Environmental Engineering Reference

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A

a

L

V
(free wind velocity)

B

Figure 3.5

Airfoil section

F
lift

Force

-ve pressure

F
drag

+ve pressure

Free wind speed

Figure 3.6

Lift and drag forces on an airfoil

The usual definitions are as follows:

A
leading edge

B
trailing edge

L
chord line (a straight line between the leading and trailing edges)

a
the angle of incidence between the free wind velocity and the chord line

As the wind passes over the blade its speed increases and pressure reduces on

the top surface. Conversely, the speed reduces and the pressure increases on the

lower surface. This follows from Bernoulli's theorem where for a horizontal airflow:

1

2
r
V
2

þ
p
¼
const
:

ð
3
:
7
Þ

where
V
is the free wind speed,
r
is the air density and
p
is the pressure. The

pressure difference can be represented as shown in Figure 3.6.

The force resulting from this pressure difference can be split into two com-

ponents at right angles to each other,
F
lift
and
F
drag
, the lift and drag forces. The lift

force is perpendicular to the free wind velocity and the drag force is parallel to it.

These forces are then described by lift and drag coefficients
C
l
and
C
d
. The lift

force is proportional to
C
l
and the drag force to
C
d
.

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