Aerodynamic characteristics of variously modified leading-edge protuberanced (LEP) wind turbine blades under various turbulent intensities
Introduction
Understanding the impact of freestream turbulence intensities on the aerodynamic characteristics of the aerofoils have been recognised as one of the long-term challenges in the field of wind engineering and Aerodynamics. Since the wind turbines operate under complex terrains and harsh environments, the wind turbine blades are subjected to varying levels of turbulence. IEC (IEC, 2005) design regulations for the wind turbines clearly state that the turbulence intensity has been proven to directly influence the aerodynamic performance characteristics of the wind turbine blades. Therefore, the assessment of the aerodynamic performance of the wind turbine blades is of primary importance in the field of wind engineering due to its relevance to the safety and design considerations. Especially, for developing countries like India where there is a serious concern amongst the researchers and the policymakers about the unprecedented growing demand on electricity and carbon emissions from fossil fuels leading to climatic change, there is a strong drive to increase the amount of energy generated through renewable sources like wind energy. Under such circumstances, given importance for the electricity production, the losses incurred due to the flow complexities arising out of the turbulence intensities will impose additional challenges. Several studies insisted on the importance of the effects of turbulence intensity over the wind turbine blades. However, there is still a lack of understanding of the effect of turbulence intensity on the performance of the wind turbines. Elliot et al. (Elliot and Cadogan, 1990), and Antoniou et al. (2009), reported that the negligence of the effect of the turbulence on the estimation of lift and drag of the aerofoil causes serious errors in the annual energy production estimates. This paper aims to provide some insight in this direction by experimentally investigating the aerofoils performance under various turbulence intensities. The first major study on the effect of turbulence intensity on the aerodynamic forces was measured by Stack et al. (Stack, 1931), during the year 1931. Stack et al. (Stack, 1931), measured aerodynamic forces acting over various aerofoil models both in the presence and the absence of turbulence grids in the wind tunnel and found that the turbulence increases the maximum CL and effectively delays the stall. The similar results are then later confirmed by Huang et al. (Huang and Lin, 1995), in 1995 and Devinant et al. (2002)., in 2002. Mueller et al. (1983), studied the effects of freestream turbulence intensity over the aerofoil and indicated that the increase in the turbulence intensity advances the boundary layer transition point towards the vicinity of the leading edge. Hoffmann (1991) confirmed this behaviour and additionally reported that the CLmax of the NACA 0015 aerofoil increases by 30% as the turbulence intensity increases from 0.25% to 9%. Studies by Sicot et al. (2008), and Seggidhi et al. (Seddighi and Soltani, 2007), further showed that the aerodynamic characteristics like the coefficient of lift and the stall delay behaviour have a strong dependence with the turbulence intensity. A similar relationship was observed with subsequent studies by Chamorro et al. (2015), and Lee et al. (2018). Li et al. (Li et al., 2016), extensively investigated the influence of turbulence intensity over the airfoil and reported that the increase in the turbulence tends to prevent the flow separation over the aerofoil surface by enhancing the flow mixing characteristics thereby resulting in the stall delay. Wang et al. (2014), confirmed this behaviour by investigating the effect of turbulence intensity over the airfoil at low Reynolds numbers. Herbst et al. (2017), Breuer et al. (Breuer, 2018), and Simoni et al. (2017), studied about the implication of the Laminar separation bubble (LSB) on the airfoil surface at elevated large-scale freestream turbulence and reported that thebubble length and the height reduce with the increase in the free-stream turbulence intensity. In order to study the influence of turbulence intensity on the aerodynamic characteristics of the aerofoil, wind tunnels are widely used, since it has the ability to create desired turbulence levels at controlled environments. Recently researchers adopted several methodologies to generate turbulence in the wind tunnels like passive screens, grids, barriers, aerofoil cascades, agitated bar grids, fractal grids, cross jets, tube bundles etc. as mentioned in the literature.
Generally, the wind turbine blades are designed to operate in a narrow range focused on their optimal working points. However, this aerodynamic limitation poses a serious challenge in achieving desirable energy capture in real-time turbulent wind environment (Ikeda. et al., 2018). Aiming at solving this issue, aerodynamic and wind engineering designers started looking for inspiration from “Mother Nature”. Nature has always inspired human achievements and it still continues to offer the best yet effective solution to overcome real-time practical problems. One such fascinating bio-inspired design is assumed to have the potential to improve the aerodynamic robustness of wind turbine blades by incorporating leading-edge protuberances of the humpback whales showing its capability to perform efficiently at greater operating range with effective stall delay characteristics. A marine biologist Frank E. Fish (1994) initiated the research on such protuberances present over the Humpback whales and reported that leading edge protuberanced blade possesses 25% more airflow than the conventional straight leading edged blade. Frank E. Fish continued his research and subsequently published several research articles (Fish, 1994; Fish and Battle, 1995; Fish and Lauder, 2006; Fish et al., 2008, 2011a, 2011b). Following that several researchers reported that the post-stall performance of the bio-inspired airfoil section was better in comparison with the conventional airfoil section. Miklosovic et al. (2004), experimentally investigated an idealised humpback whale flipper with and without leading-edge protuberances and noticed that increasing the angle of attack (α) on the bio-inspired leading-edge protuberanced model did not show any abrupt stall and the stall angle is increased by 40%. Several researchers like Johari et al. (2007), Custodio et al. (2012), and Zhang et al., 2013, 2014a, 2014b, conducted studies on leading-edge protuberances over NACA 63(4)-021 which closely resembles the aerofoil flippers and reported that the leading-edge protuberanced model outperforms the conventional straight blade model and is effective in delaying the stall. Recent flow visualisation study by Hansen et al., 2011a, 2011b, utilizing hydrogen bubble methodology revealed that the leading-edge protuberances generate vortices which change in direction of rotation with respect to the change in the leading-edge geometry tends to cancel out each other resulting in a shorter wake creating no additional drag penalty. Bai et al. (2015), tested and compared slotted, winglet and leading-edge protuberance incorporated wind turbines and found that the leading-edge protuberance incorporated design was effective in maintaining a stable flow even at higher speeds. Huang et al. (2015), tested the leading-edge protuberanced blades on a small scale HAWT and confirmed that these modified models show performance improvements and stall delay. Similarly, Wang et al. (Wang and Zhuang, 2017; Wang et al., 2018), studied the effectiveness of leading-edge serrations on the VAWT and reported that the power coefficient of the modified leading-edge serrated VAWT model is found to increase by approximately 18% when compared against the baseline unmodified version. Based on the vorticity distribution results, they further reported that leading-edge serration design significantly suppresses the flow separation near the peak region illustrating the plausible reason behind the stall delay characteristics associated with the modified model. Aiming at understanding the flow separation characteristics over the leading-edge protuberanced design Wei et al. (2017), performed PIV measurements and particle streak visualizations. Flow measurements indicate that the counter-rotating vortex pairs (CVPs) formed over each tubercle is responsible for the flow separation mitigation. However, it was further reported that the stability of the CVPs are closely related to the flow characteristics since the CVPs meanders and interacts with the adjacent flows. Even though the aerodynamic characteristics of the leading-edge protuberanced wind turbine blades are studied, it should be however noted that the turbulence has the tendency to effectively alter the flow characteristics over the aerofoil. Therefore understanding the impact of turbulence intensity on the aerodynamic characteristics of the leading-edge protuberanced wind turbine blades is quintessential for the efficient design, operation and working at optimum performance. Although the effect of turbulence intensity on the aerodynamic characteristics of the constant chord straight wing aerofoils have been comprehensively studied by the investigators, the influence of turbulence intensity on the modified leading-edge protuberanced aerofoil sections is still an open area for researchers.
In this paper, we present the experimental results on aerodynamic characteristics of the variously modified leading-edge protuberanced wings at different turbulence intensities obtained using MPS4264 simultaneous pressure scanner of Scanivalve make. The result shows the variation of aerodynamic force coefficients like the coefficient of lift (CL) and coefficient of drag (CD) of the variously modified leading-edge protuberanced models at different turbulence intensities along with their corresponding surface pressure distribution. Additionally, attempts were made to gain further insight into the effect of turbulence intensities on the spanwise cross-sections to better understand the flow mechanism in detail. Understanding the influence of turbulence intensity over the aerodynamic characteristics of the leading-edge protuberanced blades will be highly beneficial and could be advantageous for aerodynamic designers and wind engineers to bring such novel design into practical existence to enhance maximum wind capture at wide operating ranges.
Section snippets
Synthesis of experimental setup
The aerodynamic performance characteristics of a variously modified leading-edge protuberanced test models were experimentally evaluated in a Low-speed subsonic wind tunnel facility located at SASTRA Deemed University. The LEP test models evaluated were based on the NACA 63(4)-021 airfoil profile with a chord length of 100 mm featuring leading-edge protuberances at a fixed mean free-stream velocity of 30 m/s corresponding to Re = 2.0 × 105. All the models were tested at various angles of
Results and discussions
In this section, the aerodynamic forces like the coefficient of lift (CL) and the coefficient of drag (CD) acting over the variously modified leading-edge protuberanced airfoils obtained through surface pressure measurements by pressure integration technique were discussed in detail.
Conclusion
Based on the detailed experimental investigation the following conclusions were made as follows:
- 1.
The aerodynamic lift characteristics of all the three variously modified leading-edge protuberanced wing configuration tend to increase with the increase in the turbulence intensity and the angles of attack (α) in both the increasing and the decreasing direction of angles of attack.
- 2.
The time-averaged coefficient of drag (CD) for all the LEP wing configuration featuring turbulence is higher than that
CRediT authorship contribution statement
S. Arunvinthan: Software, Investigation, Formal analysis, Writing - original draft. S. Nadaraja Pillai: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision. Shuyang Cao: Formal analysis, Writing - review & editing, Supervision.
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.
Acknowledgement
This research work was supported by the Science Engineering Research Board (SERB), Department of Science & Technology (DST), Government of India, File No: ECR/2017/001199.
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2022, Renewable EnergyCitation Excerpt :The improved rudder seems to stall at a lower AOA than the smooth rudder, but the stall occurs more softly, and there is no rapid reduction in lift. Through a series of wind tunnel tests, Arunvinthan et al. [27] investigated the effect of turbulence inflow on the aerodynamic characteristics of the leading-edge projection airfoil under various turbulence intensities. The results showed that when the turbulence strength rose, the mean CL increased, which was connected to the stationary stall characteristics.