the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Apr 2025
On the mitigation of detrimental effects via passive flow-control devices in small-scale horizontal-axis wind-turbines operating under turbulence and surface degradation effects
Abstract. Small-scale horizontal axis wind turbines (SHAWTs) installed in distributed grids are increasingly important for achieving net-zero emissions by minimizing visual and environmental impacts. However, the understanding of SHAWTs under combined turbulence and roughness conditions, operating at chord-based transitional Reynolds numbers ranging from 1 × 105 to 5 × 105, is still limited. This study examines the combined influence of environmental turbulence and surface degradation on a transitionally-operating NACA0021 airfoil equipped with passive flow-control devices (PDs) to mitigate detrimental effects. Results show that specific PD distributions can delay stall up to 5º and reduce performance loss by enhancing the aerodynamic efficiency up to 16 % under certain flow conditions.
Using experimental data, the study estimates the annual energy production (AEP) of a standard 7.8 kW-rated SHAWT using a blade-element method code. Simulations reveal that, when SHAWT blades are affected by turbulence and surface degradation along their entire span, certain PD distributions can enhance the AEP by up to 80 % compared to their bare counterparts. These findings highlight the potential of PDs in enhancing SHAWT's performance and the importance of pitch-regulated control in mitigating adverse effects.
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Status: open (until 13 May 2025)
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RC1: 'Comment on wes-2025-36', Anonymous Referee #1, 14 Apr 2025
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The paper describes open-jet wind tunnel experiments of a NACA 0021 aerofoil at low Reynolds numbers in clean conditions, with high inflow turbulence and with sandpaper roughness wrapped around the leading edge. As mitigation strategy triangular elements are applied to the suction side, which are meant to act as vortex generators. The measured polars are then applied within a BEM framework to estimate power changes from turbulent inflow and LE roughness.
Whilst the experiments and addition of triangular elements to the aerofoil are actually a very interesting approach for small-scale wind turbine aerofoils, the scientific quality and structure of the paper is poor, especially as Hama trips (that is their name in literature) have been used before for LSB control. It seems that a lot of background knowledge within the area of low Reynolds number aerofoil aerodynamics is simply missing. Unsurprisingly important references are missing (a lot of work was done by Selig and his group on low Re aerofoil design and BL control) and boundary layer quantities are omitted, even though they are discussed to great detail within the introduction. The AEP loss assessment is interesting, but performed for a blade using a NACA aerofoil which is usually not used, as it is performing poorly with respect to other existing aerofoils. If the authors would dive further into the data and look into turbulent inflow spectra, extend the number of roughness types and also do some oil flow visualizations, maybe even change the trip height, this could become a very useful paper, however it would require a large rewrite and additional work. The authors also need to improve the language.
Some detailed comments are given in the attached document.
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RC2: 'Comment on wes-2025-36', Anonymous Referee #2, 05 May 2025
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The authors presented an experimental study investigating a passive device (PD) aimed at recovering the aerodynamic performance of wind turbine airfoils affected by leading edge (LE) erosion. Different device configurations were tested, and their impact on the Annual Energy Production (AEP) of a small-scale wind turbine was assessed. The topic is timely and relevant, as LE erosion is a critical issue for wind turbine longevity and efficiency. The reviewer acknowledges the novelty of the experimental approach and the potential value of this line of research. However, the current manuscript struggles to deliver findings that meaningfully advance the state of the art. More in detail, the following concerns strongly undermine the value of the work:
- The use of the NACA0021 airfoil raises concerns regarding the technological relevance of the study. This symmetric airfoil is not representative of modern wind turbine blade sections, which are typically cambered. In the reviewer's experience, translating these findings to cambered profiles is not straightforward and may limit the generalizability of the conclusions;
- The range of tested LE erosion and inlet turbulence intensities appears too narrow to support broader conclusions. A more extensive test matrix is necessary to capture the range of conditions experienced by operational turbines and to ensure the findings are robust;
- The absence of pressure distribution data along the airfoil surface makes it difficult to gain an insight of the aerodynamic mechanisms underlying the performance changes observed. Without such measurements, the value of the work remains limited.
Further issues are detailed below:
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Introduction: The introduction is overly broad and includes extensive background on fundamental aerodynamic concepts, such as transition mechanisms, which are well known to the readers. It would benefit from being more concise and focused on the state-of-the-art in LE erosion mitigation and how the proposed work addresses current gaps;
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Abstract: The abstract is vague and lacks specificity, particularly regarding the nature and design of the passive devices tested. A clearer summary of the methodology and key findings would greatly improve its effectiveness;
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Section 2.1.1 – Wind Tunnel Blockage: The reported wind tunnel blockage appears significant, even at low angles of attack. The manuscript should provide a more detailed discussion of the blockage correction methodology, including validation and an estimate of the associated uncertainty;
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Section 2.1.2 – Turbulence Characteristics: The selected integral length scale suggests that unsteady aerodynamic effects could be significant. The authors should clarify whether this was considered during experimental design and data interpretation. If not, this omission should be acknowledged and discussed as a potential limitation;
Citation: https://doi.org/10.5194/wes-2025-36-RC2 -
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