A Study on Relationship among Blade Camber Direction and Pitch Angle and Performance of a Small Straight-Blade Darrieus Wind Turbine by Using Scale Test Model and Gurney Flap

Straight-blade Darrieus vertical axis wind turbines are used as medium and small size wind turbine because of higher power output in vertical axis wind turbine (VAWT). In our previous study, the relationship between the performance and Reynolds number based on airfoil chord length had been investigated by using small-scale test models of lift-type VAWT, and the results showed that the performance of tested wind turbine models with small diameter was clearly lower than that of the large-scale field test machine, and its performance also varies significantly with the blade pitch angle. In this study, we focused on the performance of a small-scale straight-blade Darrieus VAWT, the relationship among the blade airfoil camber direction and the pitch angle, and the performance of the small-scale VAWT was examined experimentally by using a small-scale VAWT test model with Gurney flap which was a small flat plate. Gurney flaps with its height h, as a ratio to the blade chord length c, h/c = 0.036 to 0.055, were attached to the blades of the VAWT test model, in addition, the attaching direction of the Gurney flap on the blade was examined for both inward and outward of the rotor, and the pitch angle was also examined for a range of −5 to 10 degrees. These results are discussed comparing with the result of the VAWT without Gurney flap and considering the numerical results for the single blade with/without the Gurney flap. The results showed that the performance of the tested VAWT was reversed between the inward and outward Gurney flaps around a pitch angle of 10 degrees. That is, the inward Gurney flap was superior at a pitch angle of less than 10 degrees, while the outward Gurney flap was effective at a pitch angle of more than 10 degrees. Furthermore, for the tested small-scale VAWT model, the How to cite this paper: Tanino, T., Hara, K., Yoshihara, R. and Miyaguni, T. (2021) A Study on Relationship among Blade Camber Direction and Pitch Angle and Performance of a Small Straight-Blade Darrieus Wind Turbine by Using Scale Test Model and Gurney Flap. Journal of Flow Control, Measurement & Visualization, 9, 28-44. https://doi.org/10.4236/jfcmv.2021.92003 Received: November 25, 2020 Accepted: March 23, 2021 Published: April 25, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/

addition, the attaching direction of the Gurney flap on the blade was examined for both inward and outward of the rotor, and the pitch angle was also examined for a range of −5 to 10 degrees. These results are discussed comparing with the result of the VAWT without Gurney flap and considering the numerical results for the single blade with/without the Gurney flap. The results showed that the performance of the tested VAWT was reversed between the inward and outward Gurney flaps around a pitch angle of 10 degrees. That is, the inward Gurney flap was superior at a pitch angle of less than 10 degrees, while the outward Gurney flap was effective at a pitch angle of more than 10 degrees. Furthermore, for the tested small-scale VAWT model, the

Introduction
Recently, for the effective utilization of wind energy, various research and development activities such as floating offshore wind power generation using largescale wind turbines and wind environment assessment. In addition, in near future for the use of various types of renewable energy, promotion of local production for local consumption of electricity combining with smart grid demonstration projects in urban areas is being carried out. If the use of this local generated renewable energy promoted, also the use of medium and small wind turbines will be effective. Therefore, it is important to improve the safety and performance of these wind turbines in the future.
So far, we have studied the relationship between the Reynolds number and the performance of the straight-blade Darrieus vertical axis wind turbines (VAWTs), which are expected as medium and small wind turbines with high power output characteristics [1] [2]. In this study, field tests for the practical application of the VAWT and wind tunnel test using small-scale test models of the VAWT were conducted. The results showed that the wind turbine performance of the smallscale test model with a diameter of 0.38 m was obviously lower than that of the large-scale field test machine with a diameter of 2.6 m [2]. In relation to this, the performance of straight-blade Darrieus VAWTs with different diameters ranging from 0.30 to 0.45 m had been examined by Longhuan et al. [3]. It was showed that larger diameter showed higher maximum performance of VAWT while smaller diameter had better self-starting performance. These results suggest that the performance of the VAWTs has different characteristics at different scale.
The relationship between the performance of several types of Darrieus VAWTs and the camber of blade airfoils has been experimentally studied by Hara et al. [4], Ejiri et al. [5], and Longhuan et al. using small-scale Darrieus VAWT of less than 1 m in rotor diameter. Hara et al. had compared NACA0018 symmetrical airfoil with cambered airfoil, which was transformed by conformal mapping method between curvilinear and parallel flows, and showed that the cambered airfoil of concave-in configuration had higher performance of the power coefficient. Ejiri et al. had compared NACA0012 symmetrical airfoil with NACA4412 cambered airfoil and showed that the cambered airfoil of concave-out configuration had higher performance of the maximum power coefficient. In addition, Longhuan et al. [3] had compared NACA0021 symmetrical airfoil with DU06W200 foil of concave-in configuration showed higher performance of the power coefficient, while NACA4415 cambered airfoil did not show sufficient performance, but also mentioned that their result was different from the analytical result of Kirke et al. [6] that NACA4415 airfoil showed superior performance than NACA0015 airfoil at low speed. Thus, regarding the relationship between the blade camber direction and the performance of small-scale Darrieus VAWT, the results could differ when the airfoil shape and the experimental environment, etc. are just slightly different. Therefore, from the perspective of accumulating data on the performance and the geometrical conditions such as airfoil shape for Darrieus VAWTs, it is hoped that more research reports based on experiments will be published in the future.
Moreover, regarding the application of Gurney flaps to straight-blade Darrieus wind turbines, Haitian et al. [7] [8] and Yan et al. [9] also conducted detailed numerical analysis on straight-blade Darrieus VAWTs with diameters of more than 1 m and a pitch angle of 0 degrees. Haitian et al. [7] showed that the performance of the VAWT is better by applying the Gurney flap to NACA0021 airfoil, regardless of its attaching direction, compared to the clean airfoil. Yan et al. [9] showed that there is a clear difference in the performance of the VAWT with NACA0018 airfoils even between h/c = 0.04 and h/c = 0.05 of inward Gurney flaps. Their results are very interesting because it is different from our results using a small-scale straight-blade Darrieus VAWT as described later in section 3.3. We think that this is due to the size of the wind turbine rotor and the Reynolds number based on blade chord length.
Therefore, focusing on small-scale lift-type VAWTs, we conduct experimental studies on the effect of Gurney flaps attached to the wind turbine blades on the performance of a small-scale VAWT. In this study, we investigated the effects of the blade camber direction and the pitch angle on the performance of the VAWT by using Gurney flaps. That is, as a simple way to obtain the same effect as the blade camber, Gurney flaps were attached to the trailing edge of blade with a symmetrical airfoil as shown in Figure 1 in the next chapter, and the ef-           In addition, as shown in Figure 3, the angle of attack near the stall point does not shift to smaller value even if the Gurney flaps are applied. It is thought that this is because NACA0018 airfoil is relatively thick and the radius of curvature of the leading edge is large. In contrast, for thin airfoil such as NACA0011

Numerical Analysis Results
with the small leading edge radius, the angle of attack near the stall point tends to be smaller when a Gurney flap is applied to the airfoil [11]. The results in     Table 2, with the conditions of pitch angles of −5, 0, 5 and 10 degrees in Figure 7, a total of twenty cases of power performance tests were conducted. The wind speed in the power performance test was measured using a pitot tube mounted at the wind tunnel exit shown in Figure 8. The differential pressure  In addition, since the torque values of the small-scale VAWT test model are very small, the power performance tests for all the conditions shown in Figure 7 and Table 2 were carried out continuously at one time in order to reduce the influence of drift errors in the torque detector. The power performance test was measured three times for each condition. The average value was obtained from the measured torque and rotation speed.

Power Performance Test Method
The tip speed ratio λ and the power coefficient C p , defined in the Equations (1) and (2), were used to evaluate the performance of the VAWT test model. λ and C p are the dimensionless parameters of rotation speed and power output based on the inlet wind speed and rotor swept area, respectively.
Tip speed ratio: (1) Power coefficient: where r is the rotor radius, ω is the rotation angular velocity, V is the inlet wind speed, T is the shaft torque, ρ is the air density and A (=D × L) is the rotor swept area.   show the performance curves for pitch angles of 0, 5, 10 and −5 degrees, respectively. In each figure, the horizontal axis is the tip speed ratio λ and the vertical axis is the power coefficient C p , comparing the five conditions (NG0, GI4, GI6, GO4, and GO6) of the Gurney flap shown in Table 2.

Power Performance Test Results and Discussion
Overall, from the comparison of Figures 10-13, the best power performance is shown in the condition of the pitch angle 5 degrees (Figure 11). Since the    to 0.087, which are also higher than that of NG0. In addition, in the region of higher tip speed ratio than the peak point of performance curve, the performance curves of GI4 and GI6 are approaching the curve of NG0. Comparing GI4 and GI6 with different flap heights, the performance curves of both are almost equal in the range of lower tip speed ratio than the maximum power coefficient point, but the power coefficient of GI4 with the shorter flap is slightly higher than that of than GI6 in the higher tip speed ratio side.
On the other hand, the maximum power coefficients of both GO4 and GO6 with the flaps outward of the rotor are lower when compared to that of NG0.
The power coefficient of GO4, which flap is short, varies in much the same way as that of NG0 from the low tip speed ratio side to the point of maximum power coefficient. In the higher tip speed ratio side, the power coefficient of GO4 is lower than that of NG0. For GO6, which has the longer flap, the power perfor- Next, for the pitch angle of 5 degrees shown in Figure 11, the power performance is clearly higher for all the flap conditions, including NG0, than for the pitch angle of 0 degrees in Figure 10. For the case with no flap (NG0), the power coefficient reaches a peak value of Cp max = 0.13 at around λ = 1.05 which exceeds 1 and this maximum value is about twice that in the case of NG0 with the pitch angle 0 degrees.
In the cases of inward facing flaps (GI4, GI6), the maximum value of the power coefficient is almost equal to that of NG0 without the flaps. In the range below the tip speed ratio where the power coefficients reach their peak values, these power coefficients are higher than that of NG0. For GI4, which has the shorter flap, the power coefficient reaches its peak value at almost the same tip speed ratio as NG0. In addition, GI4 shows the slightly wider range of tip speed ratio, in which the power coefficient is as high as its maximum value, than NG0.
For the tip speed ratio range higher than where the power coefficient reaches its peak, the performance curve of GI4 almost corresponds to that of NG0. On the other hand, in the case of GI6 with the longer flap, the tip speed ratio, where the power coefficient peaks, is around λ = 1 which is slightly lower than that in the case of NG0. In the tip speed ratio λ > 1, the power coefficient of GI6 is lower than that of NG0 and GI4 and the performance curve of GI6 has a slightly sharper peak than that of NG0.
When the flaps are attached facing outward (GO4, GO6), the power coefficients for GO4 and GO6 are almost equal to or slightly higher than that of NG0 for the lower tip speed ratio λ < 0.8. However, for the higher tip speed ratio above 0.8, the power coefficients of both GO4 and GO6 are obviously lower than that of NG0. The performance curves of GO4 and GO6 reach their maximum peak values at the tip speed ratio below λ = 1. The maximum power coefficient of GO4 with the shorter flap is Cp max = 0.114 and that of GO6 with the longer flap is even lower Cp max = 0.107. In the higher tip speed ratio side than the peak position of the performance curve, as the tip speed ratio increases, the power coefficients of both GO4 and GO6 apparently decrease compared to that of NG0.
Then, for the large pitch angle of 10 degrees shown in Figure 12, when no flaps are added (NG0), the power performance is inferior to that for the pitch angle of 5 degrees in Figure 11 but better than that for the pitch angle of 0 degrees in Figure 10. The maximum power coefficient is Cp max = 0.102 near the tip speed ratio of 1. However, for the cases with the flaps at the pitch angle of 10 degrees shown in Figure 12, the relationship between the flap attaching direction and the power performance is reversed from the results at the pitch angles of 0 and 5 degrees shown in Figure 10 and Figure 11. That is, for the pitch angle of power performances become lower than that of NG0 with no flap in Figure 10 and Figure 11. In addition, we think that the results in the case of large pitch angle 10 degrees in Figure 12 show that GO4 and GO6 can suppress a separation on the blade outward of the rotor comparing with NG0 due to the effect reducing the pitch angle by the outward facing flaps.
Finally, for the pitch angle is −5 degrees shown in Figure Figure 13, by adding a long flap (GI6) facing inward of the rotor, even when the pitch angle is −5 degrees, the same effect as turning the blade to have the positive pitch angle is obtained. As a result, it is considered that the power performance is more improved compared to that with no flap.
To further consider the relationship among the flap height and the pitch angle and the power performance for the small-scale VAWT test model, Figure 14 shows the relationships between the maximum power coefficient in the performance curve and the pitch angle for each flap condition shown in Figures 10-13.
The vertical axis in Figure 14 is the maximum power coefficient Cp max and the horizontal axis is the pitch angle θ.
As is clear from Figure 14

Summary
In 2) In terms of the effect of flap height and direction corresponding to the camber effect, especially its direction, on the power performance of the small-scale VAWTs, it is considered that the effective camber direction on the power performance is inward of the rotor when the pitch angle is small and outward when the pitch angle is large. For the small-scale VAWT test model in this study, the results show that the boundary where superiority or inferiority of the power performance is reversed is between the pitch angle 5 and 10 degrees. Furthermore, for the degree of flap height, it is considered that the shorter Gurney flap, in other words, slight camber is effective in improving the power performance of the small-scale straight-blade Darrieus VAWTs.
3) For the small-scale VAWT test model in this study, the optimum pitch angle is about 5 degrees. When the Gurney flap is attached facing inward of the rotor, the power performance is higher than that without flap. For flap heights, the shorter flap h/c = 0.036 is more effective than h/c = 0.055. The shorter flap facing inward of the rotor shows good performance improvement effect in the tip speed ratio range lower than where the performance curve reaches its peak.