Page 81 - Fluid Power Engineering

Aerodynamics of W ind T urbine Blades 59

FIGURE 4-20 Coeffi-
cient of drag for C D
different values of
Reynolds number. 0.02 Re=9 million
Re=6 million
Re=3 million
0.01

α in deg
4 8 12

The relationship between Reynolds number and drag is counter-
intuitive: Higher Reynolds number leading to lower drag. A higher
Reynolds number leads to displacement of transition point (from lam-
inar to turbulent) toward the leading edge. This should cause the
separation of boundary layer closer to the leading edge and there-
fore higher drag. In reality, the opposite happens, when the transition
point is closer to the leading edge, turbulence starts early. The turbu-
lence causes energy to be exchanged between layers, which energizes
the layer close to the aerofoil surface. This reenergized layer is better
able to withstand the viscous force at the surface and higher pressure,
which leads to occurrence of separation closer to the trailing edge. 2
3
The shape of aerofoil determines the shape of the lift curve dur-
ing stall in Fig. 4-19. Thicker aerofoil with smooth curvature exhibit
a graceful decline in lift during stall, that is, the curve after stall has
a small negative slope. Thinner aerofoil has a larger negative slope,
similar to the one in Fig. 4-19. The reason for the difference is the loca-
tion of the start of separation. If separation starts at the trailing edge,
then stall is graceful. In contrast, if separation starts in the middle or
closer to the leading edge, then there is a sudden rise in pressure drag.

Drag-Based Turbines
In this section, a lift-based turbine is compared with a drag-based
turbine. As described above, lift force is perpendicular to the relative
velocity of wind as observed by the blade and the drag force is parallel
to the direction of relative velocity. Drag force is essentially the force
that pushes an object.

1 2
Drag force F d = ρ Av C D (4-33)
2