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Aerodynamics of W ind T urbine Blades 51

drop because of the actuator disk theory.

1 2 1 2
Additional pressure drop = ρv = ρ(2ωrb) (4-18)
t
2 2
Similar to the pressure drop from actuator disk theory, this additional
pressure drop will result in expansion of the wake as the tangential
speed slows in the wake. In addition, the tangential component of
velocity will cause the wind to follow a helical path in the wake.
In summary, the actuator disk theory of Chapter 2 stated that the
energy is delivered by the work done by the pressure drop across the
rotor. The exact mechanism within the rotor disk was not required
in the actuator disk theory. The rotor disk theory, on the other hand,
specifies a mechanism—impart of tangential momentum by the rotor
to the wind as it passes through the blades of the rotor. When power
from the two mechanisms is compared, the same value of power co-
efficient is obtained.
In the remainder of the chapter, basic theory of airfoils is devel-
oped.

Lift Force
This section will first begin with an explanation of lift. The fundamen-
tal mechanism for generation of lift forces in an airfoil is still debated.
In this section, the commonly stated mechanisms will be discussed—
some have been proved to be wrong.

Equal Transit Time Fallacy
This theory states that particles of air that separate at the leading edge
of the airfoil will meet at the trailing edge, hence, the equal transit time
theory. This requires that the particles at the top surface travel faster
than the lower surface. Bernoulli’s principle can then be applied: the
pressure at the top surface will be lower than the lower surface and
this causes lift.
The fallacy in this theory is the equal transit time. This has been
shown to be not true; that is, particles that separate at the leading edge
do not meet at the trailing edge. A more accurate method of explaining
lift is presented below.

Rotation Fluid Flow, Circulation, and Vortices
Generation of lift can be explained by circulation. For a circulation of
strength , the lift force is given by:

L = ρv 0 (4-19)