Page 146 - Wind Energy Handbook

120 AERODYNAMICS OF HORIZONTAL-AXIS WIND TURBINES

3.10.9 Calculated values of induced velocity

The measurement of the induced velocities of a wind turbine rotor in yaw has been
undertaken at Delft University of Technology, (Snel and Schepers, 1995). The tests
were carried out using a small wind tunnel model so that a steady yaw could be
maintained in a steady wind with no tower shadow and no wind shear. The rotor
had two blades of 1:2 m diameter which were twisted but had a uniform chord
length of 80 mm. The blade root was at a radius of 180 mm and the blade twist was
98 at the root varying linearly with radius to 48 at 540 mm radius and remaining at
48 from there to the tip. The blade aerofoil profile was NACA 0012. The rotor speed
was kept constant at 720 rev=min and the wind speed was held constant at 6:0m=s.
Tests were carried out at 108,208 and 308 of yaw angle.
Calculated induced velocities using the vortex momentum equation for the Delft
turbine are shown in Figure 3.64; these are the average values for each annulus
obtained using Equation (3.137) and (3.138).
The component velocities at each blade element, as defined in Figure 3.63, are
shown in Figure 3.65. Because of the rotational speed of the blades the tangential

0.4 0.02

Axial induced velocity 0.2 0˚ of Yaw Tangental induced velocity 0.01 0˚ of Yaw
0.3

0.1

0 30˚ of Yaw 0 30˚ of Yaw
0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1
r/R r/R
Local to the blade
Factored for tip/root loss
Figure 3.64 Azimuthally Averaged Induced Velocity Factors for the Delft Turbine

Normal velocity 1.0 ψ Tangential velocity 10 8
0.8
0.6
0.4
0.2
0 90 180 270 360 6
0 4
0.2 360
0.4 270 2 0.8 1
r/R 0.6 0.8 ψ 180 90 0 0.4 0.6
1 0 0.2 r/R

Figure 3.65 Component Velocities, Normalized with Wind Speed, at 308 of Yaw