Environmental Engineering Reference
In-Depth Information
Thus, the velocity of the device must always be less than the wind velocity. The power coef-
ficient of a wind power device is defined as the ratio of the power extracted to the wind power
over an area equal to the projected area of the device, or
P
0.5 r U 3 A P
v
U
v
U
2
(5-2a)
C P =
=
1 -
C D
where
C P = power coefficient
The power coefficient for a drag-driven device is a maximum when v / U = 1/3. Thus,
C P , max = (4/27) C D , max
(5-2b)
Figure 5-2. Translating drag device.
Figure 5-3. Translating airfoil with lift and drag forces acting.
Lifting Translator
By contrast, the lifting translator does much better. Figure 5-3 illustrates an airfoil that
is translating at right angles to the wind direction and is subject to both lift and drag forces.
This lifting surface sees a relative velocity that is the vector sum of the free-stream wind ve-
locity and the wind speed induced by translation. The angle from the direction of the relative
velocity to the chord line of the airfoil is termed the angle of attack . Lift and drag forces are
perpendicular and parallel, respectively, to the relative wind and are given by
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