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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RC1: 'Comment on wes-2025-65', Anonymous Referee #1, 30 May 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.
- 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.
Citation: https://doi.org/10.5194/wes-2025-65-RC1
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