Page 79 - Fluid Power Engineering

Aerodynamics of W ind T urbine Blades 57

In addition to the predominant component of velocity that is

paralleltothesurface,thereisarandomcomponentofvelocity
perpendicular to the surface that causes mixing.
As the fluid moves over the shoulder of the airfoil, pressure reaches
its lowest point and the viscous forces become large as the change
in velocity becomes large in the boundary layer close to the surface.
As the fluid moves away from the shoulder and toward the trailing
edge, pressure increases. Higher pressure leads to amplification of
disturbances leading to turbulence. A combination of higher viscous
force and higher pressure causes the boundary layer to separate from
surface at the separation point. This separation causes an imbalance
in the pressure forces along the x-axis resulting in another drag force
called pressure drag.
How can pressure drag be reduced? The answer is, by moving the
boundary layer separation point closer to the trailing edge. The two
primary factors that influence the location of boundary layer separa-
tion point are angle of attack and surface roughness. As the angle of
attack increases, the separation point moves closer to the leading edge.
The pressure drag is proportional to the square of the attack angle.
The drag force is traditionally expressed in terms of a drag coeffi-
cient (C D ) as:

1 2
D = ρSv C D (4-29)
0
2
C D = C fD + C pD (4-30)
where C fD , C pD are coefficients of skin-friction drag and pressure drag.
For normal wind speeds (<< speed of sound), skin-friction drag is
small and the pressure drag is larger. Since C L is a linear function of
α, and C pD is a quadratic function of α, see Fig. 4-18. The relationship
between drag and lift at normal wind speeds and small values of α

FIGURE 4-18 Drag C D
as a function of
attack angle.
0.02

0.01

α in deg
4 8 12