Page 63 - Fluid Power Engineering

CHAPTER4

Aerodynamics of

Wind Turbine Blades

We had taken up aeronautics merely as a sport. We reluctantly entered
upon the scientific side of it. But we soon found the work so
fascinating that we were drawn into it deeper and deeper.
—Orville and Wilbur Wright

Introduction
Chapter 2 used the actuator disk theory to describe the Betz limit, a
limit on how much energy can be extracted by a rotor-based wind
turbine. In this chapter, a more realistic look at flow of air over blades
will be undertaken. The chapter starts with description of airfoils,
which is the shape of the cross-section of turbine blade. The second
section describes frame of reference for velocity and force vectors on
a rotating blade. Rotor disk theory is described next, which describes
the flow of wind through the rotor, unlike the actuator disk theory.
The following section describes the basic mechanisms associated with
creation of lift and drag forces on blades and creation of torque on the
rotor. The chapter ends with a contrast between a drag- and lift-based
machine.

Airfoils
The cross-section of a wind turbine blade is an airfoil. Figure 4-1 is
a schematic of a symmetrical airfoil. Chord line connects the leading
to the trailing edge. Most airfoils used in wind turbines have a larger
area above compared to below the chord line. A line connecting the
leading and trailing edge that bisects the area of an airfoil is called a
camber line.

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