Aerodynamic characteristics of Straight-bladed Vertical Axis Wind Turbine with a curved-outline wind gathering device

Aerodynamic characteristics of Straight-bladed Vertical Axis Wind Turbine with a curved-outline wind gathering device

Energy Conversion and Management xxx (xxxx) xxxx Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.el...

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Energy Conversion and Management xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Aerodynamic characteristics of Straight-bladed Vertical Axis Wind Turbine with a curved-outline wind gathering device ⁎

Yan Lia, , Shouyang Zhaoa, Chunming Qua, Guoqiang Tonga, Fang Fengb, Bin Zhaoc, Tagawa Kotarod a

College of Engineering, Northeast Agricultural University, Harbin 150030, China College of Science, Northeast Agricultural University, Harbin 150030, China c Tibet Autonomous Region Energy Research Demonstration Center, Lhasa 850001, China d Faculty of Agriculture, Tottori University, Tottori 6808551, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Vertical axis wind turbine (VAWT) Wind gathering device Spline curve Aerodynamic characteristics Numerical simulation Wind tunnel test

In order to improve the aerodynamic performance of the Straight-bladed Vertical Axis Wind Turbine (SB-VAWT), a Wind Gathering Device (WGD) with curved-outline installed at the up and down of the rotor was proposed to obtain more wind energy. The B-spline curve construction was applied to the study of suitable curved outline of the WGD. The studied structure parameters of the WGD included the radius ratio of the upper and lower plan, inlet angle and the outlet angle. Firstly, based on the method of quadratic rotary orthogonal combination design, the static performance effects of these parameters of WGD on SB-VAWT were researched by three-dimensional numerical simulation. A combination of optimum curve parameters was obtained. The influence of the WGD with different structure parameters on the starting characteristics of SB-VAWTs was also calculated to investigate the fluid field effects of WGD on rotor. Furthermore, the wind tunnel tests were carried out on the rotor with and without WGD included the rotational speed performance and output power characteristics. The results showed that both the static and dynamic performance of SB-VAWT was increased in some degree by adding the WGD with curved-outline. The maximum starting moment of SB-VAWT could be increased by 14.8%. The improvement is more obvious under the condition of low wind speed. This research can be as a useful reference for the performance improving of SB-VAWT.

1. Introduction With the development of the distributed generation and off-grid generation, the research and application of small-scale wind turbine has been the study focus, in which the vertical axis wind turbine (VAWT) especially the Straight-bladed Vertical Axis Wind Turbine (SB-VAWT) has got a great development in recent years [1]. However, the poor starting performance of SB-VAWT is one of the most important reasons that restricted the application [2]. Researchers tried to improve the starting performance of SB-VAWT in different perspectives [3]. For example, Seki [4] developed the special airfoil of SB-VAWT blade TWT series airfoil which had been applied on the actual wind turbine in Japan. Besides, some researchers changed the airfoil shape which made the blade have certain resistance profile, and the starting performance would be improved under low wind speed [5]. Changing the wingspan shape of SB-VAWT blade has been the research focus besides changing the blade airfoil, such as wavy leading edge [6,7] wave-like trailing



edge [8] and flexible blade [9–11]. However, due to the difficulties in blade processing, these methods are still being explored and need to be further studied. Another way is to change the traditional structure of the SB-VAWT, increase the performance of the blade at high speed and reduce the dynamic stall, so as to make it play a greater aerodynamic performance. For example, Gebreel Abdalrahman et al. [12] studied the variation of blade pitch angle under different tip speed ratios. By analyzing the forces acting on the blade under different pitch angles under the same tip speed ratio in the process of rotation, the variation law of blade pitch angle corresponding to each blade in a rotating cycle was obtained. Wang ZY et al. [13] carried out a series of three-dimensional numerical simulations of helical blades and compared them with traditional SB-VAWT. Taguchi method was used to optimize the main structural factors of helical blades. It was found that by adopting helical design, the fluctuation of moment was significantly reduced, which was conducive to improving the lift of turbines. However, the working conditions of SB-VAWT blades are complex during operation. How to

Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Li).

https://doi.org/10.1016/j.enconman.2019.112249 Received 24 September 2019; Received in revised form 29 October 2019; Accepted 1 November 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Yan Li, et al., Energy Conversion and Management, https://doi.org/10.1016/j.enconman.2019.112249

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λ N D R H h c θ R1 R2 ε β1 β2

Nomenclature Acronyms VAWT Vertical axis wind turbine SB-VAWT Straight-bladed vertical axis wind turbine WGD Wind gathering device Symbols Cp Cm Mas Ms U

Output power coefficient Moment coefficient Average starting moment Static moment Wind velocity

Tip speed ratio Number of blade Rotor diameter Rotor Radius Height of blade Height of wind gathering device Blade chord Azimuth angle of rotor against wind Inner cone plane radius of WGD Outer cone plane radius of WGD Ratio of R2 to R1 Inlet angle Outlet angle

tunnel tests were carried out on aerodynamics characteristic of SBVAWT with and without WGD to compare the optimum effect of WGD and the better curve structural parameters were obtained. This research can provide a reasonable mathematical model for the processing and manufacturing of WGD in the future.

achieve accurate control still needs constant exploration. Some other new methods were also proposed [14–17], which were good investigation for improve the aerodynamic performance of SB-VAWT. Installing Wind Gathering Device (WGD) on the rotor has been the most popular research in recent years, which was mainly to improve the self-starting performance of rotor. Ji Junfeng et, al [18] proposed a kind of VAWT with wind shield-growth patterns installed around the SBVAWT along the incoming flow direction, which could guide the incoming flow and increase the wind speed. The results indicated that the wind shield-growth patterns improved the starting and power performance of wind turbine. W. T. Chong [19] proposed the VAWT with the omni-direction-guide-vane, which had improvement of utilization ratio of wind energy. Zhao Zhenzhou [20] also proposed a kind of SB-VAWT with guide vanes, the guide vanes were installed around the rotor along the incoming flow direction, and the study results showed that the guide vanes had improved the starting performance of VAWT. Similar to this, Xiaohang Wang et, al [21] presented a patented V-shape roof guide vane (VRGV) with a solar and wind power generation system mounted on an eco-roof system, from the comparative experiments, the self-starting performance and rotational speed of the VAWT mounted above a double slope roof with the VRGV have been significantly improved compared to the VAWT without the VRGV. There was a common ground that the guide devices were installed around the wind turbine along the incoming flow direction, which meant the wind guide and collection effect of XY plane coordinate system were studied, without Z coordinate direction. Kok Hoe Wong et, al [22,23] investigated the aerodynamic effects and the flow field around a flat plate deflector as a power augmentation device which was placed at the lower upstream of a micro H-rotor VAWT by lab tests and simulations, from the study, the deflector was able to induce a high velocity wind at the near-wake region, and the deflected wind flows improved the performance significantly as well as reduce the self-start velocity of the turbine. Li Yan et, al [24] proposed an innovative truncated-cone-shaped wind gathering device (WGD) which could be installed up and down of the rotor to collect more incoming flow and increase wind speed, and the main structural parameters of WGD was studied and optimized. Fig. 1 shows the concept of the rotor with WGD. Based on this research, further investigations were carried out in this study. The truncated-cone-shape was changed to curved-outline to deduce the size of WGD. The parametric construction of the WGD outline was carried out by using a technology of changing the points to control the generation of curve lines. The curved-outline of WGD was constructed by cubic fourth order B-spline curve, and the main structural parameters of the WGD were corresponded to the control points of B-spline curve. By means of quadratic orthogonal rotational combination design method, the B-spline curves with different control points at different positions were obtained. Numerical simulations and wind

2. Basic theory of wind gathering device 2.1. Basic wind gathering theory of WGD The most basic shape and structure of WGD is truncated-cone shape shown in Fig. 2 which can explain the basic wind gathering theory. To simplify the explanation the rotor is not considered. According to the flow continuity equation in the tube in-compressible fluid [24], it can improve the velocity of flow at the entrance of the rotor and collect more wind flow into the rotor by using the change of the tube area. When the rotor is considered, the wind speed increasing should be affected in some degree for the blockage effect of rotor in some degree. Because of the moment and power characteristics of the wind turbine are proportional to the square and cube of wind speed, respectively, the aerodynamic performance of SB-VAWT will be increased. 2.2. Basic theory of B-spline curve B-spline curve is derived from polynomial mixing function. Normally n + 1 vertex defines an n ordered polynomial. Its mathematical expression is: let the space have m + n + 1 vertex namely P0,P1, P2,…, P k+n(where n is the order and m is the maximum segment

Fig. 1. Concept of WGD set up and down the rotor [24]. 2

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3. Model design 3.1. Main rotor of SB-VAWT Considering the test section size (1 m × 1 m) of wind tunnel in the laboratory, a small-scale rotor model of SB-VAWT was selected and designed shown in Fig. 4. The structural parameters of the rotor is shown in Table 1. 3.2. WGD with curved-outline The installation method of the WGD is shown in Fig. 5. Based on the past researches [24], The height of the WGD was set as 150 mm. The distance between the inner diameter plane of the WGD and the tip of the blade was set as 36 mm by using the position parameters of the WGD described in reference. The curvature distribution of the WGD was defined by the cubic Bspline curve. Four control points were defined on the windward contour curve of the WGD, the schematic diagram of the points position is shown in Fig. 5. The relationship between the parameters of the curve and the position of the control point were correlated. Set the projection point of outer diameter as the initial point b1, the projection point of inner diameter as the end point b4. The inner diameter of the WGD is the same as the rotating diameter of the rotor so that the point b4 could be fixed to determine the position parameters of the WGD. The point b1 was determined by the radius ratio of the upper and lower surface of the WGD which was defined as ε, the definition is shown as follows:

Fig. 2. Schematic diagram of WGD.

number), the parametric curve determined by the following functions is the n-ordered B-spline curve. The formula is shown as follows: n

∑ Bi +k Fk,n (t )(0 ⩽ t ⩽ 1, i = 0, 1, ...,m)

Pi, n (t ) =

k=0

(1)

ε= Fk,n (t) is the basis functions of n-ordered B-spline curve defined by the Cox-De Boor relations, the formula is shown as follows:

Fk, n (t ) =

1 n!

(−1) jCnj+ 1 (t + n − k − j )n (0 ⩽ t ⩽ 1, k = 0, 1, ...,n)

j=0

(2) When the order of B-spline curves is too high, the physical meaning will become more difficult to be understood, and the computational load will become larger. It may produce fluctuations, oscillations, or even distortions. However, a single low-order polynomial can hardly be used to describe complex curves. Three times is a good compromise based on the past applications. The schematic diagram of cubic B-spline curve is shown as Fig. 3. It not only guarantees the continuous change of slope, but also guarantees the continuous change of curvature. The cubic B-spline curve, which is widely used in engineering, is a polygon composed of four control vertices and has second-order continuity. Its expression is as follows.

4. Research method 4.1. Numerical simulation Numerical simulations were carried out to optimize the curved shape of the WGD. The software ICEM was used to create the threedimensional mesh of the computational model. Then the computational model was imported into software ANSYS FLUENT for iterative calculation, so that the moment coefficients were obtained, which provided a basis for the optimization of the structural parameters of the WGD.

3

P (t ) =

∑ Bk Fk,3 (t ) = B0 F0,3 (t ) + B1 F1,3 (t ) + B2 F2,3 (t ) + B3 F3,3 (t ) t k=0

∈ [0, 1]

(5)

The angles between straight lines b1b2 and b3b4 are β1 and β2, respectively. The size of β1 indicated the minimum deflection of the air flow when the wind impacted the WGD which was called the inlet angle. The size of β2 indicated the difference of the deflection between the curve diversion and the straight diversion, which was called the outlet angle. The angle between Ob1 and Ob2, Ob3 and Ob4 was dθ, which was determined by trisection of the wrapping angle θ of the curve. Keeping the curve control points unchanged along the trisector, only changed the radial coordinates of the control points along the center of the circle to the tangent point, that is, the size of β1 and β2, to achieve the change of the curve outline.

n−k



R2 R1

(3)

The Bk (k = 0,1,2,3) are the control points of B-Spline curve. The basic functions Fk,3 (k = 0,1,2,3) are shown as follows: 3

2

⎧ F0,3 = (−t + 3t − 3t + 1) 6 ⎪ (3t 3 − 6t 2 + 4) ⎪ ⎪ F1,3 = 6 ⎨ F = (−3t 3 + 3t 2 + 3t + 1) 6 ⎪ 2,3 ⎪ t3 F3,3 = 6 ⎪ ⎩

(4)

Fig. 3. Schematic diagram of cubic B-spline curve. 3

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Fig. 6. Computational domain.

μeff = μ + μt

Fig. 4. Schematic diagram of rotor.

μt = ρCμ

Table 1 Parameter selection of computational model for the rotor. Parameter

Value

Blade airfoil Blade number (N) Blade chord (c)[m] Radius (R)[m] Blade height (H) [m]

NACA0018 4 0.125 0.3 0.5

(9)

where, Gk is the generation of turbulence kinetic energy owing to the mean velocity gradients; Gb is the generation of turbulence kinetic energy owing to buoyancy; YM is the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate. The quantities αk and αε are the inverse effective Prandtl numbers for k and ε, respectively, C1ε = 1.42,C2ε = 1.68 and Cμ = 0.0845. 4.1.2. Computational domain Considering that the WGD is installed on both sides of the blade, it is necessary to establish a three-dimensional calculation model. The Computational domain for SB-VAWT with WGD is shown as Fig. 6. The center of the reference system is the center of the rotor, and the direction of inflow is X-axis. The center axis of wind turbine rotation is Zaxis. Taking the rotating diameter D as the standard, the length of the domain adopted 10D in the direction of inflow, and the upwind area adopted 5D*5D. The influence of rotating axis, beam and other structures were not considered in the calculation. The inlet was set as a velocity inlet with a constant wind velocity profile of 10 m/s, while outlet was set as a pressure outlet with atmospheric pressure value. The side, top and bottom surfaces were fallen under the standard no-slip wall condition. In order to refine the grid and facilitate the calculation of the dynamic flow field in the future, the computational domain was divided into the static domain and the rotating domain. The interface of the computational domain adopted the sliding grid method, which was

4.1.1. Turbulence model In this study, turbulent energy dissipation model RNG k-ε was used. This model could predict the flow field of large flow separation more accurately. The transport equation and turbulent viscosity equation of the turbulence model could be seen in Eq. (6)~(9). The pressure–velocity coupling adopted SIMPLE algorithm. Set second-order upwind mode for momentum, turbulence momentum and dissipation rate, and default values were selected for other operations.

∂ (ρkui ) ∂ (ρk ) ∂ ⎛ ∂k ⎞ = + ⎜αk μeff ⎟ + Gk + Gb − ρε − YM ∂x i ∂x j ⎝ ∂x j ⎠ ∂t

k2 ε

(8)

(6)

∂ (ρεui ) ∂ (ρε ) ∂ ⎛ ∂ε ⎞ ε ε2 = + (Gk + C3ε Gb) − G2ε ρ − R e ⎜α ε μeff ⎟ + C1ε ∂x i ∂x j ⎝ ∂x j ⎠ k k ∂t (7)

Fig. 5. Vertical axis wind turbine with curved-outline WGD set up and down of rotor. 4

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0.10

Static moment coefficient Cms

more applied to the rotation of the rotor without causing the deformation of the grid. Tetrahedral unstructured mesh was used for mesh generation as shown in Fig. 7. This type of mesh has strong adaptability and high accuracy for simulating of wind turbine.

0.09

4.1.3. Grid independence verification Because the mesh density/quality might have a substantial influence on the CFD results, a grid convergence study was performed to evaluate the influence of grid resolution on the calculated static torque to ensure grid independent results and avoid prohibitive computational cost. The static moment coefficients of SB-VAWT were carried out with five different grid growth and the number of meshes obtained were 1740593, 2185039, 3072115, 4075,270 and 5765267, respectively. The results were shown in Fig. 8. According to the figure, when the number of grids was less than 4 million, the results increased with the increase of the number of grids, which indicated that when the number of grids was small, the calculation results of different grids were quite different. When the number of grids exceeded 4.07 million, the results did not change significantly with the change of the number of grids. Considering the influence of calculation time and period, 4 million were selected as the number of grids in this study, and a small adjustment was made according to the shape change of the WGD.

0.08

0.07

0.06 150

200

250

300

350

400

Number of grids

450

*10

500

550

600

4

Fig. 8. Static moment coefficients in different grid numbers (φ = 45 degrees).

by the incoming flow was called the moment coefficient, which was the evaluation parameter to measure the starting performance of SB-VAWT. For the dynamic torque measurements, the tip speed ratio of the SBVAWT defined as Eq.11 was controlled by induction motor, the tip speed ratio can be expressed as the ratio of the velocity of the blade tip to the incoming wind speed. The dynamic moment of the SB-VAWT during a period of time under the speed was measured at a certain wind speed. The power coefficient (Cp) is evaluation parameter to measure the output performance of SB-VAWT. The formula of the relationship between moment coefficient CM and power coefficient Cp was shown as Eq. (12).

4.2. Wind tunnel test 4.2.1. Wind tunnel The wind tunnel tests were conducted by using a low speed open type one with test section of 1 m × 1 m. The range of wind speed supplied by wind tunnel was from 1 m/s to 20 m/s. The averaged wind speed precision at the outlet section of the wind tunnel is ± 3%. The test rotor was placed at the same height as the center of the wind tunnel outlet and 1.5 m downstream from the outlet. 4.2.2. Test equipment and measurement method In this study, a wind tunnel test system for measuring the aerodynamic characteristics of SB-VAWT was established shown in Fig. 9. The wind speed was measured by an ultrasonic wind sensor with the precision of ± 0.1%. The rotor torque was measured by a digital torque detector (ONO SOKKI, Japan), which was located between the rotor and an induction motor equipped with a brake (MITSUBISHI ELECTRIC, Japan). The measuring range of the torque detector was 5 N•m and the accuracy was ± 0.2% with 1 ms of the sampling time. The revolution of the induction motor was controlled by a variable-frequency drive, which could be measured by an intelligent digital tester. In this study, the number of the rotor blade was 4, and the rotation period of the SB-VAWT with 4 blades was 90 degrees. Therefore, the range of rotation angle in the test process was 0–90 degrees. For the static torque measurements, the static moment M of wind turbine was measured at fixed angles every other time in a rotating cycle. The test time at each rotation angle was 30 s, and the average value of the static moment measured within 30 s was obtained. The moment coefficient (CM) defined as Eq.10 was the ratio of the M to the total moment caused

CM =

λ=

M 1 ρAU 2R 2

(10)

V ωR = U U

(11)

Cp = λ × CM

(12)

where M is moment of the rotor, N•m; ρ is density, kg/m ; U is income velocity, m/s; R is radius of the rotor, m; ω(rad/s) is angular velocity of the rotor. For the rotational speed characteristics measurements, the wind turbine was in no-load state before the test, and the coupling connecting the axis and the induction motor were disconnected. The speed sensor was used to measure the change of the speed of the rotor during the period from static to uniform rotation during the experiment, so as to judge the influence of the WGD on the speed characteristics of the SB-VAWT. 3

4.2.3. Uncertainty analysis For the experiments studied in this paper, three parameters

Fig. 7. Local grid. 5

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Fig. 9. Test system.

5.1. Level selection of factors

including wind speed, rotational speed and torque have been measured respectively. So the error transfer formula of Cp and Ct could be obtained as follows:

ΔCp Cp

=

1 1 3 ΔT + Δω + ΔU T ω U

ΔCt 1 2 = ΔT + ΔU Ct T U

In this study, three factors and five levels coding table were selected to code the factors, taking the ratio of upper and lower radius ε, the inlet angle β1 and the outlet angle β2 as the calculation factors, which are shown as Table 3. The selected range of diameter ratio ε was 1.5–2.0, the β1 and β2 were 0–20 degrees.

(13)

(14)

5.2. Results of numerical simulation

where ΔT, Δω and ΔU represent experimental errors of torque, rotational speed and wind speed, respectively. According to the error transfer formula, the uncertainty values of parameters are shown in Table 2.

Table 4 shows the simulation results of the average starting moment (Mas) for all the 23 teams of the rotor model with different structural parameters of WGD designed in this study. It could be seen from the table that the starting characteristics performance of the SB-VAWTs were different when the parameters of WGD were different. The fourth group model was the largest, the value was 0.449; the ninth group model was the smallest, the value was 0.403. Considering the influence of a single factor on the calculation results, Fig. 10 shows the variation trend of the average starting moment of the SB-VAWTs with WGDs based on ε, β1 and β2 respectively. It can be seen from the figure that when the ε was taken as the benchmark, the average starting moment of the SB-VAWT showed a upward trend with the increase of the radius ratio, which showed that the increase of the radius ratio factor has a promotion effect on the optimization effect of the WGD. The average starting moment reached the largest value when the radius ratio was 1.9. As for other two parameters, the average starting moment of SB-VAWT did not show a significant trend with the change of factor level, which indicated that these two factors interacted with other factors, and together affected the starting performance of SB-VAWT. Considering the interaction among factors, items A, B, C, AC and BC were significant items in the design of this experiment. The regression equation based on the design

5. Results and analysis The Quadratic Orthogonal Rotation Combination Design (QORCD) method was adopted in this study. 23 teams of structural parameter combination were obtained by using the design software Design-Expert 8.0. Numerical simulations were carried out on starting characteristics of SB-VAWT with different structural parameters of WGD. Starting moments of each calculation model was obtained at 15 degrees azimuth intervals in a rotating cycle, and the final results were averaged. The average starting moments were obtained (Mas) and the definition is shown in Eq.15. Through variance analysis of the calculated results, the regression equation with the average starting moment as the objective function, three structural parameters as well as the interaction of them as independent variables was derived. By using the regression equation, the optimal combination of structural parameters of the WGD was obtained. Finally, the starting moment coefficients (Cams) of SB-VAWT with an without WGD were calculated every 5 degrees of azimuth angle, and the optimization mechanism of gathering wind turbines was analyzed from the change of transverse and longitudinal flow fields of SB-VAWT. The definition of Cams in this study is shown in Eq.16.

Table 2 Certainty quantification of parameters.

i=N

Mas =

Cms =

∑i = 0 Mis N

(15)

Ms 1 2 ρARU 2

(16) 6

Parameter

Precision

Ultrasonic wind Sensor (U) Revolution Sensor (ω) Torque Sensor (T) Power Coefficient (Cp) Torque Coefficient (Ct)

± 0.1% ± 2% ± 0.2% ± 2.5% ± 0.4%

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Table 3 Table of factor level coding. Code

Radius ratio ε x1

Inlet angle β1 (°) x2

Outlet angle β2 (°) x3

1.682 1 0 −1 −1.682

2.0 1.90 1.75 1.60 1.50

20 15.95(16) 10 4.05(4) 0

20 15.95(16) 10 4.05(4) 0

Fig. 11. . Optimized structural parameters of WGD. Table 4 Result of experimental design. No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Factor

software of Mas depending on the three structural parameters was obtained as shown in Eq.17.

Results

x1 ε

x2 β1 (°)

x3 β2 (°)

y Mas/(N•m)

1.6 1.9 1.6 1.9 1.6 1.6 1.6 1.9 1.5 2 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75

4 4 16 16 4 4 16 16 10 10 0 20 10 10 10 10 10 10 10 10 10 10 10

4 4 4 4 16 16 16 16 10 10 10 10 0 20 10 10 10 10 10 10 10 10 10

0.411 0.426 0.429 0.449 0.407 0.433 0.411 0.440 0.403 0.438 0.412 0.429 0.428 0.422 0.423 0.423 0.422 0.422 0.423 0.422 0.423 0.424 0.423

yt = 0.42 + 0.011x1 + 6.225e−3x2 − 2.524e−3x3 + 2.301e−3x1 x3 − 3.922e−3x2 x3

(17)

where, x1is the radius ratio ε; x2 is the inlet angle β1 and x3 is the outlet angle β2. According to regression equation, the best combination of structural parameters of WGD was obtained, namely ε = 1.9, β1 = 16 degrees and β2 = 4 degrees as shown in Fig. 11. 5.3. Interactive terms analysis Fig. 12 shows the response surface of the influence of ε and outlet angle β2 on the average starting moment when the inlet angle was at zero level. It could be seen from the figure that when the ε was small, the increase of the β2 caused the average starting moment to decrease rapidly at first and then to stabilize. With the increase of the ε, the average moment had a significant increase, and the effect of the factor β2 on the test target was gradually weakened. This was because that the change of the direction of the flow wind was very obvious when the ε was small. The impact of the incoming flow on the WGD would cause a lot of energy loss. However, increasing the β2 aggravated this adverse effect and reduced the radial component of the incoming flow acting on the blade. When the ε increased, the diversion effect of the WGD on the incoming flow as more reflected in the increase of the volume of the wind and less in the change of the direction of the incoming wind. At this time, the effect of changing the β2 on the starting moment was weakened. Fig. 13 shows the response surface of the influence of β1 and outlet angle β2 on the average starting moment when the ε was at zero level. It

Fig. 10. Simulation results of average starting moment. 7

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Fig. 12. Response surfaces of ε and β2.

could be seen from the figure that when the β1 was small, the average starting moment of the SB-VAWT was not obviously enhanced by the increase of β2. With the increase of the β1, the influence of β2 on the test target was enhanced, which showed that the average starting moment of the SB-VAWT decreased with the increase of the β2. When the β2 was small, the increase of β1 had a significant effect on the increase of the average starting moment. With the increase of β2, the effect of the β1 on the test target was gradually weakened. It could be seen that when the size of the gathering device is fixed, the change of β1 and β2 had an interaction effect. This was because that when β1 was small, the change of direction after the incoming wind impacting the WGD was small, and the increase of the β1 can reduce the energy loss, which made the wind more effective on the wind turbine. When the β2 was too large, the increase of β1 can smooth the change of wind direction, but its effect on the radial direction of the SB-VAWT reduced, which weakened the effect on the average starting moment of the SB-VAWT.

5.4. Comparison of starting performance 5.4.1. Starting moment coefficient Fig. 14 shows the starting moment coefficient curves of SB-VAWT with and without wind gathering device at different azimuth angles. According to the figure, the fluctuation trend of the curves for two SBVAWTs is basically the same with the change of azimuth angle. The starting performance of SB-VAWT with WGD was better than that of original, which showed that WGD can improve the starting characteristics of SB-VAWT. The curves of two kinds of SB-VAWT showed two peaks and two valleys in one cycle: the original one had the maximum starting moment coefficients at 25 degrees and 70 degrees, respectively, and minimum values at 0 degree and 50 degrees. However, the position of the maximum starting moment coefficient of SB-VAWT with WGD was shifted, showing a high starting performance between 25 degrees and 35 degrees. At 30 azimuth, the static moment coefficient of the SBVAWT with WGD reached a maximum of 0.124, while that of original was only 0.108, which increased by 14.8%. At 65 degrees, the starting

Fig. 13. Response surfaces of β1 and β2. 8

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and more energy acted on the downstream blade. For the flow behind the WGD, the obvious velocity accumulation area appeared at the surface of the WGD due to the diffusing effect. The vortex behind the surface of WGD was also found and move to the far wake of rotor. This could reduce the energy loss along blade in the wake of main flow. This is one of the reasons of starting performance improvement by the WGD.

5.4.3. Characteristics of pressure distribution The pressure and velocity distributions of SB-VAWT at different heights were analyzed respectively. Fig. 18 shows the schematic diagram of the calculated position. Two special sections were selected for analysis. The z1 plane is the center line position of the blade, z = 0 m; the z2 plane is the tip height position of the blade, z = 0.25 m. Fig. 19 shows the pressure and streamlines distribution nephograms around the blades of SB-VAWT with and without WGD at azimuth of 30 degrees on the z1 and z2 planes, respectively. According to the figure, under static conditions, the flow field of original SB-VAWT had the pressure changes on different blade element sections, the maximum pressure changes were shown at the center position of the blade. The pressure difference between the back of the blade b and the blade d was more obvious, which showed that the blade was subjected to greater moments at this position. After installing the WGD, the flow fields around blades of SB-VAWT had changed in varying degrees. At z1 plane, the positive pressure area was not changed obviously by adding the WGD, however, the negative pressure area on ventral side of the blade was increased obviously. So that the pressure difference between the ventral and back sides of the four blades became more obvious and increased the moment of the SB-VAWT. As the pressure gradient increase, the vortices in the negative pressure zone increased significantly, which could cause some energy loss. At z2 plane, the positive and negative pressure areas on both sides of the four blades were enlarged, and the pressure difference was increased significantly, which made the pressure changed in the two planes more consistent. The blade b had the most influence, which could provide effective moment for the forward rotation of the SB-VAWT at the azimuth. In summary, at 30 degrees, the pressure range changed greatly at the blade tip, and the negative pressure area at the blade center section and the blade tip plane strengthens obviously. Both of them increased the force moment of the blade, promoted the rotation and enhanced the moment coefficient, which had obvious improvement effect on the starting performance of the SB-VAWT. Fig. 20 shows the pressure and streamlines distribution nephograms around the blades of SB-VAWT with and without WGD at azimuth of 65 degrees on the z1 and z2 planes, respectively. It can be seen from the graph that the pressure change at the center of the blade was greater than that at the tip plane and the pressure difference on ventral and back of blade a and c was the greatest. However, the pressure around blade b and d in the blade tip plane was almost the same. After installing the WGD, the pressure and streamline around the blades at the two cross sections has changed. At z1 plane, the positive pressure area

Fig. 14. Starting moment coefficients of SB-VAWTs with and without WGD.

performance optimization of the WGD achieved the best effect, which 33.3% higher than that of original. It was shown that the starting performance of SB-VAWT could be improved by installing curved wind gathering device. The main performance of the WGD was to expand the azimuth range to obtain high moment coefficient, so as to enhance the overall starting performance of SB-VAWT. 5.4.2. Characteristics of flow fields According to the calculation results in the previous section, the starting moment coefficient of the SB-VAWT with WGD reached the maximum when azimuth angle was 30 degrees and the optimum effects at 65 degrees. In order to further analyze the influence of curved WGD on the flow field around the blade of SB-VAWT, the pressure and velocity distributions on different cross-sections of the SB-VAWT with and without WGD were analyzed respectively at 30 and 65 degrees. Fig. 15 shows the schematic diagram of SB-VAWT at azimuth angle of 30 and 65 degrees, respectively. Fig. 16 shows the streamline distribution along the wind speed direction of the rotor with and without the WGD at 30 degrees. It could be seen that the turbulent flow of the blade wake appeared symmetrically from the center of the blade to the both sides, and the vortices were more obvious at the center, which indicated that the force at the center of the blade is the greatest and gradually weakens on the two tip sides of blade. After installed the WGD, the flow originally flowing from the upper and lower parts of the blade was guided to impact the blade. The diversion air flow acted more on both tips of the blade, which increased the force on the tip of the blade. The velocity distribution of the blade wake along the wingspan direction changed obviously. The disorder degree of the velocity along the wingspan direction decreased obviously, which reduced the shear force on the blade. At the same time, the internal vortices of the rotor had been improved to different degrees, which reduced the loss of energy, so that more energy could be applied to the downstream blades. For the flow behind the blade, the turbulence moved to the back part of WGD and flow to far wake of rotor. This can reduce the energy loss of flow and improve the aerodynamics performance of the rotor. This is the typical physics feather of flow behind WGD. Fig. 17 shows the streamline distribution along the wind speed direction of the SB-VAWT with and without the WGD at 30 degrees. It could be seen that the turbulent flow generated in the wake of the blade had no obvious difference along the wingspan direction. It had obvious vortices on the ventral of the upstream blade which made the internal velocity of the rotor to decrease obviously. After installed the WGD, the velocity attenuation of the airflow in the internal of the rotor decreased,

a) ij=30°

b) ij=65°

Fig. 15. Schematic diagram of azimuth angle of SB-VAWT. 9

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a) NWGD

b) WGD

Fig. 16. Streamlines of SB-VAWTs with and without wind gathering device (φ = 30 degrees).

of the four blades of the wind turbine with WGD did not change significantly, however, the negative pressure area on the leeward side of the blade was enlarged, which indicated that the blade absorbs more energy and enhances the aerodynamic force on the blade. At z2 plane, the pressure in the inflow direction of the wind turbine with WGD increased obviously, the flow field in the whole wind turbine appeared obvious negative pressure area, and the pressure difference between the two sides of the belly and back of the four blades with the WGD on the upper and lower sides changed obviously. The first upstream windward blade is the most affected, and the blade can provide effective rotation moment for the SB-VAWT. The pressure difference appeared on both sides of blade b and d, and the aerodynamic characteristics of the two blades began to develop under the action of WGD. In summary, when the upwind azimuth is 65 degrees, the WGD could make the wake of the wind turbine blade move backward and strengthened the negative pressure area at the blade tip plane, so that the blade could absorb more wind energy and enlarge the aerodynamic resultant force on the blade, which promoted the rotation of the wind turbine. Different blades had different characteristics, which was also the fundamental reason for the different starting performance of wind turbines under different azimuths angle.

Fig. 18. Plane locations of calculation.

greater force to impel the blade to rotate, which became the main force plane. At the z2 plane, the fluctuation of wind speed caused by wind flow passing through the wind turbine was obviously weakened, and there was only obvious velocity change near the blade. The internal flow field of the wind turbine was relatively stable, and the variation of speed was small, which indicated that the aerodynamic force generated by the blade on the plane contributes little to the rotation of the wind turbine. After installing the WGD, the wind speed passing through the wind rotor increases on the z1 plane, and the fluctuation of the speed inside the blade was more intense, however, not obvious. The effect of

5.4.4. Characteristics of velocity distribution Figs. 21 and 22 show that the velocity distribution of interior of the rotor with and without WGD on different planes at azimuth angles of 30 degrees and 65 degrees. It could be seen from the figure that the velocity gradients of wind flow through different blade element planes were obviously different for original, in which the flow field passing through the z1 plane of the blade fluctuates greatly, which indicated that the impact of wind on the blade was more obvious, and produced

a) NWGD

b) WGD

Fig. 17. Steamlines of SB-VAWTs with and without wind gathering device (φ = 65 degrees). 10

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Fig. 19. Pressure and streamlines nephograms of SB-VAWTs with and without wind gathering device (φ = 30 degrees).

the WGD on the velocity flow field on the blade element plane was weak. On the z2 plane the velocity around the blade changed obviously, and the change trend was closer to the central plane of the blade, which indicated that the blade was greatly increased by aerodynamic force on the plane. Therefore, the plane became the main action plane of the WGD affecting the velocity field of the SB-VAWT. At the same time, the force of the blade in the whole wingspan direction was more uniform due to the velocity fields uniform of the two planes. In summary, the effect of WGD on the velocity of flow field is that it increased the fluctuation range of speed, which assisted the upper and lower ends of the blade get more energy, generated greater aerodynamic force to promote the forward rotation of the wind turbine. Figs. 23 and 24 show the velocity distribution of the whole flow field of the two kinds of SB-VAWT on different plane when the azimuth angle is 30 degrees and 65 degrees, respectively. As can be seen from the figure, for original wind turbine, the wake at the central section plane of the blade was stronger, the wake width was larger than that at the blade tip plane, and the speed recovery was lower, which mean that a stronger shedding vortex disturbs the wake field at the central plane of the blade. According to Kelvin theory, the variation of circulation in incompressible non-viscous fluid is zero, so that the circulation generated at the blade will be larger. The circulation was the main reason for the lift force produce, and the wind turbine blade was more stressed on the plane. After installing the WGD, the inflow velocity of the wind turbine inlet into the rotor of the SB-VAWT increased obviously, and the increase of the inlet velocity at the blade tip plane was more obvious. In the wake region, the wake velocity of the wind turbine with WGD was obviously lower than that of the original wind turbine at both planes, which indicated that the WGD can absorb more wind energy and cause greater velocity attenuation in the wake region. At the same time, the wake width of the wind turbine with WGD is smaller than that of the original, and the wake width at the blade tip plane is smaller than

that of the central section of the blade, which mean that there was a certain pressure gradient along the axis of the wake field of the SBVAWT with WGD. The low-speed flow in the wake exchanged momentum with the high-speed flow without disturbance, which improved the flow speed around the wake and shortened the wake width. In summary, the effect of SB-VAWT with WGD on the overall flow field of wind turbine was that it could increase the inlet velocity at the blade tip plane significantly, increase the velocity attenuation in the wake region. 5.5. Results and analysis of wind tunnel tests In order to verify the accuracy of the numerical simulation result, based on the optimized structural parameters of the WGD, the wind tunnel test model of the SB-VAWT with the optimized curved shape WGD was made as shown in Fig. 25. The measured objects are rotor and wind gathering device installed on the upper and lower of the rotor. Table 5 shows the specific parameters. 5.5.1. Starting characteristics In view of the above calculation results, wind tunnel tests were carried out to measure the starting performance of SB-VAWT with and without wind gathering device. , The calculation and experimental results of starting moment coefficients of SB-VAWT with or without wind gathering device at 10 m/s were shown as Fig. 26. According to the figure, the starting moment of SB-VAWT with wind gathering device was higher than that of traditional turbine, except slightly difference in improving effect under certain upwind azimuth. The calculated results were similar to the experimental results in the variation of the upwind azimuth, which verifies the effectiveness of the wind gathering device in improving the aerodynamic performance of SB-VAWT. However, whether or not the wind gathering device is 11

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Fig. 20. Pressure and streamlines nephograms of SB-VAWTs with and without wind gathering device (φ = 65 degrees).

condition. In the initial starting stage of the wind turbine, the output power coefficients of both the SB-VAWTs increased with the increase of the tip speed ratio. When the tip speed ratio increased gradually, the improvement degree of the power coefficient by the wind gathering device was different under different wind speeds. At 5 m/s, the output power coefficient of the SB-VAWT with wind gathering device was obviously increased under each tip speed ratio, when the λ reached 1.6, the maximum output power coefficient of SB-VAWT was 0.186, 11.4% higher than that of the original SB-VAWT. At 6 m/s, the power coefficient of SB-VAWTs showed obvious optimization effect after 0.6 of λ. The range of the λ for the SB-VAWT to obtain high output power coefficient was obviously enlarged. When the sharp speed ratio reached 1.6, the output power coefficient of SB-VAWT with WGD reached the maximum value of 0.182, which is 8.3% higher than that of original SBVAWT. When the λ was greater than 1.6, the power coefficients of each SB-VAWT begin to decrease. At 8 m/s, the variation of power coefficients of SB-VAWT with and without WGD was not obvious before 1.0 of the λ. When the λ was greater than 1.0, the WGD began to show obvious optimization performance. It showed that with the increase of wind speed, the influence range of the curve shape WGD on the output power characteristics of SB-VAWT was shortened, so that it was more effective to increase the WGD for SB-VAWT running at low wind speed. When the λ reached 1.8, the output coefficient of SB-VAWT with WGD reached maximum value, it increased by 13.3%. When the λ was greater than 1.8, the output power coefficient of the original SB-VAWT began to decrease, while the SB-VAWT with WGD remained unchanged. At 10 m/s, when the λ was less than 0.8, the power coefficient of SbVAWT with WGD was higher than that of original SB-VAWT, and after the λ reached 1.2, the WGD no longer had the improving effect. This showed that the influence of the WGD on the SB-VAWT is mainly reflected in the low range of λ, but had no improving effect on the

installed, the calculation results of starting moment coefficients were higher than the test results on the whole, and there are some differences at some rotation angles. The main reasons are as follows: Firstly, the model used in numerical simulation is simpler than that used in wind tunnel test. The calculation model does not consider the influence of rotating axis, beam and flange on the results, but only considers the wind rotor as a simple model consisting of four lift blades for calculation. In the wind tunnel test, the main parts such as beam and axis are in the rotating region of the rotor, which will inevitably have a certain degree of influence on the aerodynamic characteristics and flow field of the SB-VAWT, resulting in a certain difference in the results. Secondly, the environment of wind tunnel test system is more complex than that of numerical simulation. Because of the limitation of the external environment, the wind tunnel test conditions can not set a large fluid field as the numerical simulation, which makes the wake development not necessarily sufficient and may affect the aerodynamic force of the blade. At the same time, there must be energy loss caused by friction and other factors in the process of wind turbine rotation, which makes the measurement of moment have some errors, and also the reason why the actual moment of wind turbine is too small. However, the results obtained by numerical simulation method can provide reference for wind tunnel test results, so it has a certain reference value.

5.5.2. Output power characteristics In this section, the output power coefficients of SB-VAWT with and without wind gathering device were compared to explore the influence of wind gathering device on the output performance of SB-VAWTs at different wind speeds. It could be seen from the Fig. 27 that the output power coefficient curves of SB-VAWTs with or without wind gathering device first increased and then decreased under each wind speed 12

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Fig. 21. Velocity distribution of SB-VAWTs with and without wind gathering device (φ = 30 degrees).

original, and the response time needed was shorter. After 8 m/s, the rotational speed of the two kinds of wind turbines increases obviously with time, but no longer has obvious difference. In summary, the WGD could improve the speed characteristics of the SB-VAWT. The higher the wind speed, the shorter the starting time, the faster the speed increase and the higher the stable speed. However, the effect of the WGD will be weakened with the increase of wind speed, which showed that the curved WGD had a more significant improvement effect on the aerodynamic characteristics of SB-VAWT at low wind speed.

maximum power coefficient. In conclusion, for 4-bladed SB-VAWT, the curve shaped WGD could improve the starting performance and also the output power characteristics of SB-VAWT to a certain extent. The maximum power coefficient of Sb-VAWT increased with the increase of wind speed. The output performance of SB-VAWT at low wind speed was greatly affected by the installation of curved shape WGD, and the maximum effect could reach 13.3%.

5.5.3. Rotational speed characteristics Rotational speed characteristics are also one of the main performances of SB-VAWT. The rotational speed characteristics of SB-VAWT are mainly reflected in the response time to wind speed and the value of stable rotational speed. In this section, the rotational speed of SBVAWTs with and without WGD were continuously measured from static state to stable rotational state at different wind speeds and the curves of the rotational speed varies with time were obtained as shown in Fig. 28 According to the figure, when the wind speed was 5 m/s, the original wind turbine could not start to rotate by itself. However, the SBVAWT with WGD could rotate at this wind speed, which showed that the curve gathering wind device could reduce the minimum selfstarting wind speed, and made it output effective power in the case of low wind speed. The response time of the rotor to the wind speed was longer. Under this wind speed, the changed of its speed tended to be stable when the time exceeded 600 s, and the stable speed was only 23r/min. At 6 m/s, the SB-VAWT with or without WGD can start itself, and the increase of rotational speed became more obvious. The stable speed of the SB-VAWT with WGD was slightly larger than that of

6. Conclusion Under the condition of the present study the main conclusions are as follows: 1) A Wind Gathering Device (WGD) with curved-outline installed at the up and down of the rotor of Straight-bladed VAWT was proposed and researched. The effectiveness for improving the aerodynamic characteristics is proved by numerical simulation and wind tunnel test. 2) The B-spline curve construction was applied to optimize the suitable curved outline of the WGD. A parameter combination optimized was obtained based on the simulation results on starting moment. The radius ratio of upper and lower disk surfaces is 1.9, the inlet angle is 16 degrees and outlet angle is 4 degrees. For this condition, the maximum starting moment of SB-VAWT with WGD could be increased by 14.8%. 13

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Fig. 22. Velocity distribution of SB-VAWTs with and without wind gathering device (φ = 65 degrees).

Velocity Magnitude (m/s): 0

1

2

3

4

5

6

7

8

9 10 11 12

Fig. 23. Velocity nephograms in different planes (φ = 30 degrees). 14

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Velocity Magnitude (m/s): 0

1

2

3

4

5

6

7

8

9 10 11 12

Fig. 24. Velocity nephograms in different planes (φ = 65 degrees).

Fig. 25. SB-VAWT with curved wind gathering device.

Fig. 26. Starting moment coefficients of SB-VAWTs with and without wind gathering device.

Table 5 Structural parameters of SB-VAWT with wind gathering device. Model

Curved shape WGD

Main rotor

Name

Value

material Height (h) [m] Radius ratio (ε) Inlet angle (β1) [°] Outlet angle (β2) [°] Blade airfoil Material Blade chord (c) [m] Blade number (N) Blade radius (R) [m] Blade height (H) [m]

polyurethane 0.15 1.9 16 4 NACA0018 FRP 0.125 4 0.3 0.5

output power at high tip speed ratio and expanded the range of tip speed ratio for high output power. At high wind speed, it mainly affected the output power at low tip speed ratio, so that the rotor can also output higher effective power at low rotational speed. 4) The wind gathering device could reduce the minimum self-starting wind speed required to enable it to output effective power at low wind speed. The wind gathering device had the good effect on the rotational speed characteristics at 6 m/s in this study, and the maximum rotational speed was increased by about 25%. Furthermore, further researches are considered to be carried out. The optimum structure of wind gathering device research is the main one. The outline of WGD should be optimize to reduce its size. The effects of structural parameters of WGD should be deeply researched.

3) The wind gathering device can effectively improve the rotational speed performance and output power characteristics based on the wind tunnel test results. At low wind speed, it mainly affected the 15

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0.30 U=5 m/s WGD NWGD

U=10 m/s WGD NWGD

Outpute power coefficients (Cp)

0.25

U=8 m/s WGD NWGD

U=6 m/s WGD NWGD

0.20

0.15

0.10

0.05

0.00 0.0

0.4

0.8

1.2

1.6

2.00.0

0.4

0.8

1.2

1.6

2.00.0

0.4

0.8

1.2

1.6

2.00.0

0.4

0.8

1.2

1.6

2.0

TSR Fig. 27. Output power coefficients of SB-VAWTs with and without wind gathering device. 80

500

U=5 m/s WGD NWGD

60

U=6 m/s WGD NWGD

U=8 m/s WGD NWGD

450 400

U=10 m/s WGD NWGD

RPM (r/min)

RPM (r/min)

350

40

300 250 200 150

20 100 50

0

0

0 100 200 300 400 500 600 700

60

120

180

240

300

360

0

0

40

80 120 160 200 240 280 0 20

Time t (s)

40

60

80 100 120 140

Time t (s)

2) U=5 and 6 m/s

b) U=8 and 10 m/s

Fig. 28. Rotating characteristics of SB-VAWTs with and without wind gathering device.

References

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[1] Miao Weipao, Li Chun, Wang Yuanbo, et al. Study of Adaptive Blades in Extreme Environment using Fluid-Structure Interaction Method. J Fluids Struct 2019;91:102734. [2] Liu Qingsong, Miao Weipao, Li Chun, et al. Effects of trailing-edge movable flap on aerodynamic performance and noise characteristics of VAWT. Energy 2019;190:116271. [3] Li Yan, Zheng Yufang, Zhao Shouyang, et al. A review on aerodynamic characteristics of straight-bladed vertical axis wind turbine. Acta Aerodynamic Sinica 2017, 35(3): 368-382+398. [4] Seki K. Vertical axis wind turbine designed aerodynamically at Tokai University. Periodica Polytechnic Mech Eng 1981;25(1):47–56. [5] Zamani Mahdi, Javad Maghrebi Mohammad, Rasoul Varedi Seyed. Starting torque improvement using J-shaped straight-bladed Darrieus vertical axis wind turbine by means of numerical simulation. Renewable Energy 2016:109–26. [6] Favier Julien, Pinelli Alfredo, Piomelli Ugo. Control of the separated flow around an airfoil using a wavy leading edge inspired by humpback whale flippers. CR Mec 2012:107–14. [7] Zhenyu Wang, Yuchen Wang, Mei Zhuang. Improvement of the aerodynamic performance of vertical axis wind turbines with leading-edge serrations and helical

Acknowledgement This research is sponsored by the Project 51576037 supported by National Natural Science Foundation of China (NSFC) and the project supported by Major Science and Technology Project of Tibet Autonomous Region of China (No. XZ201801-GA-03). The authors would like to give thanks to their supporters.

16

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blades using CFD and Taguchi method. Energy Convers Manage 2018:107–21. [8] Yih Lin San, Yangyou Lin, Chijeng Bai. Performance analysis of vertical-axis-windturbine blade with modified trailing edge through computational fluid dynamics. Renewable Energy 2016:654–62. [9] Butbul Jonathan, MacPhee David, Beyene Asfaw. The impact of inertial forces on morphing wind turbine blade in vertical axis configuration. Energy Convers Manage 2015:54–62. [10] Bouzaher Mohamed Taher, Hadid Mohamed. Derfouf Semch-Eddine. Flow control for the vertical axis wind turbine by means of flapping flexible foils. J Braz Soc Mech Sci Eng 2017;39:457–70. [11] Wendi Liu, Qing Xiao. Investigation on Darrieus type straight blade vertical axis wind turbine with flexible blade. Ocean Eng 2015:339–56. [12] Abdalrahman G, Melek W, Lien FS. Pitch Angle Control for Small-Scale Darrieus Vertical Axis Wind Turbine with Straight Blades (H-type VAWT)[J]. Renewable Energy 2017;114:1353–62. [13] Zhenyu Wang, Yuchen Wang, Mei Zhuang. Improvement of the aerodynamic performance of vertical axis wind turbines with leading-edge serrations and helical blades using CFD and Taguchi method. Energy Convers Manage 2018;177:107–21. [14] Zhao Zhenzhou, Qian Siyue, Zheng Yuan, et al. Enhancement approaches of aerodynamics performance of lift type vertical axis wind turbine considering small angle of attack. J Drainage Irrigation Machinery Eng 2018;36(2):146–453. (In Chinese). [15] Zhu Haitian, Hao Wenxing, Li Chun, et al. Numerical study of effect of solidity on vertical axis wind turbine with Gurney flap. J Wind Eng Ind Aerodyn 2019;186:17–31. [16] Zhu Haitian, Hao Wenxing, Li Chun. Application of flow control strategy of

[17]

[18] [19]

[20] [21]

[22]

[23]

[24]

17

blowing, synthetic and plasma jet actuators in vertical axis wind turbines. Aerosp Sci Technol 2019;188:468–80. Yan LI, Shouyang ZHAO, Chunming QU, et al. PIV visualization experiment on static flow field of Savonius wind turbine. J Drainage Irrigation Mach Eng 2018;36(2):159–65. (in Chinese). Ji JF, Deng ZY, Jiang L. Optimization design of a 5 kW lift type vertical axis wind turbine with wind shield-growth patterns. J Eng Thermophys 2012;33(7):560–4. Chong WT, Fazlizan A, Poh SC. The design simulation and testing of an urban vertical axis wind turbine with the omni-direction-guide-vane. Applied Energy, 2013,112(SI): 601-609. Huang J, Zhao ZZ, Ye F. Numerical investigation on lift-type vertical axis wind turbine with guide vanes. Renew Energy Resources 2013;3(10):53–6. Xiaohang Wang, Wentong Chong, Kokhoe Wong, et al. Preliminary Performance Tests and Simulation of a V-Shape Roof Guide Vane Mounted on an Eco-Roof System. Energies 2018;11. Wong Kok Hoe, Chong Wen Tong, Sukiman Nazatul Liana. Experimental and simulation investigation into the effects of a flat plate deflector on vertical axis wind turbine. Energy Convers Manage 2018:109–25. Wong Kok Hoe, Chong Wen Tong, Poh Sin Chew. 3D CFD simulation and parametric study of a flat plate deflector for vertical axis wind turbine. Renewable Energy 2018:32–55. Li Yan, Zhao Shouyang, Tagawa Kotaro. Starting performance effect of a truncatedcone-shaped wind gathering device on small-scale straight-bladed vertical axis wind turbine. Energy Convers Manage 2018;167:70–80.