Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 375

Figure 6-1. Typical airfoil geometry and aerodynamic forces.
can be induced by minor events, like changes in incoming flow turbulence, vibrating the
airfoil itself, or roughness on the surface [Lissaman 1983]. Fortunately, this type of
irregular airfoil behavior is not usually encountered with wind turbines, so it will not be
necessary to discuss it further.
Typical Airfoil Lift and Drag Behavior
Generally, airfoil behavior is characterized by three flow regimes, as illustrated by the
wind tunnel test data in Figure 6-2 for a representative quadrant of angle of attack, a. The
first of these is the attached regime , for angles of attack from about -15 to +15 deg; the
second is the high-lift/stall-development regime, for angles of attack between about 15 and
30 deg; and, thirdly, the flat-plate, fully-stalled , or deep-stall regime with attack angles
between 30 and 90 deg. These regimes are repeated in the other three quadrants, with
approximate symmetry for drag and anti-symmetry for lift.
Attached Flow Regime
In the attached flow regime, the general airfoil behavior is well understood. Although
it can be complicated and significantly affected by geometrical and viscous parameters like
thickness, camber, nose radius, trailing edge angle, surface roughness , and Reynolds
number , airfoil behavior in the attached flow regime can be very accurately estimated by
the wealth of theory and data accumulated in a half-century of refining two-dimensional
airfoil theory for application to wings. Methods exist for accurate analytical estimation of
the lift and drag forces in this regime, and there is also an extensive literature presenting
experimental data. The upper end of the attached flow regime, around an angle of attack