Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 95

Blade performance B = 3

0.6

0.55

0.5

0.45

E1
E2
Swirl-Glauert
Tip-Wilson
L/D 200
L/D 100
L/D 40

0.4

0.35

0.3

5 5.5 6 6.5 7 7.5 8

TSR

Figure 6: Comparison of swirl drag and profi le-drag losses against measured
values of actual turbines.

despite the fact that the otherwise neglected pressure decrease in the near wake
gives higher c P in the inboard section, Betz' limit is valid globally.
When considering drag losses, one has to imagine that a rotor blade can be regarded
as an aerodynamic device experiencing two forces drag (in fl ow direction) and lift
(orthogonal to that). The lift force and part of the drag force (per unit span) are due to
the pressure around the airfoil. Figure 7 illustrates this. The c P is defi ned as

p

c =

( 14 )

P

2
(2)

r v

where c is the chord of the airfoil.
Here:

D

( 15)

c =

2 ,
(2) · 1

D

r vc

and

L

c =

(16 )

L

2
(2) · 1

r vc

are defi ned. It can be shown that lift gives rise to no loss as it is perpendicular to
the fl ow, which is not the case for drag. A measure of effi ciency for profi les is