**3.1 Components of wind turbine**


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


### *3.1.1 Wind turbine aerodynamics and flow control*


#### **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*."

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

$$\mathbf{P} = \frac{1}{2}\rho A V^3 \tag{4}$$

Where,

*A* = *πR*<sup>2</sup> 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.

**Figure 6.** *Horizontal axis wind turbine mechanism.*


$$
\wedge = \frac{V\_{tip}}{V} \tag{5}
$$


$$
\psi = \frac{2}{3} \tan^{-1} \frac{1}{\wedge\_r} \wedge = \frac{V\_{tip}}{V} \tag{6}
$$

In Eq. (6), <sup>∧</sup><sup>r</sup> <sup>¼</sup> ∧∗ *<sup>R</sup> r* where,

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

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, (*CLmax*) =1.797

**Figure 7.** *Schematic representation of blade elements.*


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

$$\mathbf{c} = \frac{(8\pi r)(1 - \cos\psi r)}{BC\_{L,\text{design}}} \tag{7}$$

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.

$$
\beta = \psi\_r - a \tag{8}
$$

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:

$$
\theta\_{\tau} = \beta - \beta\_0 \tag{9}
$$

In this expression, *β*<sup>0</sup> 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.


**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.

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


**Figure 8.** *Computational domain of wind turbine blade.*

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

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**).

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)).

$$V = \sqrt{\frac{2(p - p\_0)}{(C\_p)\rho}}\tag{10}$$

#### **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 lift power , whereas passive flow control mechanism [13] involves a fixed surface to influence the flow purely by its

**Figure 9.** *Mesh elements of wind turbine blade.*

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**).

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

**Figure 10.** *Effect of leading-edge VG on the power curve.*

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

**Figure 11.** *Triangular Vortex generator.*

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.

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

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


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.


#### **Table 2.**

*Design risk and modification for varied dimension.*

**Figure 12.** *Influence of span-wise location of vortex generator on power output.*

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].

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

**Figure 13.** *Lift coefficient vs. angle of attack.*

**Figure 14.** *Recirculation zone behind the vortex generator.*
