Investigation of NREL Phase VI wind turbine blade with different winglet configuration for performance augmentation

ABSTRACT

A wind turbine blade’s winglet is predominantly used to reduce the induced drag generated by the blades and consequently improve the blade’s aerodynamic performance. The benchmark blade NREL Phase VI and all blades with the winglet were designed and simulated using CFD. Subsequently, the numerical model is validated against data from the NREL Phase VI experiment. In the first case, at a fixed winglet length of 0.09 m, the best results are obtained for sharp bent configurations when the winglet cant angle is 45° whereas for smooth bent configurations is at a 30° cant angle. Under the second scenario, ten winglets with different winglet heights, lengths, and cant angles and a simple linear tip extension are generated and tested numerically at wind speeds of 5, 7 and 9 m/s. Therefore, curving the tip to make a winglet shape of applicable configuration is definitely preferable to extending the tip straight.

KEYWORDS:

• NREL Phase VI blade
• winglet
• winglet configuration
• blade-tip vortices
• CFD

1. Introduction

The strategies used currently to improve the efficiency of horizontal axis wind turbines involve the use of power curve remodelling devices, such as vortex generators, winglets, and serrated trailing edges, among others. The impacts of vortex generators on the S809 airfoil were studied by Wang et al. (2016) using computational fluid dynamics. It was shown that the vortex generators could significantly reduce the thickness of the boundary layer and enhance the aerodynamic performance of the S809 airfoil. Delays occur in the stall phenomenon. As a result, the vortex generators can regulate the flow separation and effectively increase the airfoil’s lift coefficient. Similar to this, Sørensen et al. (2014) verified their findings with experimental data after using computational fluid dynamics to study the impact of vortex generators on FFA-W3-301 and FFA-W3-360 airfoils. Likewise winglets’ effects on aerodynamic performance and the characteristics pertaining to the wing tip and the blade tip are also noteworthy. Many factors determine the curvatures or geometries of winglets, including the cant angle, length, height, twist, sweep, toe, chord distribution, and also planform like rectangular, tapered, elliptical, etc., airfoil, and position, the wing suction or pressure side (Gertz, Johnson, and Swytink-Binnema 2012; Guerrero, Sanguineti, and Wittkowski 2018; Khalafallah, Ahmed, and Emam 2019; Popescu et al. 2022). The curved blade’s shape effectively divides the high and low-pressure air, enhancing lift on the blade’s upstream side and decreasing induced drag brought on by the vortices. Similarly, vortex generators are a pair of tiny fins that are located close to the wind turbine blade’s root. They reduce airflow separation, which leads to a smoother flow across the blade. This increases the torque needed to turn the rotor and generate power with less turbulence.

The shape of the winglets and their direction upstream or downstream have an impact on the performance of turbine blades, according to several researchers who evaluate the tip vortex and characteristics of blades with winglets (Al-Abadi et al. 2018; Ali et al. 2015; Khalafallah, Ahmed, and Emam 2019; Khaled et al. 2019). Other research (Guerrero, Sanguineti, and Wittkowski 2018; Gupta and Amano 2012; Khaled et al. 2019) examines the impact of blade winglet parameters on the effectiveness of wind turbine blades, specifically the modification of cant angle, sweep angle, dihedral angle, toe angle, winglet height impact, winglet curvature impact, taper ratio, and blended winglet. A study on the examination and blade tip winglets optimisation utilising the free wake vortex approach of implicit was conducted by Lawton and Crawford (2014). Thus, it was determined that adding winglets may result in a 2% increase in wind power and a corresponding increase in thrust. A winglet that slightly reduces thrust while keeping power that is quite similar to that of a regular straight blade is also suggested.

A three-bladed horizontal axis wind turbine’s entire blade and bladelet configurations were designed utilising a thorough 3D flow-field analysis in conjunction with multi-objective constrained shape design optimisation, according to a study by Reddy et al. (2019). Results for a Pareto-optimised bladelet on a specific blade demonstrate that during off-design conditions, a more than 4% increase in the coefficient of power at a minimum thrust force penalty is feasible compared to the same wind turbine rotor blade without a bladelet. It is anticipated that even higher performance of a blade and bladelet combination will be possible whenever the blade and bladelet forms are concurrently optimised, if more complex bladelet shapes are taken into account, and if more robust response or reaction surfaces and support points are employed. Likewise, for the optimisation of a curved bladelet applied to a wind turbine blade, Zouboulis, Koumoulos, and Karatza (2023) utilised a systemic computational approach. The findings demonstrate that the extra component’s aerodynamic characteristics resulted in an improved form of the original bladelet with a generated torque that was about 30% higher on the bladelet itself and 0.81% higher overall on the blade. The suggested workflow tries to take a comprehensive approach to blade tip optimisation, taking into account both structural and aerodynamic characteristics.

To determine the most effective stopping strategy of the induced flow at the tip, Al-Abadi et al. (2018) conducted experimental examinations of the turbulent free stream influence on the vortices of the tip created by blades of wind turbines and different winglet designs. The findings demonstrate that turbulence lowers tip losses by suppressing tip vortices. Encourage more research into the area surrounding the intersection of the near and far wakes to better comprehend how the exchange of energy and entrainment of the free stream support the recovery of the wake. The results of an investigation by Gaunaa and Johansen (2007) into the theoretical issues and calculation outcomes pertaining to the usage of wind turbine’s winglets revealed that winglets aligned downstream are superior to upstream ones in terms of optimising the power coefficient and that the improvement in power output isn’t as great as what could be attained by simply extending the blade radially outward. Additionally, it is claimed that downwind winglets with shorter lengths (>2%) can achieve power coefficient increases comparable to those attained by radial wing extensions. Johansen, Gaunna, and Sørensen (2008) numerically examined a number of winglet designs and revealed a number of winglet-related parameters that could be modified, such as the winglet height, curvature radius, and other winglet-related angles. According to the study, upstream-facing winglets had a power coefficient boost ranging from 0.6% to 1.4% and a thrust coefficient rise of 1.0% to 1.6%.

Furthermore, Zahle et al. (2018) investigation examined how to optimise wind turbine blades with an elongation using CFD incorporation with a surrogate model to maximise energy output. The research demonstrates that a winglet on a blade extension can enhance power generation by 2.6% while maintaining the same flap-wise bending moment at a 90% radius, but a straight blade extension could only increase power production by 0.76%. Likewise, Madsen et al. (2022) claim that a 10 MW reference wind turbine was used to aerodynamically develop a novel curved tip shape for maximum power output under the restriction that the initial steady-state loads shouldn’t be compromised. According to the analysis, a 1.12% increase in power was feasible while still adhering to the loads’ and geometry’s imposed restrictions.

Variable cant angle winglets were suggested by Guerrero, Sanguineti, and Wittkowski (2018) as a way to help aircraft achieve the optimum overall effectiveness, to reduce the induced drag due to the lift, at various values of attack angle. It is therefore recommended that for optimal performance and drag reduction, a cant angle of 15° be used. The use of winglets, according to Khalafallah, Ahmed, and Emam (2019), can increase aerodynamic efficiency to the point where at the tip speed ratio of the design for downwind-swept blades with winglets facing upwind, the power coefficient increases by 4.39% with the winglet canting at 40° and twisting at 10°. Additionally, according to Ali et al. (2015) study findings, a 26% lift-to-drag ratio increase is caused by the upwind winglet when compared to a conventional blade without a winglet, while the downstream winglet causes it to decrease by around 27%. It is also hypothesised that improved aerodynamic performance is significantly influenced by the wind turbine blade angle of the yaw. The best performance improvement, according to Khaled et al. (2019), was obtained when the length of winglets was kept at 6.32% of the wind turbine radius of the blade and canted at a 48.3° angle. The effect of winglet geometry and airfoil on wind turbine effectiveness was explored by Farhan et al. (2018), who investigated and searched for the optimal design of the winglet for the wind turbine blade. Using the model winglet design, it is predicted that the power generated by wind turbines will rise.

To predict aerodynamically the effectiveness of NREL Phase VI wind turbines, Elfarra, Sezer-Uzol, and Akmandor (2014) solved steady-state RANS equations and applied the k-ϵ Launder-Sharma turbulence model. The results of computation and the experimental data have a good deal in common. It was suggested that the final optimised winglet produced more power by about 9%. Other studies investigated and centred on the impact of winglet use on the blade’s tip of an NREL Phase VI wind turbine, and how turbulence intensity affects a blade with and without a winglet. Geometry modelling and simulation are done using CFD. The computational conclusion produced by the modelling of k-ω SST turbulence is strongly supported by the NREL experimentally measured data between wind speed of 5 m/s to 25 m/s (Farhan et al. 2018; Gupta and Amano 2012; Khaled et al. 2019; Verma et al. 2021). Similarly, Sy, Abuan, and Danao (2020) used CFD to analyze the aerodynamics of a winglet with split geometry on a wind turbine blade by modifying the NREL Phase VI blade tip. According to the study, adding the winglet boosted power generation by 1.23% averagely in between 7 m/s to 15 m/s wind speed, as opposed to extra-long blades, split winglets with split geometry boosted it by 2.53%.

Furthermore, a thorough analysis of the blade tip for a vertical axis wind turbine, including aerodynamics and the tip loss effect, the endplate design effect, investigating the impact of tip flow on the blade, and conducting a thorough analysis of the aerodynamic distribution along the span-wise direction, have been studied by several researches (Khai et al. 2022 Miao et al. 2022; Ung et al. 2022; Zhang et al. 2019;). The implications of winglet application in vertical axis wind turbines have recently also been the subject of extensive research. The studies’ key finding indicates that the shape, parameters and characteristics of the wings have a significant impact on the beneficial effect of wingtip devices. Moreover, with an increase in TSR, endplate improvements deteriorate. According to Miao et al. (2022), the only ways to increase the power coefficient over a wider range of TSRs are through the use of an innovative Winglet-H and the streamlined-shaped endplate.

According to a literature review, winglets for wind turbine blades have not received as much attention as non-rotating blades. The literature revealed that several studies had been done on the impact of winglet parameters and configurations broadly for aircraft wings and moderately on the performance of wind turbine blades, Maughmer’s (2003), but a winglet shape and airfoil profile effect for a wind turbine blade had not been thoroughly studied. The potentials of curved planform winglet profiles for performance augmentation have not been adequately investigated thereby the interaction between the winglet parameter and a wide variety of winglet geometry was not taken into account in earlier work on curved winglets. Furthermore, previous studies have mostly focused on the overall aerodynamic and underscored performance of blades with different geometric profiles, such as swept, elliptic, and rectangular shapes (Elfarra et al., 2015; Farhan et al. 2018; Khalafallah, Ahmed, and Emam 2019; Sy, Abuan, and Danao 2020; Zhang et al. 2023), still leaving a substantial research gap on curved planform winglet configurations and very little consideration given to the unique impacts of winglet curvature with various profiles. A change of focus towards a more comprehensive understanding of the winglet curvature with different profiles’ impact on the tip flow and vortices is indispensable. Moreover, previous studies have only examined a small number of winglet airfoil profiles, which has limited the range of findings.

In particular, there hasn’t been much research on or the literature report regarding the performance of curved planform winglet shapes with different airfoil profiles. As a result, the present study intended to evaluate the impact of curved planform winglet shape with variable winglet length and S809 airfoil profile on the NREL Phase VI effectiveness. As a result, the winglets’ curved design and optimal winglet length may have a substantial impact on dispersing wing tip vortices and reducing induced drag. Therefore, winglet configurations with different winglet lengths were created and looked at to evaluate the flow behaviour at the tip and tip vortices. Additionally, validation was done by comparing the predicted power, coefficients of pressure, and thrust force with data from the experiment.

2. Blade geometry modeling and methodology

The NREL Phase VI wind turbine blade was used to model and design the intended blade with a winglet. This blade, NREL Phase VI, entails extensive experiments carried out in the NASA Ames wind tunnel facilities (Hand et al. 2001). The experimental arrangements of the examinations involve several sequences, for this study the sequence S was chosen. The NREL Phase VI benchmark blade had been created in QBlade (Marten 2016), as shown in Table 1 and Figure 1. In this study, we modify the blade tip of the NREL Phase VI benchmark wind turbine using ANSYS SpaceClaim. The main focus of this study is to investigate how winglet curvature with the S809 profile affects the effectiveness of the NREL Phase VI blade by conducting a thorough examination of the aerodynamic distribution at the tip region. These winglets play a critical role in the subsequent investigation involving winglets with different airfoil profiles, which expands the scope of the findings. Moreover, the present study intended to investigate the design space of the blade tip with a methodical variation of winglet parameters to examine the impact of the blade winglet geometric parameters variation on power and thrust force. Consequently, to test the impact of incorporating a winglet, the tip section of an NREL Phase VI wind turbine blade was modified under two different conditions.

Figure 1. NREL Phase VI wind turbine blade generated in QBlade.

Table 1. The NREL Phase VI rotor: Geometry and design parameters (Hand et al. 2001).

Blades Number	2
Rotor Diameter	10.058 m
RPM	72 rpm
Cut-in wind speed	5 m/s
Rated Power	19.8 kW
Cone angle	0°
Rotor orientation	Upwind
Power Regulation	Stall Regulated
Blade tip pitch angle	$3^{\circ }$
Blade airfoil profile	S809
Blade chord	A linear taper (0.358-0.728 m)
Twist angle	Twisting nonlinearly throughout the span
Blade thickness	t/c = 21% throughout the span

Scenario 1: involved the design of winglet configuration with respect to sharp bent and smooth bent with S809 airfoil used thereby the length of the winglet was varied from 50 mm to 150 mm at fixed cant angles 45° and 90° for sharp bent, Figures 2 and 3. Likewise, the cant angle of the winglet was varied from 15° to 90° for both sharp bent and smooth bent at a fixed winglet length of 90 mm and with curvature radius distribution varying chord-wise. The winglet has also a 1% (50 mm) blade radius linear extension.

Figure 2. 2D tip region sectional view of NREL Phase VI blade with a comparable surface (wetting) area but different winglet configurations; sharp bent (a, c) and smooth bent (b, d) respectively.

Figure 3. 3D tip region view of NREL Phase VI blade with a comparable surface (wetting) area but different winglet configurations; a & b are with winglet length 90 mm and cant angle 45° oriented towards the suction side; c & d are with winglet length 90 mm and cant angle 90° oriented towards the suction side; sharp and smooth bent respectively.

Scenario 2: involved the design of winglet configuration with respect to sharp bent with arced winglet planform and S809 airfoil used thereby including the benchmark blade, the blade with simple linear extension, and the winglet-incorporated blades from NREL Phase VI wind turbine blade were then combined to create a total of ten different blade variants, Figures 4 and 5 and Table 2. The software ANSYS SpaceClaim was then used to design and create the winglets after the generated blade had been imported and the design of the winglets involved elongating (200 mm) the blade’s tip portion linearly in the direction of the span, which accounts for 4% of the benchmark blade radius, and then bending it toward the suction side with a specified curvature radius, winglet height, and rotating angle, as shown in Figures 4 and 5. Under this scenario the maximum winglet length of 40 mm to 140 mm in the step of 10 mm with the corresponding cant angle was considered; the winglet length and curve length distribution varied chord-wise but the same radius distribution was used for all designs, and the S809 airfoil used to generate the winglet. Accordingly, Table 2 and Figures 4 and 5 illustrate the generated blade sections at the maximum winglet length with the corresponding cant angle and other parameters. The simulations and analysis were performed in terms of power improvement at 5–13 m/s wind speeds and the pressure coefficient distribution at selected sections was also evaluated. The effects of various winglet configurations on the blade tip vortices, power, and axial thrust force generation of the whole blade were then investigated and finally, the better winglet configuration was selected.

Figure 4. (a) Geometric quantities used to define the winglet, The section view of the winglet at its maximum length: (b) with a sectional cant angle of 80.6° and winglet height of 70 mm, (c) with a sectional cant angle of 84.3° and winglet height 90 mm, (d) with a sectional cant angle of 88.2° and winglet height of 110 mm (e) Isometric view.

Figure 5. NREL Phase VI blade with winglet, tip region view (a) with maximum winglet length 51.47 mm & cant angle 76.9° oriented to the suction side, b) with maximum winglet length 71.09 mm & cant angle 80.6° oriented to the suction side, c) with maximum winglet length 90.62 mm & cant angle 84.3° oriented to the suction side, d) with maximum winglet length 110.3 mm & cant angle 88.2° oriented to the suction side.

Table 2. Design parameters and winglet configurations (Scenario 2).

Winglet Name	Winglet Height (mm) (%of Benchmark blade Radius)	Winglet Length (mm)	Cant Angle (degree)	Airfoil Profile
W1	40 (0.8%)	41.41	75.0	S809
W2	50 (1.0%)	51.47	76.9	S809
W3	60 (1.2%)	61.31	78.8	S809
W4	70 (1.4%)	71.09	80.6	S809
W5	80 (1.6%)	80.86	82.5	S809
W6	90 (1.8%)	90.62	84.3	S809
W7	100 (2.0%)	100.43	86.2	S809
W8	110 (2.2%)	110.30	88.2	S809
W9	120 (2.4%)	120.30	90.2	S809

2.1. Computational domain and mesh generation

The simulations were carried out using FLUENT, a computational fluid dynamics programme. Moreover, for this simulation, the RANS Equations with chosen turbulence model, k-ω SST model was used; k-ω SST is y + insensitive so that can adapt itself to any type of boundary layer mesh resolution. According to findings in the literature, the (k-ω) SST turbulence model maintains an acceptable balance between accuracy and computational effort. The (k-ω) SST model was chosen because turbulent flow over airfoils has been effectively simulated using it. Moreover, the (k-ω) SST is better than other two-equation turbulence models for external flow simulation, but typically not sufficiently precise to be used as a reference for the exact numerical prediction of aerodynamic drag and lift forces (Popescu et al. 2022; Zhang et al. 2023). The rotational axis is counterclockwise about the z-axis because the computational domain employs a Cartesian coordinate system, where the positive x-axis is in the direction of the blade span, the negative y-axis is in the vertical direction, and the positive z-axis is in the direction of the stream. The boundary conditions were defined by modelling explicitly one turbine blade during computational simulation; it explains the two NREL Phase VI turbine blades’ 180° rotation. The periodic boundary condition was used for the second turbine blade. A simulation was performed using the moving reference frame (MRF) which considers the blade’s rotating motion in the domain. Investigations were carried out at 72 rpm angular speed. Moreover, for the inlet boundary, between 5 m/s to 20 m/s wind speed values were considered, and for the outlet boundary, a zero Pascal pressure was chosen. The turbine blade surface was assigned the no-slip conditions. Since the rotational flow field simulation is based on the MRF function, a cylindrical shape of a computational domain is designated which has been made equal to 20 and 50 m from the blade in the upstream and downstream directions, respectively. The proximity and curvature sizing function was applied to mesh the model for refinements and 13 inflation layers were used so that the first layer height was kept at $2.4x{10}^{-5}$ m which keeps y + values below 5. Additionally, a rectangular local refinement region around the blade was implemented. Furthermore, to ensure accurate simulation the leading and trailing edge regions of the blade are enabled to have well-refined meshes. The volume mesh was then meshed using poly-hexcore meshing. A poly-hexcore meshes generally, the first application of Mosaic technology, results in a lower mesh count, relative to polyhedral, thereby reducing the simulation time. The resulting mesh consists of the number of cells 1.67 million, faces number 7.70 million, and nodes number 4.43 million, with an average 0.9 orthogonality quality and a 0.6 maximum skewness. Since utilising a transient solver did not significantly deviate from the steady-state solver, a pressure-based steady-state RANS solver was employed to shorten calculation times. A report definition includes the moment, force and residual on the rotor blade was considered and residual values below 10⁻⁴ were ensured for convergence. The resulting domain mesh and the mesh near the winglet and blade are shown in Figure 6.

Figure 6. (a) Physical domain, (b) a rectangular local refinement region around the blade (c) blade surface mesh, (d) a poly-hexcore domain mesh.

Taking torque and axial thrust force as the response parameters, the simulation results are checked for grid independency using mesh refinement. As revealed in Table 3, torque and thrust force at 7 m/s become stable and the deviation is less than 0.3% when cell numbers are greater than 1.5 million cells. As a result, a total of 1.67 million cell numbers are selected.

Table 3. Mesh independence test.

Cells	Torque	Thrust
$1.06\mathit{x}{10}^6$	285.81	493.55
$1.24\mathit{x}{10}^6$	368.79	571.82
$1.67\mathit{x}{10}^6$	370.75	570.28
$1.86\mathit{x}{10}^6$	371.01	569.02

3. Results and discussion

The NREL Phase VI blade tip is modelled using SpaceClaim direct modeller. These include the maximum winglet height and length, the corresponding cant angle and arc radius, and Fluent output parameters like the moment and axial thrust force. To assess how these inputs influence the effectiveness of the winglet, a range of input parameters is chosen thereby a winglet height between 0.04 and 0.14 m and a cant angle between 15° and 90° have been examined. Furthermore, the simulation is carried out over selected wind speeds of 5, 7, 9 11, and 13 m/s. Two of the main emphasis parameters of the investigation are the torque and the axial thrust force. As a result, comprehensive CFD simulations are used to compare and validate them. Therefore, the simulation results were divided into two categories. The first is the validation part which is intended to validate and assess the computational model’s capability to predict the NREL Phase VI experimental data. The second is the CFD simulation results that were obtained by the modelled and designed blade winglets. The NREL Phase VI benchmark simulation was conducted at wind speeds of 5-10, 13, 15, and 20 m/s. The tip-speed ratio of 1.51–7.52 is considered which corresponds to the angular speed of 72 rpm; so the design tip-speed ratio is 5.20 which corresponds to the design wind speed of 7.20 m/s and blade radius of 5.029 m. In particular, as shown in Figure 7, these CFD simulation findings were contrasted with data from the NREL Phase VI turbine blade experiment and related works published in the literature.

Figure 7. Aerodynamic power comparison of the Benchmark blade CFD, QBlade prediction, published work, and experimental data (NREL).

The equation used to compute the pressure coefficient is (Khalafallah, Ahmed, and Emam 2019; Popescu et al. 2022; Zhang et al. 2023): (1) $C_{p,i}={\displaystyle \frac{p-p_{\mathrm{ref}}}{0.5\rho (V^2+{(\omega r_i)}^2)}}$(1) Where i stands for the i^th blade portion, p denotes local static pressure, p_ref indicates a reference, the free stream static pressure at the domain inlet in this instance, $\rho$ denotes air density, V denotes inlet velocity, $\omega$ denotes rotor rotational velocity, and r_i denotes i^th section radial position. Likewise, integrated aerodynamic torque can directly be calculated either from CFD post-processer or solution report stage.

The wind turbine aerodynamic power is calculated by. (2) $P=\mathrm{\tau \omega }$(2) where P is the computed aerodynamic power (W); $\tau$is integrated torque (Nm); $\omega$ is the rotor angular speed (rad/s).

3.1. Validation of NREL Phase VI CFD simulation

The simulation results are depicted by comparing the aerodynamic power at different wind speeds as well as the chord-wise pressure coefficient distributions at different blade sections span-wise; Hence, Figure 7 depicts the comparison between the simulated power and the sequence S experimental data of NREL Phase VI so that there is a good agreement between the simulated power and data from the experiment. Further, the simulated results of the benchmark blade show that the CFD power prediction is in good agreement with the experimental results compared to the QBlade power prediction.

Furthermore, related works in the literature showed that wind turbine blade winglet performance can be reasonably predicted using the CFD simulation (Madsen et al. 2022 Zahle et al. 2018; Zouboulis, Koumoulos, and Karatza 2023;). Therefore, as studied by (Elfarra et al., 2015; Farhan et al. 2018; Sy, Abuan, and Danao 2020; Verma et al. 2021 and Zhang et al. 2023), the NREL Phase VI wind turbine blade tip modification and winglet incorporation under various operating conditions can be designed and predicted accurately using CFD. Many of these computational studies encountered numerous problems at higher wind speeds, where stall dominates the flow, with an escalating tendency for divergence from the experimental data and disparities, Figure 7. In our view and from the literature, the less successful attempts can be primarily due to inadequate (low-quality) meshing, poor model selection for turbulence, or other types of user errors. Some studies also experienced mesh density issues, which were likely brought on by a lack of sufficient computational capacity (Madsen et al. 2022; Popescu et al. 2022; Zhang et al. 2023).

Likewise, the comparisons of measured and simulated pressure coefficients for different wind speeds can be illustrated at the blade span-wise location of 30%, 47%, 63%, 80%, and 95%. Equation 1(1) $C_{p,i}={\displaystyle \frac{p-p_{\mathrm{ref}}}{0.5\rho (V^2+{(\omega r_i)}^2)}}$(1) was used to calculate the pressure coefficients at this span-wise location of the blade; a robust agreement exists between predicted and data from the experiment at all span-wise sections for the pressure coefficient distributions of 7 m/s speeds where the flow attachment dominate and 9 m/s wind speed as depicted in Figures 8 and 9. However, the variation between the experimental data and simulated pressure coefficient distributions is noticeable, particularly in the root region, for instance, the 30% inboard span of the blade. This discrepancy is attributed to a strong stalled, flow separation and the transition flow which occurred at this speed in this region. Literature surveys showed similar results obtained by several studies (Elfarra et al., 2015; Farhan et al. 2018; and Sy, Abuan, and Danao 2020).

Figure 8. Comparison of pressure coefficient measured experimentally and CFD simulation at 7 m/s which corresponds to a 5.41 tip speed ratio.

Figure 9. Comparison of pressure coefficient measured experimentally and CFD simulation at 9 m/s which corresponds to a 4.21 tip speed ratio.

3.2. Winglet simulation results and analysis

To provide clarity on how each of the winglet shape characteristics contributes to the performance of the blade, initially, each winglet shape is examined separately. On this track, two scenarios were considered.

3.2.1. Scenario 1: design of winglet with sharp and smooth bent configurations

3.2.1.1. Effect of cant angle

This scenario focuses on how the winglet’s cant angle and length affect its performance. Under this scenario, the impact of the cant angle on the performance of the winglet is considered and illustrated for the tip region only. Consequently, the effect of cant angle on the winglet performance was examined at fixed winglet lengths 0.09 m with different cant angles 15°, 30°, 45°, 60°, 75°, and 90°. Figure 10 shows the computed torque for fixed winglet lengths of 0.09 m at 9 m/s wind speed and various cant angles. Thus, as seen in Figure 10, both torque and axial thrust force rise as the cant angle increases until it reaches its maximum value at 45°, after which it decreases until it reaches its minimum value at 90° for sharp bent configuration. Similarly, both torque and axial thrust force also rise as the cant angle increases until it reaches its maximum value at 30°, after which it decreases until it reaches its minimum value at 90° for a smooth bent configuration, however, the smooth bent configuration shows a comparable torque and axial thrust force increments at 15° and 75°. Briefly, the cant has a significant impact on the winglets’ performance. Compared to the baseline without winglet, the torque shows an increment from 24.96% to 36.07% for smooth bent winglet configurations while an increment of 19.35% to 35.87% for sharp bent configurations is observed. However, the thrust force shows an equivalent increment for both cases. The results indicate that the performance improvement is greater in the case of smooth bent than in the sharp bent case. Accordingly, the winglet with the smooth bent configuration shows better performance than the winglet with the sharp bent configuration at the same wind speed.

Figure 10. The influence of cant angle on the winglet performance at 9 m/s, the torque and the axial thrust force simulation, for the tip region only.

3.2.1.2. Effect of winglet length

Similarly, two winglet configurations with varying winglet lengths from 0.05 m to 0.15 m and fixed cant angles of 45° and 90° were examined at 9 m/s wind speed for sharp bent case only. Consequently, the torque increases with the increase in winglet length at fixed cant angles 45° and 90° as illustrated in Figure 11. The axial thrust force also shows a similar trend. However, as winglet length increases, both torque and axial thrust force increase until they reach their maximum values. Depending on the cant angle, these peak values occur at different winglet lengths, as shown in Figure 11. Following that, the thrust force and torque both decrease. Notably, the performance of the winglets is greatly impacted by even little variations in winglet length. Compared to the baseline without winglet, the torque shows an increment from 24.86% to 44.27% at a fixed 45° cant angle and −1.71% to 21.19% at a fixed 90° cant angle. Likewise, the thrust force shows an increment from 21.12% to 40.54% at a fixed 45° cant angle and 11.74% to 25.58% at a fixed 90° cant angle. Briefly, the findings show that, compared to the 90° cant angle case, the performance improvement is larger when winglet length is varied at a fixed 45° cant angle.

Figure 11. The influence of winglet length on the winglet performance at 9 m/s, the torque and the axial thrust force simulation, for the tip region only.

3.2.2. Scenario 2: design of winglet with simple linear extension and sharp bent with arced winglet planform

3.2.2.1. Power and axial thrust force

The scenario involved the design of winglet configuration with respect to sharp bent with arced winglet planform thereby including the benchmark blade, the blade with simple linear extension, and the blade with sharp bent arced winglet were then combined to create a total of ten different blade variant, Table 2. Accordingly, the investigations compare a linearly 0.2 m blade extension without a winglet to a blade elongation with a winglet to determine the possible power boost as a function of varying winglet length and the benefit of tip extension with a winglet. The most effective winglet configuration has been obtained based on the simulation findings of the optimum winglet design parameter combination. The winglet shape is fixed to have a 0.2 m stretched blade’s tip portion linearly in the direction of the span blade and subject to varying winglet height subsequently winglet length and cant angle also vary. This allows the influence of winglet length and cant angle to be studied.

The extension with winglet examination: Figures 12 and 13 show the cumulative effect of winglet length and cant angle on the blade’s extension with winglet, so the plots depict resultant power and thrust force for wind speeds of 5, 7, 9, 11, and 13 m/s. The findings indicate that using a tip extension with a winglet as opposed to a tip extension alone can result in a larger increase in power. The increase in thrust force also demonstrates similar trends. Consequently, there is essentially a considerable difference in the winglets’ performance as the winglet’s height increases between 0.050 and 0.11 m, however, winglets with a height above 0.11 m show a significant reduction both in power and thrust force. Consequently, blade with winglet W2 results shows a power boost of up to 18.37%, 16.05%, and 7.07% which was observed at 5, 7, and 9 m/s wind speeds, respectively while the effect is minimal at higher wind speeds 11 and 13 m/s. In contrast, winglet W6 performs marginally better than W2 at a wind speed of 9 m/s. On the other hand, the performance of the winglets at higher wind speeds 11 and 13 m/s resulted in a significant decrease in power.

Figure 12. Comparison of the blade with W2, W4, W6, and W8, and extension alone (baseline) blade in terms of power generated at 5–13 m/s for the whole blade.

Figure 13. Cumulative axial thrust force coefficient, at 9 m/s.

In general, the analysis and comparison of Figure 12 indicates that the power produced by the extended blade without a winglet versus the power produced by the extended blade with a winglet exhibit, for lower wind speeds, the W2 winglet shows an increase in power of up to 18.37%; however, at higher wind speeds, the effect is minimal. Correspondingly winglet W2 exhibits lower axial thrust force, which is an advantage of having a short winglet length and demonstrates that axial thrust force decreases with decreasing winglet length as illustrated in Figure 13. Furthermore, Figures 12 and 13 also reveal a greater fall in power and thrust force as a function of winglet length when the performance of the winglets assessed above 0.11 m in length. Additionally, Figure 13 demonstrates the impact of adding a winglet on span-wise axial thrust force distribution, particularly in the area near the tip where most axial thrust force is generated. It is worth noting that the winglet has not been optimised for a wide range of operating conditions, and that further performance improvements can be made if it is optimised for a wide range of operating conditions.

3.2.2.2. Tip flow and vorticity

It is essential to look into the flow around the tip and vorticity distribution close to the blade winglets in order to understand the power boost mechanism of winglets in various configurations. Thus, on an operational wind turbine blade, the subsequent pressure differential results in flow that is inward along the suction side of the blade and outward along the pressure side towards the tip, so induced drag is created at the trailing edge where vorticity is created. A winglet is mostly used to lower the span-wise flow, diffuse and relocate the tip vortex away from the blade surface, which lessens the induced drag on the blade. Furthermore, vortices that come from the blade tip have a tendency to enhance induced drag, which consequently results in less lift production. Utilising a well-designed and optimised winglet is a practical solution to reduce this impact. The boost in power generation realised is not predominantly attributed to the winglet. The winglet’s primary function is to dilute and disperse the vortices of the tip, thereby lowering the induced drag.

Typically, a plane behind the trailing edge is used to examine the vortices created around the tip region of the blade. In order to analyze the vortices formed around the tip region of the blade, a ZX-plane that cuts the blade right at the tip of the trailing edge was used, as shown in Figure 14. This allows the vortices to be observed using local pressure distribution, velocity magnitude (vortex-induced), vorticity, and axial velocity line vector representation, as illustrated in Figures 14–18 for 9 m/s wind speeds.

Figure 14. ZX-plane that cuts the blade right at the tip of the trailing edge and the sliced portion an – enlarged view.

Figure 15. Velocity magnitude (vortex-induced), at 9 m/s around the tip region: blade with simple linear extension and blade with winglets.

Figure 16. The distribution of axial velocities obtained from the line passes through r/R = 0.99 and core centre for the baseline and winglet cases, at 9 m/s.

Figure 17. The vorticity distributions obtained from the line passes through r/R = 0.99 and core centre for the baseline and winglet cases, at 9 m/s.

Figure 18. Static surface pressure contour, at 9 m/s: blade with simple linear extension and blade with winglets.

The results are essentially different for the four blades when the vortices, axial velocities, and velocity magnitude of the baseline blade and blade with the winglet are compared. Additional information regarding the vortex centre and rolling direction is depicted in Figures 15 and 18. Although the air is still swirling in all settings, the vortex core distance from the blade surface is quite different so that the blade with simple linear extension’s vortex core is the closest while the blades with the winglets have both wide and relatively far vortex cores, as shown in Figures 15 and 18. Furthermore, Figure 15 compares the extension only (baseline) and winglet instances for the magnitude of the vortex-induced swirl velocity. The vortex-induced swirl velocity magnitude has a symmetric distribution in relation to the vortex core centre for both baseline and winglet situations. When the winglets are used, the vortex core diameter is substantially greater. Also, the increased core diameter causes the vorticity levels to be dispersed over a broader area around the core.

Figure 16 displays the extracted distributions of axial velocity along the line at r/R = 99% of the span and perpendicular to the plane of rotation to carry out more quantitative comparisons. The distributions demonstrate the considerable impact of adding a winglet, particularly in the area surrounding the core near the tip, as illustrated in Figure 16. The axial velocity reaches the value of the free-stream velocity when it is far from the tip vortex areas. Additionally, Figures 16 and 17 show that the axial velocity distribution resembles a bell shape and the vorticity is greatest at the centre of the vortex and approaches zero outside of it. The findings also show that the vortex centre (shown in dark blue in Figure 18) nearly matched with the place of largest velocity drop (core axial velocity).

The tip vortex’s compact, dense core is where the majority of the vorticity turns out to be concentrated. Inevitably, the vortex rotates from the blade’s pressure side to its suction side, producing a significant amount of negative vorticity. The vorticity distributions shown in Figure 17 demonstrate that, in the winglet situation, the levels of vorticity within the vortex cores are considerably decreased, signifying the declining strength of the tip vortices. The variations in the geometrical differences in winglet geometries are most likely responsible for the variations found in the vortex centre positions and vortex structure from one winglet to the others, as illustrated in Figures 15–18.

3.2.2.3. Pressure and flow field

The flow characteristics responsible for the increased power and axial thrust force shown in the preceding section are also characterised by visualising the pressure and flow field. Accordingly, the contours of surface pressure on the blades show that the local pressure distributions across the blade’s surface essentially show a considerable variation in magnitude and intensity around particular locations so these noticeable differences are observed at the blade tip region, Figure 18.

The result of the winglet’s addition so clearly demonstrated that it considerably reduces the size of the low-pressure region at the blade tip region that was created as a result of blade tip vortices. Additionally, the induced drag is decreased by this dispersion of the blade tip vortices. Figure 18 shows the location of the vortex core at the blade tip by pressure contour and highlights it with a dark blue colour. This makes it easy to see the tip vortex. As a result, the blades with winglets effectively lower the tip vortex effect by displacing the vortex’s centre farther from the blade’s tip, while the blade with a simple linear extension does not displace effectively the core away from the tip.

To realise the effect of the winglet configurations, the pressure coefficients close to the blade tip, based on the linearly extended blade (baseline) at section r = 5.3234 m which is r/R = 98% of baseline blade, the baseline, W2, W4, W6, and W8 winglets were examined, as shown in Figure 19, inspecting the results show that substantial pressure difference on the suction side but a slight variance in pressure on the pressure side except for all winglets at the trailing edges where the pressure difference is significantly small. These variations are predominantly attributed to the effect of the vortices at the blade tip as illustrated in Figures 15–18. The pressure coefficient plots in Figure 19 at a wind speed of 5, 7, and 9 m/s show that the blade with winglet W2, W4, W6, and W8 demonstrates the pressure coefficient distribution enhancement from small increment to more increment on the span-wise suction side, respectively. The primary goal of adding a winglet to a wind turbine rotor is to reduce the overall drag generated by the blades and consequently raise the turbine’s aerodynamic efficiency. However, the whole blade performance of the blade with winglet W2 outperforms others; this is due to the increased drag from the winglet being smaller than the decrease of the induced drag on the rest of the blade thereby overall drag is reduced. Therefore, the improvement in pressure coefficients shows that the use of winglets causes the blade to extract greater energy from the fluid flow.

Figure 19. Pressure coefficients at section 98% of the span (r = 5.3234 m) of the baseline and blade with winglets at a) 5 m/s, b) 7 m/s, and c) 9 m/s comparison.

Conclusions

The possibility for improving the aerodynamic performance of an NREL Phase VI blade through blade tip extension with winglet, simple extension alone, and winglet parameters influence scenarios are examined in this work. Further investigations are also conducted on the effects of winglets on the flow field near the blade tip, on the tip vortex characteristics such as vortex rolling direction, vortex core size, and core displacement, as well as the induced drag on the blade which is caused by these effects.

The first scenario involves the investigation of winglet design with sharp and smooth bent configurations and examines how the winglet’s cant angle and length affect its performance. With fixed cant angles of 45° and 90°, the winglet length is adjusted between 0.05 and 0.15 m. Following that at a fixed winglet length of 0.09 m the cant angles are adjusted from 15° to 90°. The wind speed used for the simulation was 9 m/s. The findings show that the inclusion of winglets causes observable increases in torque and thrust force. Accordingly, the winglet with the smooth bent configuration shows better performance than the winglet with the sharp bent configuration at the same wind speed. Therefore, when the winglet cant angle is 45° at a fixed winglet length of 0.09 m, the performance is improved the most for sharp bent while for the smooth bent configuration, the greatest performance occurs when the winglet cant angle is 30°. Likewise, both the torque and axial thrust force rise as the winglet length increases until it reaches its maximum value, after which it decreases. Notably, the performance of the winglets is greatly impacted by even little variations in winglet length. The length at which the maximum value occurs depends on the cant angle. Under the second scenario, a blade tip elongation with a winglet is proven to be capable of boosting power production significantly. Accordingly, the blade with winglet W2 results shows a power boost of up to 18.37%, 16.05%, and 7.07% at 5, 7, and 9 m/s wind speeds, respectively compared to a design with a simple linear tip extension. In contrast, the results also demonstrate a 3.21% to 44.36% increase in axial thrust force, generated by a blade with a tip elongation with a winglet when compared to a plain extension alone. In conclusion, this suggests that a winglet is advantageous for the refurbishment of existing blades for a power boost and for improving a wind turbine blade’s effectiveness when the thrust force meets the required constraints. It is revealed that the winglet length with a curve that bends toward the suction side is as predicted beneficial with the effective design parameters and space relating to the winglet shape. Therefore, even though the designs did not considerably reduce the axial thrust, increasing the winglet length up to 0.09 m shows a beneficial effect. In general, the findings indicate that, as opposed to a straight extension of the tip along the span-wise location, it is certainly advantageous to curve the tip to form a winglet shape of different variants.

Moreover, these winglets are essential to our further research involving winglets with various airfoil profiles, which broadens the scope of the findings, future efforts will focus on adding additional parameters and a variety of winglet configurations, as the current paper only covers a limited number of winglet parameters and configurations.

Acknowledgement

Partial support from Jimma Institute of Technology Center of Excellence is gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).