the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Dec 2025
Experimental Investigation of the Rotor–Tower Interaction of a Modern Multi–Megawatt Wind Turbine
Abstract. The increasing demand for a reduction in energy costs per kilowatt hour is leading to larger wind turbines, resulting in longer rotor blades that are also more slender, lighter, and more flexible. These structures are more dynamically responsive and more sensitive to excitations. When a rotor blade passes the tower, an aerodynamic interaction occurs between the blade and the tower, known as rotor–tower interaction. This interaction induces fluctuating loads on both the blade and the tower. Understanding the fluctuating loads on both blade and tower is essential for the design of larger and therefore more dynamically active wind turbines.
To assess how and to what extent the influence of the rotor–tower interaction impact the structure of modern multi–megawatt wind turbines, in this study, the rotor–tower interaction was investigated by means of pressure measurements on the tower of a modern 4.26 MW upwind wind turbine. For the measurements, a pressure belt, equipped with 36 differential pressure sensors was mounted on the tower at mid-rotor height. The measurements were conducted over two months with the aim to measure transient surface pressure fluctuations induced by the passing rotor blades. The blade root bending moments recorded by the wind turbine were also examined for selected operating points.
The results show a clear periodic fluctuation of the aerodynamic loading of the tower at the 3P blade-passing frequency. Aerodynamic phenomena at the tower, such as velocity excess, stagnation point displacement, and synchronized vortex shedding, which had been predicted in earlier numerical studies, are confirmed by these measurements. The maximum dynamic loads on the tower occur when the turbine reaches its rated power, where the aerodynamic load on the blades is at its highest. The Investigation of the blade root bending moment shows that the blade is also influenced by the tower. A fluctuation in the flapwise bending moment of approximately 1 % of the maximum flapwise bending moment is observed when the blade passes the tower. These findings show that the effect of rotor–tower interaction occur in modern multi-megawatt wind turbines and can be measured, even if it is only minor in this particular wind turbine type due to the large blade–tower clearance.
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Status: final response (author comments only)
- RC1: 'Comment on wes-2025-262', Anonymous Referee #1, 30 Jan 2026
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RC2: 'Comment on wes-2025-262', Anonymous Referee #2, 31 Jan 2026
This manuscript presents a valuable full-scale experimental investigation of rotor–tower interaction on a modern 4.26 MW upwind wind turbine, based on more than two months of field measurements. The use of a pressure belt equipped with 36 differential pressure sensors mounted on the tower at mid-rotor height represents a substantial experimental effort and provides rare insight into transient pressure fluctuations induced by blade passage under real operating conditions. Given the scarcity of long-term full-scale measurements on multi-megawatt turbines, this work is inherently valuable for the wind energy community.
A particularly strong contribution of the study is the presentation of a dedicated measurement methodology based on a purpose-designed pressure belt. To the reviewer’s knowledge, this represents one of the first studies demonstrating a practical approach for experimentally characterizing rotor–tower interaction effects on a modern multi-megawatt wind turbine using surface pressure measurements. The manuscript also usefully discusses in the conclusions potential improvements to the belt design based on the experience gained during the measurement campaign, highlighting that this work should be regarded as a first step toward a progressively refined experimental tool rather than as a definitive dataset.
For this reason, the reviewer considers the primary contribution of the manuscript to be methodological rather than the provision of a reference database for CFD validation, especially since the authors state that the measurement data sets cannot be made publicly available due to confidentiality and non-disclosure agreements between the project partners.
In this context, the manuscript would benefit from a clearer and more explicit discussion of how the investigated turbine compares with configurations studied in previous CFD and small-scale experimental works available in the literature. Related to this, the motivation for focusing on an upwind turbine configuration—where rotor–tower interaction effects are expected to be less pronounced—is stated clearly, but its implications could be discussed in more depth. While the authors note that the interaction effects are measurable but minor for the investigated turbine, it would strengthen the manuscript to more clearly explain how the measured results, despite the reduced interaction strength compared to existing literature, can still inform the design of future upwind turbines, particularly with respect to blade–tower clearance re-evaluation studies.
Overall, the manuscript presents a valuable and technically challenging experimental study and introduces a promising methodology for investigating rotor–tower interaction in modern wind turbines. Clarifying the scope of the contribution, strengthening the comparison with existing literature, and better articulating the design implications of the results would further improve the clarity and impact of the paper.
Minor Comments and Editorial Suggestions:
- Abstract, l. 18 “The Investigation”
- Data processing and analysis, l. 147 “distribution: Based”
- Results and discussion, Wake skewness, l. 210 “thus an pressure”
- Results and discussion, Velocity excess, Figure 7: in the text, references are made to Fig. 7f, Fig. 7c, and Fig. 7d. However, Figure 7 appears to use alphabetical labels a), b), c), and d) to indicate the four velocity cases, and there is no visible subfigure f). This makes the figure references difficult to follow. For improved readability, the figure labeling and corresponding text references should be clarified and made consistent. Figure 20 provides a clearer and more intuitive example of subfigure indexing.
- Figure placement: attention should be paid to the placement of figures within the manuscript. In several instances, figures are located far from the sections in which they are discussed, and in some cases, they even appear in the preceding section relative to their first reference in the text. This makes it difficult for the reader to follow and correctly interpret the results.
Citation: https://doi.org/10.5194/wes-2025-262-RC2 -
AC1: 'Comment on wes-2025-262', Philipp Wölk, 02 Mar 2026
Dear reviewers,
We would like to thank you for your constructive and insightful comments. These comments have significantly helped us to sharpen the argumentation and to clarify the scope of the paper. Also, we appreciate the recognition of the experimental effort involved and of the relevance of long-term full-scale measurements for the wind energy community. Such feedback is very encouraging and confirms the importance of conducting field-based investigations of this kind. We have addressed the major comments of reviewer 1 (RC1) and reviewer 2 (RC2) in the text below. Reviewer comments are reproduced in italics, followed by our responses.
RC1:
That said, some aspects of the experimental setup and analysis limit the broader impact and interpretability of the results. Some relatively straightforward reference measurements—such as a reference pressure measurement inside the tower—and fully consistent rotor position time stamping are stated to be missing. The absence of these quantities introduces additional uncertainty and necessitates assumptions or data anchoring during post-processing that could potentially have been avoided. This complicates the interpretation of the measured trends.
We agree with the reviewer that the absence of reference measurements and fully synchronized rotor position time stamping represents a limitation of the current data set. Measures are already being implemented to address these issues in future measurement campaigns. However, the quantified pressure amplitudes and the derived magnitude of rotor–tower interaction effects presented in this manuscript are not affected by this limitation.
The authors state that the measurement data sets cannot be made publicly available due to confidentiality and non-disclosure agreements between the project partners. While this constraint is understandable in the context of industrial collaborations, it limits the possibility for independent validation and reuse of the data by the wider research community.
We fully understand the reviewer’s concern regarding data availability and agree that public access to full-scale measurement data would be highly beneficial for the research community. However, we are currently not permitted to share the raw data set. Within these constraints, we have aimed to provide as much quantitative information as possible.
In this context, complementary CFD analysis becomes critical, especially since the manuscript emphasizes that the blade–tower clearance in the present study is higher than typically reported in the literature, and this is used to explain discrepancies with previously published results. Without dedicated CFD simulations for the present configuration, this explanation remains largely qualitative. Moreover, in some cases, the observed measurement trends do not follow the same behavior reported in existing CFD studies from the literature. Including CFD simulations tailored to the present geometry and operating conditions would substantially strengthen the manuscript. Such analysis could help support the interpretations attributed to increased clearance, bridge the gap with existing literature, and enhance the scientific value of the study, particularly given the limited availability of the raw experimental data.
We agree that complementary CFD analyses would provide valuable additional insight into the observed trends. However, dedicated CFD simulations were beyond the scope of the present experimental study. We consider this an important direction for future work, whereas in the aim of the paper at hand is the publication of the experimental results. We agree that such analyses would further strengthen the quantitative interpretation of the results. In response to this and as suggested in RC2, we have expanded the comparison with the existing literature and clarified more explicitly that the primary contribution of this work is methodological.
RC2:
For this reason, the reviewer considers the primary contribution of the manuscript to be methodological rather than the provision of a reference database for CFD validation, especially since the authors state that the measurement data sets cannot be made publicly available due to confidentiality and non-disclosure agreements between the project partners.
We thank the reviewer for this valuable perspective. We agree that the primary contribution of the manuscript is methodological, in that it demonstrates a full-scale measurement approach for quantifying rotor–tower interaction under real operating conditions. We have therefore clarified this focus more explicitly in the introduction, when defining the study's objectives, and reiterated it in the conclusions.
In this context, the manuscript would benefit from a clearer and more explicit discussion of how the investigated turbine compares with configurations studied in previous CFD and small-scale experimental works available in the literature. Related to this, the motivation for focusing on an upwind turbine configuration—where rotor–tower interaction effects are expected to be less pronounced—is stated clearly, but its implications could be discussed in more depth. While the authors note that the interaction effects are measurable but minor for the investigated turbine, it would strengthen the manuscript to more clearly explain how the measured results, despite the reduced interaction strength compared to existing literature, can still inform the design of future upwind turbines, particularly with respect to blade–tower clearance re-evaluation studies.
We thank the reviewer for this helpful comment. This comment has helped us to further clarify the positioning of the manuscript within the existing literature and to sharpen the overall focus of the study. In an additional paragraph at the end of the “Results and Discussion” section, we compared the present findings with existing literature to provide context. In this discussion, we clarified how the measured results can inform the design of future upwind turbines, particularly with regard to re-evaluation of the blade–tower clearance. Additionally, the introduction has been revised to provide a clearer and more comprehensive comparison of the turbine configurations examined in previous studies. Furthermore, we provided a more detailed comparison of the effects of rotor–tower interaction in upwind and downwind turbines. Finally, the conclusions have been adapted to this revision.
Clarifying the scope of the contribution, strengthening the comparison with existing literature, and better articulating the design implications of the results would further improve the clarity and impact of the paper.
We again thank the reviewer for the positive overall assessment of our work and for the constructive suggestions. In the revised manuscript, we have clarified the scope of the contribution, strengthened the comparison with the existing literature, and more explicitly articulated the design implications of the results, as outlined in our previous responses. We believe these revisions have improved the clarity and overall impact of the paper.
Citation: https://doi.org/10.5194/wes-2025-262-AC1
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- 1
Conducting a full-scale experimental campaign on an operating wind turbine over an extended period (more than two months) is inherently valuable for the wind energy community, and the effort required to acquire and process this data set is clearly significant.
That said, some aspects of the experimental setup and analysis limit the broader impact and interpretability of the results.
Some relatively straightforward reference measurements—such as a reference pressure measurement inside the tower—and fully consistent rotor position time stamping are stated to be missing. The absence of these quantities introduces additional uncertainty and necessitates assumptions or data anchoring during post-processing that could potentially have been avoided. This complicates the interpretation of the measured trends.
The authors state that the measurement data sets cannot be made publicly available due to confidentiality and non-disclosure agreements between the project partners. While this constraint is understandable in the context of industrial collaborations, it limits the possibility for independent validation and reuse of the data by the wider research community.
In this context, complementary CFD analysis becomes critical, especially since the manuscript emphasizes that the blade–tower clearance in the present study is higher than typically reported in the literature, and this is used to explain discrepancies with previously published results. Without dedicated CFD simulations for the present configuration, this explanation remains largely qualitative. Moreover, in some cases, the observed measurement trends do not follow the same behavior reported in existing CFD studies from the literature.
Including CFD simulations tailored to the present geometry and operating conditions would substantially strengthen the manuscript. Such analysis could help support the interpretations attributed to increased clearance, bridge the gap with existing literature, and enhance the scientific value of the study, particularly given the limited availability of the raw experimental data.