Experimental assessment of scale-effects on the aerodynamic characterization of a transitionally-operating airfoil working under clean flow conditions

Zarketa-Astigarraga, Ander; Peñalba, Markel; Martin‐Mayor, Alain; Martinez‐Agirre, Manex

doi:10.1016/j.measurement.2021.110414

Cited by 4 publications

(12 citation statements)

References 40 publications

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“…If the

c_{l_{max.}}

trends indicate a progressive lowering on the power outcome of transitionally‐operating blades, the

E_{max.}

parameter shows that such a decrease is accompanied by a higher ratio of the input/output energies of the system, leading to a higher cost per kW produced.

{bold-italicα}_{boldstall}

evolution: it is representative of the strong synergistic effect undergone by the Real configurations. Indeed,

α_{stall}

experiences a sustained increase in the case of the Clean paradigm, in accordance to the acute scale‐effect explained in 18 that leads to an enlarged pre‐stall regime. The Turbulent paradigm is effective in delaying stall, showing a wider pre‐stall region that barely changes with the Reynolds number.…”

Section: Resultssupporting

confidence: 80%

“…As for the Turbulent and Real cases, their effect can only be considered within the inwards half of the span, due to the technical limitations of the wind‐tunnel facility that constrain the Reynolds number to values below

1.2 \times 1 0^{5}

when the turbulence‐generating‐grid is present. A previous work by the authors has shown that the aerodynamic features of the NACA0021 airfoil tested herein evolve non‐linearly within the transitional region, 18 being unable to determine their variations via extrapolation with acceptable precision. Accordingly, the polars of the outer half of the blade sections, when they are operating under either Turbulent or Real conditions, are considered to be those that correspond to the respective data sets obtained at

Re = 1.2 \times 1 0^{5}

, as represented by the straight dashed line that parts from the mid spanwise dimension in Figure 4B.…”

Section: Case‐studymentioning

confidence: 73%

“…Given the scale‐effect observed in transitionally operating airfoils 18 and the noticeaeble Reynolds variation that, according to Figure 4B, ensues along the span, the blade is discretized into

N = 100

equispaced elements for considering the Reynolds dependency of the polars. The polars of the blade elements subjected to Reynolds numbers that fall between the testing values are obtained by interpolation, which cover the entire range in the Clean cases.…”

Section: Case‐studymentioning

confidence: 99%

“…Databases comprising a wide number of airfoils operating at such Reynolds numbers have been largely used, 13,14 with updated versions of lift, drag and pressure coefficient curves being widely employed nowadays 15,16 . Under such Reynolds conditions, the flow is known to lie within the so‐called transitional regime , with complex structures being formed upon its surface 16–18 . The transitional regime stands between the laminar and turbulent regimes and shows an interplay between the fluid structures formed upon both of them.…”

Section: Introductionmentioning

confidence: 99%

“…The usual picture of an airfoil subjected to a transitional flow consists of a short laminar extent near the leading‐edge of the airfoil, followed by a separated shear‐layer that reattaches to the surface in the form of a turbulent boundary‐layer downstream, producing what is known as a laminar separation bubble in between 19,20 . Such a bubble is known to be highly sensitive to flow conditions, its position depending strongly on the Reynolds number, the geometry of the airfoil and the freestream conditions 18,20,21 . This sensitivity of the bubble gets reflected on the aerodynamic behaviour of the airfoil, whose lift and drag coefficients show a noticeable dependence on the Reynolds number, even at Reynolds variations as low as 20k 18 .…”

Section: Introductionmentioning

confidence: 99%

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Impact of turbulence and blade surface degradation on the annual energy production of small‐scale wind turbines

Zarketa‐Astigarraga,

Penalba,

Martin‐Mayor

et al. 2023

Wind Energy

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Small‐scale horizontal axis wind‐turbines (SHAWTs) are acquiring relevance within the regulatory policies of the wind sector aiming at net‐zero emissions, while reducing visual and environmental impact by means of distributed grids. SHAWTs operate transitionally, at Reynolds numbers that fall between . Furthermore, environmental turbulence and roughness affect the energetic outcome of the turbines. In this study, the combined effect of turbulence and roughness is analysed via wind tunnel experiments upon a transitionally operating NACA0021 airfoil. The combined effects cause a negative synergy, inducing higher drops in lift and efficiency values than when considering the perturbing agents individually. Besides, such losses are Reynolds‐dependent, with higher numbers increasing the difference between clean and real configurations, reaching efficiency decrements above 60% in the worst‐case scenario.Thus, these experimental measurements are employed for obtaining the power curves and estimating the annual energy production (AEP) of a 7.8‐kW‐rated SHAWT design by means of a BEM code. The simulations show a worst‐case scenario in which the AEP reduces above 70% when compared to the baseline configuration, with such a loss getting attenuated when a pitch‐regulated control is assumed. These results highlight the relevance of performing tests that consider the joint effect of turbulence and roughness.

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“…If the

c_{l_{max.}}

trends indicate a progressive lowering on the power outcome of transitionally‐operating blades, the

E_{max.}

parameter shows that such a decrease is accompanied by a higher ratio of the input/output energies of the system, leading to a higher cost per kW produced.

{bold-italicα}_{boldstall}

evolution: it is representative of the strong synergistic effect undergone by the Real configurations. Indeed,

α_{stall}

Section: Resultssupporting

confidence: 80%

1.2 \times 1 0^{5}

Re = 1.2 \times 1 0^{5}

, as represented by the straight dashed line that parts from the mid spanwise dimension in Figure 4B.…”

Section: Case‐studymentioning

confidence: 73%

N = 100

Section: Case‐studymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Impact of turbulence and blade surface degradation on the annual energy production of small‐scale wind turbines

Zarketa‐Astigarraga,

Penalba,

Martin‐Mayor

et al. 2023

Wind Energy

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View full text Add to dashboard Cite

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A proper orthogonal decomposition analysis upon aerodynamic structures under clean and rough conditions

Quiroga,

Zarketa-Astigarraga,

Cárdenas-Rondón

et al. 2023

Physics of Fluids

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A transitional flow regime is known to induce complex flow structures upon aerodynamic geometries such as airfoils, and the dynamics of laminar separation bubbles (LSBs) pose a relevant research field. In addition to being affected by the Reynolds value itself, LSBs are shown to be equally sensitive to the surface roughness of the airfoils. The study analyses wind tunnel-derived surface-pressure distribution datasets obtained for a particular airfoil of the standard family developed by the National Advisory Committee for Aeronautics (NACA), namely, the NACA0021, subjected to the range 0.8×104<Re<1.6×104 at different angles of attack under two flow configurations that correspond to a clean and roughened surface. The analysis is undertaken via the proper orthogonal decomposition (POD) technique. The results show that the decomposition of the temporal series of surface-pressure data and the processing of the most energetic POD modes recovers the position of the LSBs, properly capturing the closure point of the separation bubbles and, hence, the turbulence transition location. Some of the most energetic POD modes observed are closely related, in terms of shape, to the POD modes present at the reattachment point on a 5:1 rectangular cylinder. This could indicate there is a recognizable pattern in coherent structures of pressure fluctuations when it comes to a reattached flow. Therefore, a principal component analysis such as the POD presented in this study can be used to determine the reattachment position of the flow or the transition point in the presence of a LSB.

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Investigation of aerodynamic performance of Clark-Y airfoil with more realistic tubercle model and internal slots

2023

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An experimental study is conducted to research the combined effect of leading-edge (LE) tubercles and internal slots on the Clark-Y airfoil at Reynolds numbers (Re) of 5.0×104, 7.5×104, and 1.0×105 for angles of attack ranging from 0° to 30°. The Clark-Y is a cambered airfoil that performs well at low and medium Reynolds numbers, having a maximum thickness of 11.7% at 28% of the chord. Five different test models are the subjects of experiments, one of which is the baseline. The other test models are the Wavy model, which has LE tubercles, and the Wavy with three different slot positions: the Wavy-LES (leading-edge slot), the Wavy-MS (middle slot), and the Wavy-TES (trailing-edge slot). The LE tubercle geometry is formed by the sum of two sinusoidal wave functions with a realistic approach. In the present work, experimental studies are performed through force measurements, and detailed information on a 3-dimensional flow field is obtained by a surface-oil flow visualization technique. Force measurements indicate that the Wavy model delays stall and exhibits smoother stall behavior compared to the baseline. Moreover, the findings show that the lift coefficient (CL) of the Wavy model deteriorates in the pre-stall region with the inclusion of the internal slots but improves significantly in the post-stall region. Maximum improvement in CL in the post-stall region was achieved by 60% in Wavy-LES at Re of 5.0×104 as compared to the baseline. At Re of 5.0×104, Wavy and Wavy-MS present a better lift-to-drag ratio (CL/CD) than the other models in the pre-stall region, whereas the baseline is the best at Re of 1.0×105. The best CL/CD is achieved by Wavy-LES in the post-stall region, regardless of the Reynolds numbers. The variation of flow characteristics relevant to aerodynamic performance is revealed by surface oil flow visualization for all tested models.

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Experimental assessment of scale-effects on the aerodynamic characterization of a transitionally-operating airfoil working under clean flow conditions

Cited by 4 publications

References 40 publications

Impact of turbulence and blade surface degradation on the annual energy production of small‐scale wind turbines

Impact of turbulence and blade surface degradation on the annual energy production of small‐scale wind turbines

A proper orthogonal decomposition analysis upon aerodynamic structures under clean and rough conditions

Investigation of aerodynamic performance of Clark-Y airfoil with more realistic tubercle model and internal slots

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