Aerodynamics of Wind Turbine

Introduction:

Wind turbine blades are designed according to the aerofoil structure which is commonly used in the design of aeroplane wings. Wind turbine blades are long and slender structures where the span wise velocity component is much lower than the stream wise component, and it is therefore assumed in many aerodynamic models that the flow at a given radial position is two dimensional and that 2-D aerofoil data can thus be applied. Two-dimensional flow is comprised of a plane and if this plane is described with a coordinate system, the velocity component in the z-direction is zero. In order to realize a 2-D flow it is necessary to extrude an aerofoil into a wing of infinite span. On a real wing the chord and twist changes along the span and the wing starts at a hub and ends in a tip, but for long slender wings, like those on modern gliders and wind turbines, Prandtl has shown that local 2-D data for the forces can be used if the angle of attack is corrected accordingly with the trailing vortices behind the wing. Blades are selected according to the aerofoil section which in turn depends on many facts like angle of attack, pitch angle, lift and drag etc. which are associated with a particular aerofoil.

Nomenclature of Aerofoil and basic definitions:

In this section, the nomenclature of aerofoil and all the terms related to it are explained. A key characteristic of an aerofoil is its chord and the others are as follows:

1. Leading edge: This is the point at the front of the aerofoil that has maximum curvature.
2. Trailing Edge: This is defined similarly as the point of maximum curvature at the rear of the aerofoil.
3. Chord Line: This is a straight line connecting the leading and trailing edges of the aerofoil.
4. Chord Length: Chord Length, is the length of the chord line and is the characteristic dimension of the aerofoil section.
5. Pitch angle (α): The angle between the chord of the aerofoil section and the plane of rotation, also called as setting angle.
6. Angle of Inclination (I): The angle between the relative velocity vector and the plane of rotation is called as angle of inclination
7. Angle of incidence (i): The angle between the relative velocity vector and the chord line of the aerofoil. It is also called as angle of attack. It is clear that i = I –α.
8. Lift force: It is the component of aerodynamic force in the direction perpendicular to the relative wind. It is given by FL = (ρ Ab w2 Cl)/2 Newton, where Cl is the dimensionless lift co-efficient and Ab is the blade area in square meters.
9. Drag force: It is the component of aerodynamic force in the direction of relative wind. It is given by Fd = (ρ Ab w2 Cd)/2 Newton, where Cd is the dimensionless lift co-efficient and Ab is the blade area in square meters.
10. Total force (F): The total aerodynamic force of a blade is the sum of the lift force and the drag force.

NACA Profiles

NACA stands for "National Advisory Committee for Aeronautics" which deals with testing and research related to wind turbine blades and aero plane wings. All the blades are given a separate code for example NACA0012, NACA2406 etc. These are four digit series and the digits after the letters NACA indicates the following parameters:

1. First digit: Gives maximum camber as percentage of the chord.
2. Second digit: Gives the distance of maximum camber from the airfoil leading edge in tens of percent of the chord.
3. Last two digits: Gives the maximum thickness of the airfoil as percent of the chord.

For example: The NACA 2412 airfoil has a maximum camber of 2% located 40% (0.4 chords) from the leading edge with a maximum thickness of 12% of the chord. Four-digit series airfoils by default have maximum thickness at 30% of the chord (0.3 chords) from the leading edge. The NACA 0015 airfoil is symmetrical, the 00 indicating that it has no camber. The 15 indicates that the airfoil has a 15% thickness to chord length ratio: it is 15% as thick as it is long.

To make the turbine to rotate the lift force should be more than that of drag force. Here to explain how the drag and lift force varies according to the angle of attack, NACA2404 is considered and graphs are drawn for different angle of attack and are as below:

For angle of attack of 8 degrees the lift co-efficient is maximum and the drag co-efficient is minimum. Hence to get the maximum output from the available energy the angle of attack of 8 degrees is to be maintained. Hence there will be a pitch control in every wind turbine to maintain the angle of attack constant and to extract maximum power from the available energy sources.

Source : Portal Content Team