Wind Turbine Aerodynamics and Flow Control

Abstract

Aerodynamics is one of the prime topics in wind turbine research. In aerodynamics, the design of a flow control mechanism lays the foundation for an efficient power output. Lift generation in the airfoil section leading to rotary motion of blade and transfer of mechanical to electrical power generation through gearbox assembly. The primary objective of a flow control mechanism in wind turbine blades is to delay the stall and increase the lift, thereby an efficient power generation. Flow control is classified into active and passive flow control mechanisms. Active flow control works on an actuation mechanism that comes into action when required during varied operating conditions. Passive flow control devices are designed, developed, and fixed on the surface to extract the required output through effective flow control. Vortex generators are the simplest, most cost-effective and efficient passive flow control devices. These devices influence the power of wind turbine blades in various ways, such as placement of generator along the chord, distance between pairs of a generator, angle of inclination of a generator with the blade surface, the height of generator. Flow control device needs to be optimized with the aforementioned parameters for efficient stall delay and power generation.

1. Introduction

Wind is an abundant resource available in the earth’s atmosphere, and the need for renewable energy is demanding due to climate change and the energy crisis. Wind energy is low carbon footage leads to importance in research increasing the efficiency and use of the wind resource even in low wind speed. In the case of renewable and carbon-free emission energy production; firstly, solar power gains less attraction due to the less efficient and cannot produce energy on a night or cloudy days. Secondly, hydropower depends on rainfall; has a high impact on river ecosystems and forest environments. Additionally, tidal power and geothermal energy are far away from mass energy production. At last, carbon-free energy production can be achieved in nuclear energy but gain a vast life risk during a disaster and handling nuclear waste is a big challenge. Therefore, wind energy gains significance in the technological and political community in fighting climate change without compromising the modern depend and national economy.

In Denmark, 28% of wind energy is generated at total consumption in 2018 [1], and wind energy capacity almost doubled in 2020, where China had a major part of 72 GW [2]. The wind farm located in China (Gansu Wind Farm) with a capacity of 7965 MW is the world’s largest and the second-largest is located in India (Muppandal Wind Farm) with a capacity of 1500 MW [3]. Wind energy production is increasing globally by installing wind turbines in large offshore farms located in agricultural lands, valleys and hills. In addition, onshore wind turbines on the sea bed and new initiatives for installing wind turbines in urban areas (University Campus or highway street lights) [4].

The power extraction from the wind is by converting the wind energy into useful mechanical energy by rotating the turbine or through vibration. The latest research trends in wind energy are in the construction of horizontal wind turbines (liftbased rotation), vertical wind turbines (drag-based rotation) and bladeless wind turbines (aero-elastic-based vibration). The other significant research focused on the pattern of sitting wind to gain more aerodynamic efficiency to get more power output in farm and urban areas, the aerofoil and flow control mechanism in the blade increase the power output efficiency and decrease the cutoff wind velocity. The major challenges in wind energy are turbine transportation and installation, especially in the hilly area, bird’s attack in turbines, the need for extensive land acquisition and recycling of retired wind turbines.

1.1 Wind and wind turbine history

The wind played an important role in ancient civilization in developing sailing boats, kites, agriculture, and metrology. In the ancient period, there are a lot of myths about wind being raised and a hole in the sky which blew it from the sky to earth. In Greek mythology, the God of the Sea, Aeolus, is a guardian of the wind. Feng Po (Wind God) had a sack with an opening that controlled the wind in China. In 3500 BC, Egyptians used wind power to sail the boat in the Nile river, and in Persia, 500 BC millstone, the water pump is driven using wind power. In 1300–1850 AD windmill was designed for water pumping and large-scale milling, which is similar to modern wind turbine design [5]. In 1887, a wind turbine was firstly used to generate electricity built by Prof. James Blyth in Scotland. In 1900, 30 MW of power was generated with around 2500 windmills in Denmark. A Smith Putnam 75—feet wind turbine blade generated 1.25 MW of power for local energy needs gained colossal importance and possibilities in the wind energy sector. In 1975, a wind turbine was developed by NASA—with a composite material blade with pitch control, steel tube tower installed with aerodynamics and structural design ignited more possibilities in the max power output and led to building large wind turbines for energy production [6]. Today, the Sea Titan three-bladed wind turbine can generate 10 MW of power with a rotor diameter of 190 m.

2. Aerodynamics basics

Aerodynamics is a branch of fluid dynamics, the study of the motion of air with forces and moments that act on the body. Aerodynamics plays a vital role in the flight of the aeroplane and helicopter, rocket technology, designing high speed and fuel-efficient cars, reducing the drag on the athlete in sports events and a lot more engineering applications. For example, the aerodynamics of the wind turbine is an important area to increase power output and design a large turbine blade.

2.1 Aerodynamics forces and moments

The forces and moments on the body are due to pressure and shear stress distribution (Figure 1). The pressure acts perpendicular to the surface, which acts as a load on the wind turbine and shear stress is the frictional force tangential to the surface. The pressure difference between the bottom of the blade and the top of the blade generates the lift force (Eq. (1)) (perpendicular to freestream velocity) the wind turbine blade generates the power by rotating the generator.

Figure 1.

Aerodynamic forces in the aerofoil.

$$\mathbf{L}=\frac{1}{2}\rho V^{2}S{C}_L$$E1

Where,

L = lift force (N)

$\rho$ = density of air (Kg/m³)

V = wind velocity (m/s)

S = blade span area (m²)

C_L = coefficient of lift

The lift force on the wind turbine blade is proportional to the square of the wind velocity gains essential parameters in the wind energy generation. The blade span area depends on the length and width of the blade throughout the cross-section and C_L depends on the shape (aerofoil selection) and orientation (pitch angle) of the blade. Aerofoil is a streamlined, cross-sectional blade shape that produces high lift compared to other shapes. The aerofoils were first used in the aeroplane wings to generate lift and are now widely used for energy production. Terminologies define the shape of the aerofoil, the frontal part is the leading edge and rearward part is the trailing edge, and the chord line is the straight line connecting the leading edge to the trailing edge. A mean locus between the upper and bottom of the aerofoil is the mean camber line and the maximum distance between the chord and camber line is called camber. The well-known and basic aerofoil series is NACA aerofoil. NACA stands for National Advisory Committee for Aeronautics, which designed aerofoil with a series to understand the shape [7] easily. For example, in a NACA-4 series aerofoil, the first digit represents maximum camber at 0–9.5% chord, the second digit represents the location of maximum camber at 0–90% chord and the last two-digit represents the thickness of the aerofoil at 1–40% chord. NACA-0012 is a symmetrical aerofoil with zero camber at 12% chord thickness. NACA-2412 is asymmetrical aerofoil with 2% chord of maximum camber, location of camber at 40% of chord, and 12% thickness of chord.

2.2 Reynold’s number

Reynolds number (Re) is a non-dimensional number used to predict the behavior of the fluid at varying environments and used to model the scale-down model [8]. The Reynolds number is named after Irish-born Osborne Reynolds, who predicted the different flow patterns by inducing die in the pipe flow. Reynolds number (Re) is the ratio of inertial force to viscous force (Eq. (2)).

$$\mathbf{Re}=\frac{\mathit{\rho VD}}{\eta }$$E2

Where,

Re = Reynolds number

$\rho$ = density of air (Kg/m³)

V = wind velocity (m/s)

D = characteristic length or diameter (m)

η´ = dynamic viscosity of air (Pa.s/Kg m⁻¹ s⁻¹)

The flow pattern is differentiated into laminar flow and turbulent flow. Both possess different characteristics in nature. Laminar flow is a smooth and regular streamline pattern, whereas turbulent flow is a random and irregular flow pattern. The critical Reynolds number is 5 × 10⁵ transition between the laminar to turbulent flow over a flat plate.

2.3 Boundary layer

In 1904, Ludwig Prandtl developed the theory boundary layer [9], the flow field around the body had two areas where flow is frictional and non-frictional. The boundary layer is the area where the friction of the flow is considered due to viscous characteristics. The thickness of the boundary layer is a distance between the surface to freestream velocity of flow, the velocity at the surface is zero (V = 0) due to shear stress with the surface and air. The velocity will increase with the increase in thickness (Figure 2) in the boundary layer and attain the freestream velocity on point and outside the boundary layer; the flow is considered a non-frictional flow regime. The laminar boundary layer is less thick than the turbulent boundary layer and the turbulent boundary layer will have high kinetic energy and mixing rate.

Figure 2.

Velocity profile in boundary layer.

2.4 Pressure coefficient

Pressure is a dimensional quantity (Eq. (3)) (SI unit N/m²) and important variable to express the force that acts on the body. The pressure must be expressed in the dimensionless quantity pressure coefficient (C_p) like Reynolds number for similarity in aerodynamics.

$$C_p=\frac{P-P_{\infty }}{q_{\infty }}$$E3

To measure the pressure coefficient, the pressure tapping is distributed around the model’s surface in the wind tunnel. To measure the pressure coefficient, on the surface of the model in the wind tunnel the pressure tapping will be distributed around the surface. The tubes will be connected to multi-tube manometer or pressure sensors to measure the pressure difference at the tappings (p) in the surface and the free stream pressure (p_∞). The dynamic pressure is measured through freestream quantity ($\left(q_{\infty }=1/2{\rho }_{\infty }{V}_{\infty }^2\right)$, where $\rho$_∞ is freestream or sea-level density and V_∞ is freestream velocity.

2.5 Generation of lift

Aerodynamics lift is a complex topic for understanding, the lift generated by wings made the heavier than air flight possible. There is much debate on how the wing or turbine creates lift with aerofoil cross-sections. When the fluid flow over an object, the force exited due to the fluid motion where the lift is perpendicular to the freestream and drag is parallel to the freestream. Concentrating on the lift produces a high lift with minimum drag on the streamlined body like an aerofoil.

The aerofoil shape is used in aeroplane wings, wind turbines and propellers to generate the lift and based on the application and need the different aerofoil profiles are used. Consider a wind turbine aerofoil where the wind flow over it causes a pressure distribution with high pressure in the bottom and low pressure on the top cause a lift generation on the turbine to rotate the generator to produce electricity. The shape of the aerofoil creates an uneven pressure when fluid moves over it to generate the lift, but how is the uneven pressure distribution formed on the aerofoil? It is a tricky question to answer. We discuss two widely accepted explanations of lift generation in the aerofoil. The following explanation is based on Newton’s third law of motion, where the fluid nature is considered in lift generation. When fluid flows over an aerofoil, the fluid will suddenly experience the aerofoil where the flow moves upward, called upwash and downward called downwash. Due to the large fluid volume displacement, every action has an equal and opposite reaction, the aerofoil creates lift as a reaction force by turning down the incoming air. In conclusion, the lift is created due to uneven pressure distribution, but the pressure distribution is complex and has a different explanation based on the approach.

We will now discuss how the aerofoil shape and orientation affect lift generation. At freestream velocity V (relative wind) over an aerofoil will generate a lift, drag and moment due to pressure and shear stress distribution. The angle of attack (ρ) is the angle between chord (c) and the relative wind velocity (V_∞) of the aerofoil. The coefficient of lift (C_L) will increase with an increase in the angle of attack till C_Lmax and stalls (lift decrease) due to flow separation on the upper surface. The symmetrical aerofoil (NACA 0012) has a similar shape on both sides of the chord line and C_L = 0 when the angle of attack is zero because pressure distribution will same on both but asymmetrical aerofoil (NACA 4412) will generate lift even at a zero angle of attack (Figure 3a). There are two kinds of stalls based on the aerofoil thickness: leading-edge and trailing-edge stall. Let us compare NACA 4412 and NACA 4421 (Figure 3b), both aerofoils have the same mean camber line and camber location but the thickness varies, NACA 4421 have 10% extra thickness to NACA 4412. In both cases, the aerofoil has the same lift slope, C_L increases with increasing angle of attack but C_Lmax various. In NACA 4412, the flow separation occurs at the leading edge of the aerofoil where the stall will be sudden and cover the entire upper surface, the phenomenon is known as a leading-edge stall. On the other hand, NACA 4421, where the aerofoil thickness is high enough to make the separation occur in the trailing edge and stall will be gradual. The lift curve evident that the NACA 4412 C_Lmax is increased little compared to NACA 4421 where the curve bend over at C_Lmax means that stall was soft and gradual at maximum lift (Figure 3b).

Figure 3.

(a) C_L vs. α for symmetrical and asymmetrical aerofoil. (b) C_L vs. α for thin aerofoil (NACA 4412) and thick aerofoil (NACA 4421).

The flow control technique (flaps and slats) alters the lift slope and increases the C_Lmax. The flaps and slats are also known as high lifting devices, which increase the lift higher than the actual aerofoil lifting capacity. The flap (Figure 4a) is a moving element in the trailing edge that moves up and down when the flap deflects downward camber of the aerofoil increase to shift the lift curve upward to increase C_Lmax. The slat (Figure 4b) will be fixed in the leading edge of the aerofoil which moves front to allow the airflow between the slat and aerofoil to the upper surface to delay the flow separation thereby increasing the C_Lmax in lift slope which is evident that the stall is delayed in high angle of attack.

Figure 4.

(a) C_L vs. α tailing edge flap effect in C_L. (b) C_L vs. α for leading edge slat effect in C_Lmax.

3. Wind turbine

A wind turbine is a mechanical device that converts the kinetic energy of the incoming airflow striking the blade surface, producing considerable lift on the airfoils; thereby, rotation of blades is effected and successfully converted to electrical power through gearbox assembly. According to the mode of operation, wind turbines can be classified as follows.

Each type of wind turbine mentioned in (Figure 5) above can be summarized as:

1. Horizontal axis wind turbine: It is a type of wind turbine in which the rotor’s axis of rotation is parallel to wind flow.

   1. Dutch type grain grinding windmill: It operates at the thrust exerted by wind, and the number of blades in a turbine is four. Wooden slats have been used for making the blades of the turbine.

   2. Multiblade water pumping windmill: Blades of this type of turbine are made of metal or wood and the selection of a site depends on the water availability of the area. It operates at low velocities and is also called a fan mill.

   3. High-speed propeller-type wind machines: The working of this turbine is only dependent on the aerodynamic force generated when wind flows on the airfoil surface of the blade section. They find their applications in the electricity generation of our modern era. The selection of the airfoil section forms the core of the blade design of modern wind turbines.

2. Vertical axis wind turbine: It is a type of wind turbine in which the rotation axis is placed vertical or perpendicular to the ground.

   1. The Savonius rotor: This wind turbine consists of a drum cut into two halves and attached opposite to the vertical shaft. The rotor torque is generated due to wind flow on concave and convex surfaces.

   2. The Darrieus turbine: This type of wind turbine has two or more blades made flexible and attached in the shape of a bow to the vertical shaft. The rolling action of blades generates the torque.

Figure 5.

Classification of wind turbines.

3.1 Components of wind turbine

• Rotor: Rotor blades of wind turbines work under the principle of an aircraft wing. The airflow on their surface creates pressure difference; blades rotate to produce electrical power.

• Nacelle: It forms the housing, which contains gearbox, generator, drive train, brakes, etc.

• Blades: Blade is a critical part of any wind turbine design as they are responsible for lift and power by rotation. The blade section close to the rotor is the hub, whereas the section away from the rotor is the tip of the blade. Hub is designed thicker, and the blade’s tip is thinner to facilitate the airflow.

• Tower: It is designed to hold the rotor blades and whole assembly off the ground. Usually, a tower is constructed 50–100 m above the ground surface or water (in the case of offshore wind turbines).

3.1.1 Wind turbine aerodynamics and flow control

• Brake: The braking system is specifically designed to stop the whole machine when there is a flaw or damage in a component of the turbine. The braking system demands higher cycle rates and reliability. The brake pad of the modern turbine is coated with Kevlar to ensure longevity and robustness.

• Gearbox: The gearbox is used to is to increase the rotational velocity of the low-speed rotor to an electrical generator by gearing arrangement. The gearbox ratio varies from 15:1 to 30:1, depending on the power output of turbines.

• Anemometer: Instrument used to measure the velocity of incoming wind flow, and it transmits the wind speed to the controller.

• Controller: A wind turbine controller is a series of systems connected to monitor the operation of the wind turbine and adjacent turbines (wind farm). It is responsible for the initiation and shutdown of the system in adverse conditions.

• Yaw system: The orientation of the wind turbine towards the incoming wind is done by the yaw system. It has two systems; active and passive yaw systems and comprises mainly of yaw drive, yaw brake, and yaw bearing.

3.2 Design of horizontal axis wind turbine blade

Horizontal axis wind turbine (Figure 6) blades demand a pre-requisite of specific terminologies and mathematical formulas, which converge to a critical section called blade element momentum theory [10]. The preliminary step in blade element momentum theory is dividing the blade into equal sections and let each sectional element has a radius “r.”

Figure 6.

Horizontal axis wind turbine mechanism.

The output power (P) of the blade is determined by (Eq. (4)):

$$\mathbf{P}=\frac{1}{2}\mathit{\rho A}{V}^3$$E4

Where,

A = πR² is the rotor’s surface area (m₂)

V = velocity of incoming wind flow (m/s)

Betz law states that “The power extracted from the wind is independent of wind turbine design in the open flow. Therefore, it is impossible to capture more than 59.3% of Kinetic energy from the wind.” From the Betz law, power is validated from the above equation.

• The angle of attack (α) is defined as the angle between the chord line and incoming wind. The optimal angle of attack of a wind turbine falls in the range of 25°–35°.

• Tip speed ratio: Tip speed ratio of the wind turbine is defined as the ratio of blade tip velocity to the wind velocity as mentioned in (Eq. (5)).

• The tip speed ratio of wind turbines should be greater than 4 for electrical power generation applications. The optimum value for TSR is 6 for a horizontal axis wind turbine blade.

$$\wedge =\frac{V_{\mathit{tip}}}{V}$$E5

• The number of blades (B) is an essential criterion in the power performance of blades. In horizontal axis wind turbines, the number of blades is chosen to be three as it is 40% more efficient when blades are reduced (wobbling) or increased (high drag). In the case of vertical axis wind turbines, blade number varies from 2, 3, or 4 depending on the operating conditions.

• Once the number of blades is fixed, the immediate next step in blade design is evaluating the relative wind angle (r). It is done by:

$$\psi =\frac{2}{3}{tan}^{-1}\frac{1}{{\wedge }_r}\wedge =\frac{V_{\mathit{tip}}}{V}$$E6

In Eq. (6), $\wedge \mathrm{r}={\wedge }_{\ast }\left(\frac{R}{r}\right)$ where,

R = rotor radius, r = radius of element (refer (Figure 7)).

Figure 7.

Schematic representation of blade elements.

The design lift coefficient is measured from the properties of airfoil used in a wind turbine blade. For example, if the analyst uses NACA 4418 airfoil [10] for the wind turbine analysis, the aerodynamic properties of an airfoil can be extracted from the lift curve and lift-drag curve.

• Maximum lift coefficient, (C_Lmax) =1.797

• Critical angle of attack, (α_critical) =15°

• Zero lift angle, α_L = 0 = −4°

• ${\left(\frac{C_L}{C_D}\right)}_{\mathit{max}}$ = 44.44, which occurs at an angle of attack α = 6.5°

• Design lift coefficient, C_L,_design = 1.209

The next step in the design process is the evaluating the chord length of airfoil sections in the blade by using (Eq. (7)) below:

$$\mathbf{c}=\frac{\left(8\mathit{\pi r}\right)\left(1-cos\mathit{\psi r}\right)}{B{C}_{L,\text{design}}}$$E7

Pitch angle (β) is measured theoretically from 0°, and they form the crucial part in the design of blades. For example, the optimum pitch angle for low velocity such as 15 m/s is 20°, and it varies depending on the conditions.

Mathematically pitch angle is calculated using (Eq. (8)) by the difference between blade angle and angle of attack.

$$\beta ={\psi }_r-\alpha$$E8

The twist angle at each section of the blade is calculated using (Eq. (9)) by subtracting the blade pitch with the pitch at the tip:

$${\mathit{\text{θ}}}_{\mathit{\text{τ}}}=\beta -{\beta }_0$$E9

In this expression, β₀ is the blade pitch angle at the tip.

The twist angle reduces from the hub to zero at the tip. From the above data, we can create a table for the geometric design of the horizontal axis wind turbine blade, as shown in Table 1. The geometrical modeling of the blade can be done using commercial software ANSYS (or) SOLIDWORKS.

S. No.	Radius of element (r) [mm]	Chord length (c) [mm]	Twist angle (θT0)
1	r1	c1	θT1(Hub)
2	r2	c2	θT2(Hub)
…	…	…	…
10	r10	c10	θT10(Tip)

Table 1.

Blade geometry.

3.3 Computational analysis

Computational analysis (3D) of the blade is a tedious process as modeling of the blade is a complex process to the core. The computational domain involves a stationary element and a rotational element to perform the moving reference frame approach, as shown in (Figure 8). Moving reference frame involves varied translation and rotational velocities of individual cell zones of the mesh. Stationary equations are generated and solved for stationary element. The rotating element is solved by moving reference frame equations such as centripetal acceleration and Coriolis acceleration in the momentum equation. The flow variables in one zone are extracted to calculate the adjacent zone by transforming the local reference frame in the interface between the cell zones.

Figure 8.

Computational domain of wind turbine blade.

Usually, the computational domain for horizontal axis wind turbine blade is designed as follows.

• Diameter of inner cylinder = 1.5 D

• Length of inner cylinder = 0.5 D

• Diameter of outer cylinder = 5 D

• Length of outer cylinder = 20 D

• Distance between the cylinder and upstream domain = x = 5 D where “D” is the diameter of the rotor.

The meshing of domain involves creating unstructured mesh [11] around the domain with tetrahedral elements as they give good results during the simulation. The exploded view of mesh and meshing elements around the blade (Figure 9).

Figure 9.

Mesh elements of wind turbine blade.

Simulation of the turbine blade is done using commercial software such as ANSYS-FLUENT/CFX. The turbulence model suitable for external flows [12] such as wind turbine flows is the k-ω SST (shear stress transport). In this model, “k” denotes turbulent kinetic energy, and “ω” denotes a specific dissipation rate. This turbulence model is widely used for wind turbine blades as it predicts the flow separation more efficiently than other RANS (Reynolds average numerical solution) models. It also performs well in adverse pressure gradients as it takes the principal shear stress transport into account while solving the equations.

Experimental analysis of wind turbine blades involves modeling and fabrication of blade setup as its preliminary step. Fabrication of blade is done using 3D printing of reinforced composite material.

The velocity profile of the rotor is extracted by fixing a pitot tube with equal holes in the X and Y-axis along the surface. Then, the pressure difference readings can calculate the velocity using (Eq. (10)) derived from (Eq. (3)).

$$V=\sqrt{\frac{2\left(p-p_0\right)}{\left(C_p\right)\rho }}$$E10

3.4 Flow control

Flow control [12] is one of the essential phenomena to be addressed in aerodynamics. As the name says, the flow control mechanism aims to control the flow of wind, thereby delaying the flow separation leading to the generation of lift and power output. Flow control is primarily classified into two types: active flow control and passive flow control mechanism.

Active flow control mechanism involves an instantaneous change in the design of the installation the installed device to increase the $\frac{\text{lift}}{\text{power}}$, whereas passive flow control mechanism [13] involves a fixed surface to influence the flow purely by its geometrical characteristics. The passive flow control mechanism requires a more efficient design process as it does not have the luxury of displacement. This book will discuss one of the simplest and most effective passive flow control devices called vortex generator [14]. The design methodology [15], parameters influencing the design of vortex generators [16] and the aerodynamic effects of the vortex generator [17] are discussed in detail, taking a sample analysis for reference.

Vortex generator was introduced by Taylor [18] during 1947 as thin plates arranged in a spanwise manner projecting on the airfoil surface. Intensive research in Vortex generators had its roots in the 1970s when Kuethe [19] performed analysis on wave-type vortex generators with (h/δ) of 0.27 and 0.42 using the Taylor-Gortler mechanism [20] to create a vortex stream when a concave surface experiences and incoming flow. NASA performed much qualitative research on the design, development and testing of vortex generators [21] and preliminary analysis results suggested vortex generators (Figure 10) as a passive add-on control for Carter model airfoils resulting in efficient momentum transfer. The installed vortex generators on the surface have to be at the height of 1–2% of chord length and length should be approximately 2–3% of chord length with the angle of attack (α) varying from 150 to 200. The vortex generators are placed on the inboard span, outboard span, midspan, and whole span along the surface and the resulting power output is compared as shown in (Figure 11).

Figure 10.

Effect of leading-edge VG on the power curve.

Figure 11.

Triangular Vortex generator.

The performance comparison is shown in (Figure 12) depicts the increment in output power due to the addition of vortex generators. The vortex generators placed in the airfoil surface’s whole span predominantly produce a 6% increase in power output with a mean wind speed of 7.15 m with a counter-rotating arrangement. The optimum dimensions suggested are pair width of vortex generators should be 0.1 c and pair spacing between generators is 0.15 c where “c” is chord length of airfoil. It also deduces vortex generators used to suppress the sensitivity of the blade to dirt accumulation on the leading edge. The following research step is optimizing the design and performance prediction of turbines [22] installing vortex generators [23]. Integrating vortex generators in wind turbines is the next giant leap in aerodynamic research.

Figure 12.

Influence of span-wise location of vortex generator on power output.

Design risks and modifications in the vortex generators are studied [24] thoroughly for different radius as tabulated in Table 2.

Radius	Radius	Modification
0–30 m	Laminar flow is observed at 25% radius. Forward placement of VG leads to early transition and increased drag penalty.	VGs are placed aft outboard of the blade.
0–45 m	Vortex generators are positioned to stall at a velocity range of 14.3–15.6 m/s where a portion of the blade is installed sharply, leading to adverse effects.	The slope of the chordwise VG locations is increased, leading to the smooth progression of the stall.
5–60 m	A stall angle closer to maximum peak rotor power may lead to an unwanted increase in the rotor power. As a result, outboard sections are less significant and sometimes lead to additional drag.	Removal of unwanted outboard vortex generators will compromise the drag penalty.

Table 2.

Design risk and modification for varied dimension.

The design of the vortex generator depends on parameters such as:

1. Height of vortex generator: In most analyses, the boundary layer thickness (δ) of the flow is taken as the height of the generator as it proves to be in good accordance with the results.

2. Spacing between generators: The spacing between a pair of vortex generators depends on the chord length of the airfoil element of the surface and flow characteristics.

3. Position of vortex generator: The position of the vortex generator is fixed by the prediction of flow separation point in the blade surface extracted from the CFD analysis of the blade.

A triangular vortex generator [25] is designed for a wind turbine blade as a sample analysis as it is simple and effective under varied operating conditions.

In a preliminary analysis, one of the airfoil elements in BEM analysis is taken, and the vortex generator is placed at different locations in the chordwise direction. The meshing of an airfoil with VG involves special near-wall mesh. The flow can be captured on the surface without any jumps in this mesh type.

From the wall shear analysis, we can predict the flow separation point, forming the underlying basics for consequent 3-dimensional analysis.

The flow separation point is decided by fixing the vortex generator in different positions on the elemental surface and it is evident from CL vs. angle of attack (Figure 13) and recirculation zone (Figure 14) that the highest lift is obtained when the vortex generator is placed on the flow separation point [26]. The experimental analysis is validated from the CFD analysis to get qualitative results [27].

Figure 13.

Lift coefficient vs. angle of attack.

Figure 14.

Recirculation zone behind the vortex generator.

4. Conclusion

Wind turbine aerodynamics is one of the intriguing sections in the field of aerodynamics with much varied scope in the future years. Wind turbine blade analysis is practically a tedious and challenging area as the design parameters are vast, and each of them has a specified impact on the turbine performance either directly or indirectly. Effects of climatic change, terrain location, the wind rose of a particular area, environmental effects of the wind turbine, impact of blade materials in performance, height of tower and impact of the surrounding environment on the turbine’s performance. Research on offshore turbines and bladeless turbines has started and improvement of performance with considerable cost will be the key objective. The effect of ocean currents, ecosystem, and airflow in the ocean are exciting areas to ponder as energy conservation will be the prime focus for the future. Wind energy, the cheapest energy source, will be looked upon in the immediate future. The chapter gives a preface to the concept of aerodynamics and explains wind turbine terminologies to briefly explain the design and analysis of turbines to form a formidable and appealing pre-requisite for researchers to begin their work on wind turbine analysis.