the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2025
Experimental study of transonic flow over a wind turbine airfoil
Abstract. For the largest wind turbines currently being designed, operation at cutout conditions can lead to the tip airfoil experiencing transonic flow conditions. To date, this phenomenon has primarily been explored through numerical simulations, but modelling uncertainties limits the reliability of these predictions. In response to this challenge, our study marks the first experimental investigation of a wind turbine airfoil under transonic conditions, for which we consider the FFA-W3-211 airfoil. Measurements are conducted in the high-subsonic range (Mach 0.5 and 0.6), utilizing Schlieren visualization and Particle Image Velocimetry (PIV) to characterise the airfoil across a range of angles of attack expected at cutout conditions. Unsteady shock wave formation is observed for the higher Mach number, with the shock oscillation range increasing with steeper angles of attack. Also, it is found that the presence of a local supersonic flow region does not consistently result in a shock wave. Our findings reveal that while calculations based on isentropic flow theory are reasonably effective in predicting the onset of transonic flow, they fail to predict the intensity of transonic flow effects, in particular, the formation and unsteady nature of shock waves. This underscores the need for higher-fidelity tools and experiments to capture the dynamic transonic flow effects on wind turbine airfoils.
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Status: final response (author comments only)
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RC1: 'Comment on wes-2025-65', Anonymous Referee #1, 30 May 2025
- AC1: 'Reply on RC1', Abhyuday Aditya, 11 Jul 2025
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RC2: 'Comment on wes-2025-65', Anonymous Referee #2, 16 Jul 2025
This paper presents an insightful experimental investigation into transonic flow behavior over the FFA-W3-211 airfoil, a profile commonly used near the blade tip region of large wind turbine rotors. The study is carried out using the TST-27 blowdown wind tunnel at TU Delft, at freestream Mach numbers of 0.5 and 0.6 and Reynolds numbers around 1×10⁶. Schlieren imaging and Particle Image Velocimetry (PIV) are employed to visualize and analyze shock formation and unsteady flow phenomena across a range of negative angles of attack, intended to replicate blade tip conditions near cut-out operation (Region III) for IEA 15MW and 22MW reference turbines.
The experimental methodology is well executed, and the authors provide compelling visualizations and robust data.
In general, the chosen Mach numbers (0.5 and 0.6) are higher than typical operational conditions for large wind turbines—tip Mach numbers are generally closer to 0.3 for the IEA-22MW reference turbine. While this limits immediate applicability, the reviewer does not question the potential future relevance of these findings, which may indeed open new directions in transonic research on thick airfoils.
However, several important points must be addressed to strengthen the manuscript:
Major Comments:
- The experiments are conducted at Reynolds numbers (~1×10⁶) that are significantly lower than those in real turbine operations (~10⁷). A more in-depth discussion of how this lower Reynolds number affects the results obtained would strengthen the interpretation of the results and help assess their applicability to real-world conditions.
- The study presents only experimental data, without any supporting numerical simulations. This makes it difficult to validate or generalize the observations. While the experiments are of high quality, some comparison with CFD simulations would reinforce the conclusions. At present, the lack of numerical support limits the completeness of the study. This issue is reinforced by the fact that, as also commented on by the authors, the measurements shown in Figures 5 are in contrast to studies previously published by the same authors and summarized in Figure 4. Further analysis (through, for example, CFD simulations), might better explain this mismatch.
Minor Comments and Editorial Suggestions:
- Abstract, l. 1 and 6 “cutout” while throughout the text cut-out is used.
- The Methodology section could benefit from including photos of the wind tunnel setup with the airfoil installed, if available.
- Table 1 is not referenced in the text; please clarify its relevance and refer to it explicitly.
- Experimental Design, l. 169 “Furthermore, The”
- Results, Local Mach Number Trends, l. 202 “(Figure 5f”, missed )
- Results, Local Mach Number Trends, l. 217 “(Fig. 6a”, missed )
- Results, Probability of Local Supersonic Flow, l.251 and 259 “Mainfty” appears to be a typo or LaTeX conversion error
- In Figure 4, the figure caption tells “ The transonic envelope showing the separation between complete subsonic flow and transonic flow for an FFA-W3-211 airfoil”. but the graph indicates supersonic conditions with value of Mach less than 1. This could confuse the reader; consider clarifying the graph and/or caption.
- In Figure 7, consider thickening the dashed line to improve visibility against grayscale. Also, Figure 7d appears difficult to interpret and compare to the others—additional commentary explaining its significance would be helpful.
- Results, Occurrence of Shock Waves, Mach 0.55 is introduced without explanation. Given that earlier sections focus only on 0.5 and 0.6, please justify this intermediate condition.
Final Recommendation:
The experimental data are of high quality and represent a technically interesting investigation into transonic flow phenomena on a relatively thick airfoil, and this point is definitely of interest for the scientific community. This is especially true given that these experiments demonstrated that the envelope derived using basic isentropic flow theory combined with low-fidelity airfoil design tools such as Xfoil is unable to accurately predict the frequency and intensity of transonic flow effects.
However, in its current form, the paper is not yet ready for publication due to the points highlighted before. Given the high quality of the experimental work and its potential significance, the reviewer strongly recommends that the authors consider submitting this study to a conference journal, such as TORQUE 2026 and then improve the present work by adding additional numerical support. Moreover, this would be an excellent venue to present the experimental findings to the wind energy community and, in the future, could form the basis for a journal publication once supported by complementary numerical analysis.
Citation: https://doi.org/10.5194/wes-2025-65-RC2 - AC2: 'Reply on RC2', Abhyuday Aditya, 25 Jul 2025
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RC3: 'Comment on wes-2025-65', Anonymous Referee #3, 13 Aug 2025
This paper addresses a relatively unexplored aspect of transonic flows by investigating airfoils with high thickness-to-chord ratios. The reviewer appreciates this novel contribution. The experimental methodology is clearly described, and the results are well analyzed and documented.
The reviewer also acknowledges the authors’ attempt to relate the findings to the possibility of transonic flow occurring at high negative angles of attack near the blade tip of a wind turbine. However, there are significant concerns regarding this interpretation. The primary issue is the influence of the open atmosphere. In the case of wind turbines, the surrounding unconfined air can respond to localized flow accelerations, allowing for upstream and downstream adjustments that may mitigate the extreme local accelerations observed in confined wind tunnel conditions. It is possible that, in an unconfined environment, the incoming flow may decelerate over a wider region, thereby reducing or preventing the occurrence of extreme suction peaks and transonic regions.
While the reviewer values the experimental investigation of thick airfoils under transonic conditions, extending these findings to system-level conclusions for wind turbines—based on low-fidelity tools—appears premature. For such generalizations to be justified, high-fidelity simulations of full-scale systems operating in open atmospheric conditions would be necessary.
In conclusion, the reviewer recommends that the scope of the manuscript be either limited to the experimental investigation of high-thickness airfoils under transonic conditions, which in itself provides valuable insights, or extended with comprehensive high-fidelity CFD studies to support the broader implications for wind turbine applications. Without such additions, system-level generalizations remain insufficiently supported.
Citation: https://doi.org/10.5194/wes-2025-65-RC3 -
AC3: 'Reply on RC3', Abhyuday Aditya, 16 Aug 2025
We sincerely thank the reviewer for the evaluation of our work and for recognizing the novelty of investigating transonic flow phenomena on thick airfoils. We also appreciate the constructive feedback regarding the interpretation of our results in the context of wind turbine operation.
We duly note the reviewer’s concern that, in an unconfined atmospheric environment, the flow may adjust upstream and downstream, potentially mitigating the extreme accelerations observed in confined wind tunnel conditions. Furthermore, we acknowledge that our analysis is conducted at the 2D airfoil level, while the response of a real wind turbine blade tip is influenced by additional three-dimensional effects and the surrounding atmospheric flow.
To address this point, we will explicitly include a disclaimer in the conclusions of the revised manuscript, clarifying that our present study is limited to 2D thick airfoils under transonic conditions. While the results provide valuable insights into the fundamental aerodynamic behavior of such airfoils, system-level generalizations for wind turbines cannot be made without further high-fidelity 3D simulations in open atmospheric conditions.
We appreciate the reviewer’s perspective and believe this clarification will improve the scope of our work.
Citation: https://doi.org/10.5194/wes-2025-65-AC3
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AC3: 'Reply on RC3', Abhyuday Aditya, 16 Aug 2025
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WES-2025-65
Experimental study of transonic flow over a wind turbine airfoil
The paper describes an experimental investigation into beginning transonic flow behaviour of a thick FFA-W3-211 airfoil commonly used in the blade tip region of wind turbine rotors. The experiments are conducted in the TST-27 transonic-supersonic blowdown wind tunnel at TU Delft. Freestream Mach numbers of 0.5 and 0.6 are considered. Shock waves are identified via optical flow diagnostics (Particle Image Velocimetry, PIV). Some observations are made concerning the formation and unsteady behaviour of shock waves on the lower airfoil surface at negative angles of attack relevant to high-speed (close to cut-out) conditions in Region III of notional large IEA 15MW/22MW reference turbines.
The experiments appear to be original, and the uncertainty quantification of both the wind tunnel conditions and PIV measurements appear adequate. Overall though, there is no clear scientific contribution that is relevant to the wind energy science community. The experimental conditions chosen (and feasible) in the TST-27 wind tunnel are paradoxically disconnected from previous studies of the same research group on the general scaling of tentative transonic conditions in Region III [de Tavernier & v. Terzi, 2022] and recent numerical investigations of transonic flow around the same airfoil [Vitulano et al., WES 2025]. Similarly, the discussion of observations with respect to shock formation reveals a lack of knowledge on part of the authors in both compressible flow and scientific literature on transonic flows with respect to shock boundary-layer interactions (SBLI) and the physics of buffeting. Consequently, the paper is neither a contribution to wind energy science nor to the literature on transonic flows.
The recommendation is therefore to REJECT the paper in its current form.
Below some further comments and questions.
In summary, the reviewer acknowledges the originality of the work but has strong reservations as to the scientific relevance to the areas of wind energy science and the compressible flow community. In its current form, the paper is of conference quality but not worthy of publication in a highly ranked peer-reviewed journal. A future investigation of an integrated numerical and experimental investigation with implications and recommendations for wind turbine design and operation would be a valuable contribution to wind energy.
The reviewer hopes that the comments are helpful to the authors to advance their work in the future.