the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Apr 2025
On the prediction of underwater aerodynamic noise of offshore wind turbines
Abstract. The growing demand for offshore wind energy has led to a significant increase in wind turbine size and to the development of large-scale wind farms, often comprising 100 to 150 turbines. However, the environmental impact of underwater noise emissions remains largely unaddressed. This paper quantifies, for the first time, the underwater aerodynamic noise footprint of three large offshore turbines (5 MW, 10 MW, and 22 MW) and wind farms composed of these turbines. We propose a novel methodology that integrates validated wind turbine noise prediction techniques with plane wave propagation theory in different media, enabling turbine designers to predict and mitigate underwater noise emissions. Our results confirm that aerodynamic noise from offshore wind farms presents a potential environmental challenge, with negative effects on marine life. Addressing this issue is crucial to ensuring the sustainable expansion of offshore wind energy.
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RC1: 'Comment on wes-2025-40', Anonymous Referee #1, 16 Apr 2025
Review on the manuscript entitled “On the prediction of underwater aerodynamic noise of offshore wind turbines” by Botero-Bolívar et al. (wes-2025-40)
The manuscript presents a numerical study on wind turbine noise generated from offshore wind turbines for underwater environmental impact consideration. The NREL 5MW turbine, DTU 10 MW turbine and IEA 22 MW turbine were used for wind turbine noise generation and the spherical and cylindrical sound propagations were used for the parts of sound propagation in air and in water, respectively. The manuscript is well-written and discusses an interesting problem about wind turbine noise from offshore wind turbines. This reviewer recommends the acceptance of the manuscript for publication when the following issues are addressed.
Special comments:
- Results should be summarized in the abstract.
- Line 27, it is not precise to say increasing with the rotor diameter, but better with the tip speed.
- Line 43, not only mechanical noise, also the structure borne of aerodynamic noise.
- Line 91, “the transition was fixed at 5% of the chord”, why didn’t you use free-transition?
- Figure 1, caption: Aerodynamic noise includes more than leading-edge and trailing-edge noise.
- Line 95: check the Doppler effect factor for Spp.
- Line 104, “at the same relative location with respect to the observer”, not a precise estimate.
- Figure 2, the symbol φ is not consistent with the one in the main text.
- Line 153, d1 or d2?
- Line 155, how do you consider the attenuation of sound in water?
- Table 1, the numbers for IEA 22MW are different from the original definition.
- Line 176, the definition of low-frequency sound is different from the standard definition.
- In figures, “Noise Amplitude”, should it be “Overall Sound Pressure Level”?
Citation: https://doi.org/10.5194/wes-2025-40-RC1 - AC1: 'Reply on RC1', Laura Botero Bolivar, 09 May 2025
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RC2: 'Comment on wes-2025-40', Anonymous Referee #2, 01 May 2025
The paper presents a numerical model to predict the generation of noise from off-shore wind turbines due to aeroacoustic sources and the transmission of the noise into the water. The effect of this noise on the marine life is investigated. The authors conclude that aerodynamically generated noise by the wind turbines blade is a potential environmental challenge and mitigation measures such as trailing edge serrations should be applied.
The authors use largely simplified models throughout the modelling chain. The uncertainty of the model is not discussed and validation is very sparse. It seems that the authors do not understand the limitations of their model. The conclusions are bold. I think a proper scientific manuscript would deserve more careful judgement and critical discussion. I recommend performing a validation of each element of the modelling chain before using it. I also recommend that the authors collaborate with marine biologists to set the predicted noise levels into the context of the underwater environment and judge the harmfulness for marine life.
I strongly recommend against accepting this article for publication.
Below I have listed some detailed criticism about the modelling.
Air water transmission model:
The model is based on plane wave refraction theory. This is the most simple analytical model available in the literature. The plane wave approximation is known to be inaccurate at low frequencies. The analytical model for the transmission of spherical sound waves between two media [Salomons, E. M.: Computational Atmospheric Acoustics, Springer Science C Business Media, B.V., https://doi.org/10.1007/978-94-010-0660-6, 2001.] is usually more accurate than the plane wave transmission model.
According to the plane wave model, total reflection occurs at an air-water interface when the angle to the surface normal is above 13 degrees. Within this narrow angular range there is a steep dependency on the angle of incidence. The authors assume that the water can be modelled as a perfectly flat surface. This assumption needs to be further verified. Due to the steep gradients of the model sensitivity, even small surface waves on the water could alter the transmission loss significantly. Therefore I assume that actual transmission loss would be higher than one predicted with the flat surface model.
The transmission model is crucial in the modelling chain. It should be validated against measurements or high-fidelity numerical models.
Wind farm noise model:
The wind farm model assumes that all wind turbines are located at the same position with respect to the observer location. It is used to compute the effects of having 100 or 150 very large wind turbines in a wind farm. Such a wind farm would stretch over several kilometers (at 10x10 squared wind cluster with 4.5 rotor diameter distance between each 10 MW wind turbine would cover a space of about 8x8 km). It is unlikely that the assumption is justified for such a wind farm. At the same time the added level by this model is crucial in whether the predicted sound exceeds the hearing threshold or not.
Wind turbine noise prediction code:
The prediction code has been validated by means of a single data set of noise measurements on a 2.3MW wind turbine. The differences between a 2.3MW and a 22MW wind turbine are significant. It is doubtful that one can trust the prediction model.
Trailing edge noise: The Stalnov trailing edge noise model was tuned to fit measurements for low Reynolds numbers, see figure 8(a)-(d) in [Stalnov, O., Chaitanya, P., and Joseph, P. F.: Towards a non-empirical trailing edge noise prediction model, Journal of Sound and Vibration,372, 50–68, https://doi.org/10.1016/j.jsv.2015.10.011, 2016.]. The best fit with measured data occurs at Reynolds numbers 0.26 and 0.52 million. The agreement with the measurements becomes gradually worse for Reynolds numbers 0.78 and 1.04 million and is more than 3 dB off. The blade sections at the outer part of the rotor of the 22MW reference turbine operate at Reynolds number of 15-20 million. Therefore it is very unlikely that the trailing edge noise will be predicted with sufficient accuracy.
Effect on marine life:
The authors conclude that any sound that is above the threshold of hearing can be perceived. They do not assess underwater background noise levels. A more detailed study is necessary to assess the audibility of the predicted sound.
Statement in the conclusions:
The authors claim they have identified trailing edge noise as the dominant aerodynamic noise source to effect marine life. However, the model only considers leading edge noise and trailing edge noise. Other mechanisms are not modelled. Mechanical noise is not modelled either. Due to modelling assumptions the dominant noise sources are already determined beforehand.
Citation: https://doi.org/10.5194/wes-2025-40-RC2 - AC2: 'Reply on RC2', Laura Botero Bolivar, 16 May 2025
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