Wind Turbine Aerodynamics and Flow Control

*Karthik Jayanarasimhan and Vignesh Subramani-Mahalakshmi*

### **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.

**Keywords:** aerodynamics, wind turbine, boundary layer, flow control, vortex generator

### **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 fuelefficient 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.

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

Where,

*L* = lift force (N) *ρ* = density of air (Kg/m<sup>3</sup> ) *V* = wind velocity (m/s) *S* = blade span area (m2 ) *CL* = 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 *CL* 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

**Figure 1.** *Aerodynamic forces in the aerofoil.*

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)).

$$\mathbf{Re} = \frac{\rho \text{VD}}{\eta} \tag{2}$$

Where, Re = Reynolds number *ρ* = density of air (Kg/m<sup>3</sup> ) *V* = wind velocity (m/s) *D* = characteristic length or diameter (m) η´ = dynamic viscosity of air (Pa.s/Kg m�<sup>1</sup> 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 � <sup>10</sup><sup>5</sup> 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.

*Wind Turbine Aerodynamics and Flow Control DOI: http://dx.doi.org/10.5772/intechopen.103930*

#### **2.4 Pressure coefficient**

Pressure is a dimensional quantity (Eq. (3)) (SI unit N/m<sup>2</sup> ) and important variable to express the force that acts on the body. The pressure must be expressed in the dimensionless quantity pressure coefficient (*Cp*) like Reynolds number for similarity in aerodynamics.

$$C\_p = \frac{P - P\_{\infty}}{q\_{\infty}} \tag{3}$$

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 ( *<sup>q</sup>*<sup>∞</sup> <sup>¼</sup> <sup>1</sup>*=*2*ρ*∞*V*<sup>2</sup> ∞ , where *ρ* <sup>∞</sup> is freestream or sea-level density and *V*<sup>∞</sup> 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 (*CL*) will increase with an increase in the angle of attack till *CLmax* 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 *CL* = 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, *CL* increases with increasing angle of attack but *CLmax* 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 *CLmax* is increased little compared to NACA 4421 where the curve bend over at *CLmax* means that stall was soft and gradual at maximum lift (**Figure 3b**).

**Figure 3.** *(a)* CL *vs.* α *for symmetrical and asymmetrical aerofoil. (b)* CL *vs.* α *for thin aerofoil (NACA 4412) and thick aerofoil (NACA 4421).*

*Wind Turbine Aerodynamics and Flow Control DOI: http://dx.doi.org/10.5772/intechopen.103930*

**Figure 4.** *(a)* CL *vs.* α *tailing edge flap effect in* CL*. (b)* CL *vs.* α *for leading edge slat effect in* CLmax*.*

The flow control technique (flaps and slats) alters the lift slope and increases the *CLmax*. 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 *CLmax*. 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 *CLmax* in lift slope which is evident that the stall is delayed in high angle of attack.

## **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:

	- a. 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.
	- b. 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.
	- c. 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

#### **Figure 5.**

*Classification of wind turbines.*

electricity generation of our modern era. The selection of the airfoil section forms the core of the blade design of modern wind turbines.

	- a. 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.
	- b. 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.
