Wind Turbine Aerodynamics Part A: Basic Principles - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 321

Figure 5-24. Vortex generators on the 3.2-MW Mod-5B HAWT. (a) Typical vane
dimensions (b) Installing counter-rotating pairs of VGs on the low pressure side of the
blade, with ± 20 deg angles of incidence at 10 percent of chord.
Vortex generators provide a simple method of increasing this mixing rate. These devic-
es were first developed at the United Aircraft Corporation and are credited largely to Taylor
[1947] and Bruynes [1951]. In their simplest form, they consist of small vanes projecting
normal to the low-pressure surface of the airfoil at an angle of attack to the incoming flow
(Fig. 5-24). In this configuration they behave like half-wings and generate trailing vortices
from their tips. These vanes can be parallel to one another or arranged with alternate positive
and negative angles of incidence, producing corotating or counterrotating vortices, respec-
tively. They have been used to suppress or delay separation in diffusers, pipe bends, and
wings. It should be noted that by creating an energetic vortex, vortex generators necessarily
incur a drag penalty when separation is not imminent.
The first application of vortex generators to wind turbine blades was performed by the
Boeing Engineering and Construction Company on the 2.5-MW Mod-2 HAWT [Boeing 1982].
This installation and that on the later 3.2-MW Mod-5B HAWT were of the counter-rotating
type. Wind tunnel tests verified that vortex generators would delay stall and increase the maxi-
mum lift coefficient of the moderately-thick airfoils used in these rotors. In addition, vortex
generators helped solve a control instability problem in the Mod-2 caused by intermittent stall-
ing of the airfoil sections near the gap between the fixed and moveable sections of the rotor.
Shown in Figure 5-25 are power curves for the Mod-2 HAWT with and without vor-
tex generators on the blades [Sullivan 1984]. Without vortex generators, control instability
forced a non-optimum pitch setting during operations in below-rated winds, particularly at
the “knee” of the power curve. This resulted in a substantial loss in power when wind speeds
were between 11 and 15 m/s. Vortex generators over the fixed sections of the rotor (the in-
board 70 percent of span) eliminated the control instability and permitted a more-optimum
pitch setting, reducing the rated wind speed by almost 2.5 m/s, compared with previous op-
erations. When vortex generators were also added to the tip sections, improvements in the
lift coefficient further increased power in below-rated winds.