1. Introduction
Today, the potential and characteristics of wind energy have already been thoroughly investigated in several nations worldwide. The International Energy Agency’s (IEA)’s 2019 report on Morocco highlights the ambitious energy transition of the country to renewable energy sources. Wind energy is anticipated to be the second most common method of electricity generation after hydro-power.
The many difficulties this sector faces are not obscured by the success of wind turbines as a source of green and renewable energy [
1,
2]. Grasping and evaluating the operation of wind turbines requires a thorough understanding of wind dynamics. Additionally, the location has a big impact on the frequency and speed of the wind. In order to determine the performance of wind, the statistical features of the wind speeds at the location of installation of the wind farm must be considered. Indeed, the market value of wind power declines during periods of heavy winds, and the exchange price for wind power tends to be near zero on windy days as the amount of wind power in the energy system increase [
3,
4]. Modern wind turbine blades have increased in size by a factor of 100 versus a drop in energy by a factor of 5 in the past 30 years. This rise comes at a cost for the companies. Due to the size, the blade geometry must be created during the design phase while considering both aerodynamic and acoustic requirements. The future of wind turbines should be focused more on the improvement of these machines for better power output in unfavorable conditions.
A potential solution to improve existing wind turbines is the addition of flow-control devices to the rotor blades. Flow control devices can effectively prevent or delay flow separation and suppress turbulence resulting in improved aerodynamic and aeroacoustics performance, load reduction, fluctuation suppression, and ultimately increased wind turbine power output [
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15].
Through learning from its miraculous nature, researchers and engineers have proposed techniques that have revolutionized modern machinery [
10,
16]. Birds, in particular, have inspired human flight since the early days of flight machine development and continue to inspire the aerospace industry on different levels. Owls have been extensively researched as potential star raptors, and it was discovered that the owl’s exceptional ability to fly is mostly due to its distinctive feather structure. Particle Image Velocimetry (PIV) experiments [
17,
18] of owl-like surfaces have demonstrated its ability to delay the transition, reduce the separation bubble and decay the vortical structures along the wing. To understand the unique morphological characteristics and the biological mechanisms of the owl’s flight, Bachmann et al. [
19] compared the wing of a barn owl to that of a pigeon with an emphasis on the distinctive features of the owl’s feathers, both on a macroscopic and microscopic level. They reported that owl feathers had some characteristic features, such as serrations at the leading edge of the wing, fringes at the edges of each feather, and a velvet-like dorsal surface. This study also pointed out the notably larger area of owl wings compared to pigeons with the same body mass, allowing the owl to generate enough lift to glide at a relatively low speed. The wings of owls are significantly different from those of all other groups of birds in terms of the intricate pattern and surface roughness. Typically, the feathers of an owl have directional textures, velvet-like surfaces, leading edge combs, and trailing edge fringes.
Extensive literature [
19,
20,
21,
22,
23,
24,
25,
26,
27] detailing the owl wing morphological structure in general and trailing edge shapes in particular is available.
Figure 1 shows the placement of serrations and fringes on a sample feather. Serrations are found on some of the primary feathers of an owl, especially towards the tip of the feather. At the same time, thin fringes can be found on the trailing edge of each primary feather. The thickness of the feather decreases towards the end of the fringes. These fringes reveal variations in size and direction within a single feather as well as between many feathers on a single wing. From the base of the feather toward the tip, the length of the fringed region often decreased.
Investigation into the aerodynamic effect of these distinct owl wings features [
18,
22,
28,
29,
30] has shown that serrations cause the flow near the wing leading edge to turn toward the wingtip forming a stationary leading edge vortex that delays separation and producing non-linear lift on the outer half of the owl’s wing. Additionally, they discovered that transition was moved upward over the wing as a result of the separation bubble’s size being reduced on the velvet surface that mimicked down feathers. Finally, as the flow passes over the wing and sheds in the trailing edge, the fringes result in a smoother lower wing reducing the sharp edges and allowing for the decay of the vortical structures.
Inspired by the owl wing, serrated trailing edges are rising in popularity. Industries have adopted these shapes as they are rather simple to manufacture, and their installation and maintenance costs are plausible. The fundamental reason this control device has gained so much traction is the ability to retrofit already-running wind turbines to comply with noise laws. Saw-tooth serrations and sinusoidal serrations are the two main types of bio-inspired simulation of the owl wing serrations [
16]. In general, there are two ways to apply trailing edge serrations: either cutting forms directly from the sharp trailing edge or adding thin serrated flat plate inserts to the current trailing edge. The ability to trail edge serration to reduce the airfoil tonal noise is confirmed through a large existing literature [
31,
32,
33,
34,
35,
36,
37,
38,
39].
Gharali et al. [
40] looked into the impact of a dynamic serrated airfoil. Unlike researchers who tested static serrated airfoils, the dynamic serrated airfoil is exposed to angles of attack that oscillate sharply, which is typical behavior for a wind turbine when working in the field. The primary conclusions of their research are that lift values for the serrated case increased closer to the dynamic stall angle, from 18.5 to 19.5 degrees, while lift values for the unserrated case generally remained consistent with the original airfoil at low angles of attack.
More recently, Zhou et al. [
35] tested trailing edge serrations on two airfoil profiles and drew a theoretically derived optimum for the amplitude, the wavelength, and the flap angle. The obtained optimal configuration was tested on a full-scale wind turbine on terrain where it was found that the trailing edge serrations increased the annual power output by less than 0.7%. Overall, a carefully thought-out configuration of the serrations is possible to significantly reduce the loss of the aerodynamic efficiency and power generation of a wind turbine in particular operating conditions.
The focus of this research is to obtain an optimum configuration of serrations for the application of wind turbines. Even less work dives into the effect of these serrations on the aerodynamics and, eventually, the power generated by the wind turbine. This work is a detailed numerical study of the aerodynamic performance of different trailing edge serrations configurations on a sample horizontal axis wind turbine. An optimal configuration is sought to obtain the best results for the particular conditions of the National Renewable Energy Laboratory (NREL) phase VI wind turbine blade. Three-dimensional simulations of the clean blade and the modified blade with the trailing edge serrations are carried out. The results are finally used to derive an optimum configuration. The primary goal of this research is to use a mix of Computational fluid dynamics (CFD) and Taguchi methodologies to comprehend how a horizontal wind turbine’s trailing edge serration parameters interact with one another under various flow conditions.
5. Conclusions
The present work highlights the impact of trailing-edge serrations on horizontal wind turbine aerodynamics through a CFD investigated on a 3D wind turbine blade. To evaluate the impact of different trailing edge saw tooth serration designs on the performance of the wind turbine, the Taguchi experimental design is employed, and the annual output power is measured using Weibull’s distributions of three selected cities. The optimal design is tested and validated for the selected regions through the Modified additive Taguchi model. The purpose of this study is to shed light on the aerodynamic capability of trailing edge serration for further improvement of the design approaches used in horizontal axis wind turbines. The major findings of the present work can be summarized as follows:
The Taguchi-modified additive model coupled with CFD is effective in estimating the interactions between the factors and deriving the optimal design for the horizontal axis wind turbines.
The optimal design predicted by the Taguchi model and the modified additive model is the trailing edge serration of wavelength is equal to 5% tip chord line, the amplitude of 20% chord line, and serration plate thickness of 1 mm.
The optimum design was found to be consistent for the three different regions.
The instantaneous torque of the modified wind turbine results in a shift of the rated wind speed from 10 m/s on the clean blade to 8 m/s for a majority of the configurations.
The instantaneous torque increases pre-rated wind speed and decreases post-rated wing speed for all configurations.
In the post-stall region, the serrations result in an increased wake and advancement of the separation towards the leading edge.
The results obtained in this study show that with an optimal design, the serration can increase performance in certain important regions. In addition, the evaluation of the performance of these wind turbines should be conducted in a manner that accounts for the terrain, as specific instantaneous parameters of the wind turbine are not enough. Further studies in this area can cover the mechanical aspect and aero-acoustic impact of the trailing edge serrations, as well as the correlation between acoustic emission, power generation, and aerodynamic forces in the improvement of an overall wind turbine performance.