Environmental Engineering Reference
In-Depth Information
6
Wind Turbine Airfoils and Rotor Wakes
Peter B. S. Lissaman, Ph.D.
Adjunct Professor of Aerospace Engineering
University of Southern California
Los Angeles, California
Introduction
In this chapter we will discuss the detailed characteristics of the
lifting surfaces
which
we know as
airfoils
,
in order to provide guidance in the selection of airfoil shapes that will
perform satisfactorily over the broad operating environment of a wind turbine. In addition,
the fundamental fluid-dynamic principles involved in modeling the structure of rotor wakes
and integrating wake-induced effects over an array of turbines are presented in this chapter.
Wind turbines operate by the action of the
relative wind
(the natural wind plus wind
caused by rotor motion and rotor-induced flow), which creates aerodynamic forces on the
rotating blades. These can normally be grouped into
lift-like forces
and
drag-like forces.
Lift forces operate through the generation of
circulation
and do not involve large viscous
losses in the flow and the associated loss of
total head
,
while drag forces function through
flow separation
on the blade and the loss of total head. In the previous chapter, the relative
merits of lift and drag power devices were discussed, with lift being the clear preference
for wind turbine rotors. When considering the potential impact of wakes on the total output
of a wind power station, a lift-type device is again preferred, since a drag-type unit develops
a greater wake and less energy is available to downwind units. Thus, the array efficiency
of a cluster of drag-type units will be lower than that of lift-type turbines.
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