several shortcomings that make it not suitable for publication:
- First, the title does not reflect the manuscript content. No wake control is performed, but modifications of the TSR and observations of the consequences on the wake properties.
- The use of Qblade for the rotor aerodynamic analysis and then, of OpenFAST for the coupling with the CFD is questionable, and not justified in the manuscript. It would be much more consistent to use the same tool (OpenFAST here, since it is already coupled with SOWFA) for the overall study.
- The content presents a lot of redundant sentences and redundant results (presented in different ways) that extend the document unnecessarily. It is built as a mix of basic wake flow descriptions and wind turbine performance analyses. The key messages are therefore too diluted. A thorough restructuration and optimisation of the manuscript is needed.
- The values of the TSR investigated in the manuscript change regularly. It is ranged from 5 to 12 (figs. 5 and 6), then from 6 to 10 (fig. 7), then from 6 to 11 (Figs. 8 and 9), then 5 to 11 in the simulations for two turbines. These changes do not make the manuscript consistent.
- The simulations are performed in a low-turbulent uniform incoming flow. The referee agrees on the fact that initiating a parametric study with these simplistic inflow conditions are essential to observe the wake details. However, in the present manuscript, in which the authors primarily aim to address the wake interactions between two turbines, these inflow conditions cannot be the only ones considered. The authors write “we simplify the problem to more clearly observe wake details that would otherwise be obscured in the background turbulence.” The wake details would not be obscured but strongly modified by the background turbulence, making the present conclusions inadequate. Particularly for the two-turbine simulations, where the authors, with the argument that it is a classical distance in full scale onshore wind farms, choose a distance of 6D. The present wakes have even not reached their far-wake states, their interactions with another wind turbine is not representative at all of any situations that could be encountered on real wind farms.
- The LES simulations seem to be not completely converged.
- The positioning of the present study in the existing literature may be not comprehensive. Most of the cited journal articles are quite “old” (older than 2020), whereas the wake control and wind farm control are very hot topics at the moment.
Other major comments:
- Figures 5 and 6 (and also 12 and 13): you present instantaneous velocity and vorticity distributions (by the way, it is not written in the caption that they are instantaneous distributions). Considering the global objectives of the present manuscript, the presentation of the time-averaged distributions would have been more interesting. Additionally, the TKE is presented but only along a profile, whereas the TKE distributions present strong gradients in the wake. Why do the authors not present the TKE distributions?
- Figure 7: A curve representing the envelope of TKE maxima is displayed. Lines 276-279 “As for TKE strength and location, the existence of an envelope is likely. It can initially be taken from the TKE slopes and the locations of TKE maxima. Such an envelope, describing a possible operation range for TSR modifications and their impact on TKE magnitude and location, is shown in Figure 7.” The reviewer does not see the added value of this information in the context of the present manuscript. Could you please elaborate on this idea?
- Figure 10 : The time histories show that the results are not converged for TSR 6. Difficult to check for the other cases since the signal is much noisier.
- Figure 10 and lines 346-347: “The spectral content for a TSR of 6 is mainly located between 0.1 Hz and 0.2 Hz.” This statement is completely incompatible with the figure. Additionally, the high power spectral density (it is not the velocity variance, which is the integration of the PSD) for the lowest frequencies is an indicator of non-converged times series. Furthermore; the scatter in the PSD is an indicator of non converged spectra. It makes comparison between PSDs difficult to interpret.
- Line 386-388: “The spatial and temporal resolution in the simulation is regarded as sufficiently high, because more than 80% of the turbulent energy in the wake is captured, and typical flow features like tip and root vortices are resolved. This statement is not proven in the manuscript.
- 6.2 Simulation results for a two-turbine array: you mentioned some deviations of the TSR from the targeted values (4.6 instead of 5, 10.3 instead of 11). Why these deviations appear for the Two-turbine configurations and not for the single-turbine ones?
- Line 413-414: For TSR 6, “Turbine 2 creates a strong wake where additional downstream meandering appears”. One can see on the instantaneous vorticity distributions (Fig. 13) that a kind of periodic structuration of the flow downstream of the second wind turbine appears. However, how do you define the meandering here? How do you quantify it? What would be the source of meandering?
- Lines 458-459: “The increased power production of the waked turbines shows some low-frequency dynamics.” The reviewer does not see the link between these two aspects. Could the authors elaborate on this statement?
- Figure 14: the plot show that the time series of the wind turbine power are not converged. Additionally, since the study is not supposed to address transient configurations, there is no need to show such time histories. The steady-state results are the only relevant ones.
Minor comments:
- Line 49-51 : give the key messages provided by these overviews of wake management. Does the wake control by TSR variation is mentioned? What are the drawn conclusions?
- Line 63 : “For both higher and lower TSRs, the power…” : higher and lower than what? To the design one?
- Lines 67-68: “Furthermore, the effect of turbulence intensity of the inflow is addressed.” : it is too vague. Give more details and position your present work in perspective
- Line 95: “The rotor is suitable for these wake studies because of its performance and robustness.” What does it mean exactly?
- Table 1 : add the design TSR
- Line 122: “anisotropy” instead of “anisotropic”
- Figure 1: a threshold of C_P> 0.4 is displayed on the graph but no reference to it appears in the caption and in the main text of the manuscript
- Lines 154-156: check the grammatical construction of the sentence. Mention the physical reasons of “the flow over the section is likely more unstable compared to the original operational point”: higher risk of flow separation and stall
- Lines198-199: “The DTU 10 MW rotor is aerodynamically suitable for the operation in the TSR range of approximately 5 to 10 to manipulate the wake”. What does it mean? What are the criteria used to make this statement? Why wouldn't another turbine be suitable?
- Line 205: “A range between TSR 6 and 10” instead of “5 and 12”
- Tables 2 and 4 : give also the grid dimensions in values normalised by the rotor diameter
- Figure 4 right: normalise the velocity by the free wind speed. Where (y and z coordinates) is the location of the U-min?
- Figure 4 caption: change “wake velocity development” with something like “streamwise evolution of the minimum of the time-averaged wake velocity”.
- Figure 8: normalise the mean velocity by the free wind speed.
- Line 302: “High-TSR operation establishes a different wake development with more turbulence and mixing effects closer to the rotor.” This part of the manuscript refers to Fig. 8, for which the turbulence and mixing effects cannot be interpreted.
- Figure 9: add the x label and normalise it : “y/D”
- Line 469: “The parameter study identifies alternative…” instead of “The parameter studies identified alternative…” ?
- Figure 15 is redundant with Table 5
Citation: https://doi.org/10.5194/wes-2025-86-RC1 -
RC2: 'Comment on wes-2025-86', Anonymous Referee #2, 29 Jul 2025
The paper investigates the benefits of running a turbine above design tip speed to reduce wake effects in a two-turbine configuration. Overall the topic is of interest, however the approach is not novel and unfortunately the paper does not add substantially to the existing knowledge base. That TSR, CT and TI play a role in wake breakdown is already encapsulated in the relationship published by Vermeulen in 1979. Since then several others have been added, but they all use those basic parameters. In this particular study actually the effect of TI has been excluded, though, however without it the wake breakdown occurs at a very different location than in a real offshore wind farm, for which the presented approach is apparently beneficial. The paper would be of greater value if the authors would relate them back to the existing research on this topic, but also relate it to existing papers on induction control. Additionally the structure of the paper should be improved, as it is difficult to understand why certain choices have been made by the authors. The abstract is more of a summary and the introduction is incomplete, including a lot of unnecessary literature. The numerical approach needs to be more detailed and be verified. It is unclear whether the wake breakdown changes when using the two-turbine configuration, as the extent of the high resolution grid is greater. Also results should include blade load variations and data should be normalised. The CT needs to be reported as otherwise it is difficult to evaluate how the induction is changing.
Some detailed comments are given in the attached document.
- RC3: 'Comment on wes-2025-86', Anonymous Referee #3, 29 Jul 2025
Status: closed
-
RC1: 'Comment on wes-2025-86', Anonymous Referee #1, 23 Jul 2025
The manuscript entitled “Wake control through tip-speed ratio adaptation” deals with the characterisation of the wake flow for a wind turbine that does not operate at its design Tip-Speed-Ratio (TSR), and the consequences for a downstream wind turbine. The tools of investigation are first the multi-physics simulation tool Qblade for the rotor aerodynamics analysis, then the CFD, in particular Large-Eddy-Simulations (LES), coupled with the multi-physics simulation tool OpenFAST that models a DTU 10MW wind turbine for the wake flow field analysis and the wind turbine interactions.
Despite the fact that the investigated topic is of interest for the wind energy scientific community, the present manuscript and its content present several shortcomings that make it not suitable for publication:
- First, the title does not reflect the manuscript content. No wake control is performed, but modifications of the TSR and observations of the consequences on the wake properties.
- The use of Qblade for the rotor aerodynamic analysis and then, of OpenFAST for the coupling with the CFD is questionable, and not justified in the manuscript. It would be much more consistent to use the same tool (OpenFAST here, since it is already coupled with SOWFA) for the overall study.
- The content presents a lot of redundant sentences and redundant results (presented in different ways) that extend the document unnecessarily. It is built as a mix of basic wake flow descriptions and wind turbine performance analyses. The key messages are therefore too diluted. A thorough restructuration and optimisation of the manuscript is needed.
- The values of the TSR investigated in the manuscript change regularly. It is ranged from 5 to 12 (figs. 5 and 6), then from 6 to 10 (fig. 7), then from 6 to 11 (Figs. 8 and 9), then 5 to 11 in the simulations for two turbines. These changes do not make the manuscript consistent.
- The simulations are performed in a low-turbulent uniform incoming flow. The referee agrees on the fact that initiating a parametric study with these simplistic inflow conditions are essential to observe the wake details. However, in the present manuscript, in which the authors primarily aim to address the wake interactions between two turbines, these inflow conditions cannot be the only ones considered. The authors write “we simplify the problem to more clearly observe wake details that would otherwise be obscured in the background turbulence.” The wake details would not be obscured but strongly modified by the background turbulence, making the present conclusions inadequate. Particularly for the two-turbine simulations, where the authors, with the argument that it is a classical distance in full scale onshore wind farms, choose a distance of 6D. The present wakes have even not reached their far-wake states, their interactions with another wind turbine is not representative at all of any situations that could be encountered on real wind farms.
- The LES simulations seem to be not completely converged.
- The positioning of the present study in the existing literature may be not comprehensive. Most of the cited journal articles are quite “old” (older than 2020), whereas the wake control and wind farm control are very hot topics at the moment.
Other major comments:
- Figures 5 and 6 (and also 12 and 13): you present instantaneous velocity and vorticity distributions (by the way, it is not written in the caption that they are instantaneous distributions). Considering the global objectives of the present manuscript, the presentation of the time-averaged distributions would have been more interesting. Additionally, the TKE is presented but only along a profile, whereas the TKE distributions present strong gradients in the wake. Why do the authors not present the TKE distributions?
- Figure 7: A curve representing the envelope of TKE maxima is displayed. Lines 276-279 “As for TKE strength and location, the existence of an envelope is likely. It can initially be taken from the TKE slopes and the locations of TKE maxima. Such an envelope, describing a possible operation range for TSR modifications and their impact on TKE magnitude and location, is shown in Figure 7.” The reviewer does not see the added value of this information in the context of the present manuscript. Could you please elaborate on this idea?
- Figure 10 : The time histories show that the results are not converged for TSR 6. Difficult to check for the other cases since the signal is much noisier.
- Figure 10 and lines 346-347: “The spectral content for a TSR of 6 is mainly located between 0.1 Hz and 0.2 Hz.” This statement is completely incompatible with the figure. Additionally, the high power spectral density (it is not the velocity variance, which is the integration of the PSD) for the lowest frequencies is an indicator of non-converged times series. Furthermore; the scatter in the PSD is an indicator of non converged spectra. It makes comparison between PSDs difficult to interpret.
- Line 386-388: “The spatial and temporal resolution in the simulation is regarded as sufficiently high, because more than 80% of the turbulent energy in the wake is captured, and typical flow features like tip and root vortices are resolved. This statement is not proven in the manuscript.
- 6.2 Simulation results for a two-turbine array: you mentioned some deviations of the TSR from the targeted values (4.6 instead of 5, 10.3 instead of 11). Why these deviations appear for the Two-turbine configurations and not for the single-turbine ones?
- Line 413-414: For TSR 6, “Turbine 2 creates a strong wake where additional downstream meandering appears”. One can see on the instantaneous vorticity distributions (Fig. 13) that a kind of periodic structuration of the flow downstream of the second wind turbine appears. However, how do you define the meandering here? How do you quantify it? What would be the source of meandering?
- Lines 458-459: “The increased power production of the waked turbines shows some low-frequency dynamics.” The reviewer does not see the link between these two aspects. Could the authors elaborate on this statement?
- Figure 14: the plot show that the time series of the wind turbine power are not converged. Additionally, since the study is not supposed to address transient configurations, there is no need to show such time histories. The steady-state results are the only relevant ones.
Minor comments:
- Line 49-51 : give the key messages provided by these overviews of wake management. Does the wake control by TSR variation is mentioned? What are the drawn conclusions?
- Line 63 : “For both higher and lower TSRs, the power…” : higher and lower than what? To the design one?
- Lines 67-68: “Furthermore, the effect of turbulence intensity of the inflow is addressed.” : it is too vague. Give more details and position your present work in perspective
- Line 95: “The rotor is suitable for these wake studies because of its performance and robustness.” What does it mean exactly?
- Table 1 : add the design TSR
- Line 122: “anisotropy” instead of “anisotropic”
- Figure 1: a threshold of C_P> 0.4 is displayed on the graph but no reference to it appears in the caption and in the main text of the manuscript
- Lines 154-156: check the grammatical construction of the sentence. Mention the physical reasons of “the flow over the section is likely more unstable compared to the original operational point”: higher risk of flow separation and stall
- Lines198-199: “The DTU 10 MW rotor is aerodynamically suitable for the operation in the TSR range of approximately 5 to 10 to manipulate the wake”. What does it mean? What are the criteria used to make this statement? Why wouldn't another turbine be suitable?
- Line 205: “A range between TSR 6 and 10” instead of “5 and 12”
- Tables 2 and 4 : give also the grid dimensions in values normalised by the rotor diameter
- Figure 4 right: normalise the velocity by the free wind speed. Where (y and z coordinates) is the location of the U-min?
- Figure 4 caption: change “wake velocity development” with something like “streamwise evolution of the minimum of the time-averaged wake velocity”.
- Figure 8: normalise the mean velocity by the free wind speed.
- Line 302: “High-TSR operation establishes a different wake development with more turbulence and mixing effects closer to the rotor.” This part of the manuscript refers to Fig. 8, for which the turbulence and mixing effects cannot be interpreted.
- Figure 9: add the x label and normalise it : “y/D”
- Line 469: “The parameter study identifies alternative…” instead of “The parameter studies identified alternative…” ?
- Figure 15 is redundant with Table 5
Citation: https://doi.org/10.5194/wes-2025-86-RC1 -
RC2: 'Comment on wes-2025-86', Anonymous Referee #2, 29 Jul 2025
The paper investigates the benefits of running a turbine above design tip speed to reduce wake effects in a two-turbine configuration. Overall the topic is of interest, however the approach is not novel and unfortunately the paper does not add substantially to the existing knowledge base. That TSR, CT and TI play a role in wake breakdown is already encapsulated in the relationship published by Vermeulen in 1979. Since then several others have been added, but they all use those basic parameters. In this particular study actually the effect of TI has been excluded, though, however without it the wake breakdown occurs at a very different location than in a real offshore wind farm, for which the presented approach is apparently beneficial. The paper would be of greater value if the authors would relate them back to the existing research on this topic, but also relate it to existing papers on induction control. Additionally the structure of the paper should be improved, as it is difficult to understand why certain choices have been made by the authors. The abstract is more of a summary and the introduction is incomplete, including a lot of unnecessary literature. The numerical approach needs to be more detailed and be verified. It is unclear whether the wake breakdown changes when using the two-turbine configuration, as the extent of the high resolution grid is greater. Also results should include blade load variations and data should be normalised. The CT needs to be reported as otherwise it is difficult to evaluate how the induction is changing.
Some detailed comments are given in the attached document.
- RC3: 'Comment on wes-2025-86', Anonymous Referee #3, 29 Jul 2025
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