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.

• It is unclear why recent numerical investigations on the same airfoil by the same research group [Vitulano et al., WES 2025] have not been applied to the experimental conditions investigated in this paper. The reviewer thinks that this is a missed opportunity for an easy-to-do validation study.
• The reviewer very much appreciates the TORQUE 2022 paper by the same research group [de Tavernier & v. Terzi, 2022] that brought attention to the possibility of transonic flow in high-speed Region III where blade pitch control results in order -10deg angle of attack near the blade tip. In this regard, Fig. 3d of [de Tavernier & v. Terzi, 2022] has both all relevant information and actually also the solution to the problem with respect to blade design (e.g. reduce tip chord to increase angle of attack, or lower both tip speed and blade pitch to achieve the same rated power at higher angle of attack). This conclusion is not changing for the IEA-22MW turbine as the associated increase in tip speed compared to the IEA-15MW turbine is not going to ever result in Mach numbers higher than 0.5. Therefore, the TST-27 is simply not the best wind tunnel to conduct relevant experiments. In other words, the experimental conditions chosen (see Fig. 4 of present paper) are simply not relevant.
• Also, Xfoil is not suitable for thick airfoils. This is known and was one of the objectives for TU Delft to develop Rfoil. Why are TU Delft authors not using Rfoil ?
• Missed V&V opportunities. As mentioned above, it is curious why the same research group would not conduct numerical simulations (RANS, URANS, DDES, steady and unsteady) of the experimental conditions considered in this paper and benchmarked against research in the compressible flow community.
• The lack of lift and drag data also makes the present work less relevant as the drag rise and associated loss in airfoil cl/cd would have a quantified effect on tentative performance loss. Once more, Ma > 0.5 are not relevant to utility-scale turbines, and transonic flow conditions can be avoided both by rotor design and operation in high-speed Region III.
• There is a wealth of literature available on shock boundary-layer interaction and shock buffeting. The reviewer only sees one reference to work in this area. This is unfortunate as it leaves the present work without contribution in that area.

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.

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:

1. 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.
2. 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.

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.