# The Influence of Winglet Pitching on the Performance of a Model Wind Turbine: Aerodynamic Loads, Rotating Speed, and Wake Statistics

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Setup

## 3. Results and Discussion

#### 3.1. Rotation Speed and Thrust Force

#### 3.2. Wake Characteristics

## 4. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**(

**a**) schematic of the experimental setup illustrating the PIV system, force sensor, and bottom wall roughness; (

**b**) photograph of the experimental setup in BLAST; (

**c**) details of the turbine rotors highlighting the definition of winglet pitching angle from lateral (left) and leeward (right) side views. The orange arrow indicates the clockwise direction of turbine rotation from the leeward sight; (

**d**) photographs of blade tips with winglet with pitching angles at $\alpha ={0}^{\circ},{30}^{\circ},{60}^{\circ}$ and ${90}^{\circ}$ (from top to bottom). The leeward surface of the turbine was painted black and the winglet was paint white.

**Figure 2.**Main characteristics of the incoming turbulent boundary layer flow. (

**a**) time-averaged velocity $U/{U}_{hub}$; (

**b**) streamwise turbulence intensity ${\sigma}_{u}/{U}_{hub}$; (

**c**) kinematic shear stress $-{u}^{\overline{\prime}}{v}^{\prime}/{U}_{hub}^{2}$. The vertical distances are normalized by the turbine hub height ${z}_{hub}$.

**Figure 3.**(

**a**) comparison of the normalized mean tip speed ratio ($\lambda /{\lambda}_{o}$, blue bar) and thrust coefficient (${C}_{T}/{C}_{{T}_{o}}$, red bar) of the turbine subjected to various winglet pitch angles; (

**b**) schematic of the blade tip vortices with winglet pitching.

**Figure 4.**Spectra of instantaneous turbine rotation speed ${\Phi}_{\omega}$ under different $\alpha $. The subplot shows the spectra of incoming velocity at hub height. The vertical dashed line separates the turbulence from energy containing sub-range (${R}_{1}$) to inertial sub-range (${R}_{2}$).

**Figure 5.**Integrated energy ${\sigma}_{\omega}^{2}$ of large-scale rotation speed fluctuations subjected to various $\alpha $.

**Figure 6.**Normalized mean streamwise velocity distribution, $U/{U}_{hub}$, in the central plane for the turbines with winglet pitch angle under $\alpha =$ (

**a**) ${0}^{\circ}$, (

**b**) ${30}^{\circ}$, (

**c**) ${60}^{\circ}$, and (

**d**) ${90}^{\circ}$.

**Figure 7.**Profiles of normalized mean streamwise velocity deficit, $({U}_{inc}-U)/{U}_{hub}$, for the wake flow at $x/{d}_{T}=$ (

**a**) 2, (

**b**) 3, and (

**c**) 4. The dash-dotted lines refer to positions of the upper and lower blade tips.

**Figure 8.**(

**a**) normalized stream-wise mean velocity profile, $U/{U}_{hub}$ along the hub height for various $\alpha $. The solid lines represent those measured from experiment and the dashed lines are velocities predicted by model from Bastankhah and Porté-Agel [39]; (

**b**) determination of ${k}^{*}$ by fitting the measured and modeled velocities of a traditional turbine without winglet.

**Figure 9.**Distribution of turbulence kinetic energy, $TKE=<{u\prime}^{2}+{w\prime}^{2}>/2{U}_{hub}^{2}$, in the central plane for the turbines under $\alpha =$ (

**a**) ${0}^{\circ}$, (

**b**) ${30}^{\circ}$, (

**c**) ${60}^{\circ}$ and (

**d**) ${90}^{\circ}$.

**Figure 10.**Profiles of turbulence kinetic energy, for the wake flow at $x/{d}_{T}=$ (

**a**) 2, (

**b**) 3, and (

**c**) 4.

**Figure 11.**Normalized streamwise wake velocity fluctuation ${I}_{u}$ under $\alpha =$ (

**a**) ${0}^{\circ}$ and (

**b**) ${90}^{\circ}$. The dash-dotted lines refer to positions of the upper and lower blade tips.

**Figure 12.**Normalized streamwise wake velocity fluctuation ${I}_{v}$ under $\alpha =$ (

**a**) ${0}^{\circ}$ and (

**b**) ${90}^{\circ}$.

**Figure 13.**Compensated velocity spectra difference $\Delta \left(f\varphi \right)$ normalized by its local maximum between the wake and incoming flow for $\alpha =$ (

**a**) ${0}^{\circ}$, (

**b**) ${30}^{\circ}$, (

**c**) ${60}^{\circ}$, and (

**d**) ${90}^{\circ}$.

**Figure 14.**(

**a**) cross-correlation function $\eta \left(\tau \right)$ for selected case under $\alpha ={0}^{\circ}$; (

**b**) normalized time-delay of wake transport across various downstream distances and $\alpha $.

**Table 1.**Comparison of measured and modeled $\frac{d(U/{U}_{hub})}{d(x/{d}_{T})}$ across various pitching angles.

$\mathit{\alpha}={0}^{\circ}$ | $\mathit{\alpha}={30}^{\circ}$ | $\mathit{\alpha}={60}^{\circ}$ | $\mathit{\alpha}={90}^{\circ}$ | |
---|---|---|---|---|

$\frac{d(U/{U}_{hub})}{d(x/{d}_{T})}$ (Mea) | 0.100 | 0.097 | 0.091 | 0.097 |

$\frac{d(U/{U}_{hub})}{d(x/{d}_{T})}$ (Mod) | 0.0893 | 0.0812 | 0.0724 | 0.0747 |

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**MDPI and ACS Style**

Aju, E.J.; Suresh, D.B.; Jin, Y.
The Influence of Winglet Pitching on the Performance of a Model Wind Turbine: Aerodynamic Loads, Rotating Speed, and Wake Statistics. *Energies* **2020**, *13*, 5199.
https://doi.org/10.3390/en13195199

**AMA Style**

Aju EJ, Suresh DB, Jin Y.
The Influence of Winglet Pitching on the Performance of a Model Wind Turbine: Aerodynamic Loads, Rotating Speed, and Wake Statistics. *Energies*. 2020; 13(19):5199.
https://doi.org/10.3390/en13195199

**Chicago/Turabian Style**

Aju, Emmanuvel Joseph, Dhanush Bhamitipadi Suresh, and Yaqing Jin.
2020. "The Influence of Winglet Pitching on the Performance of a Model Wind Turbine: Aerodynamic Loads, Rotating Speed, and Wake Statistics" *Energies* 13, no. 19: 5199.
https://doi.org/10.3390/en13195199