Environmental Engineering Reference
In-Depth Information
HAWT operational and design features are presented, including the teetered rotor ,
yawing and yaw stability , blade- and tip-pitch controls , ailerons , transient aerodynamics ,
and vortex generators . Power outputs and aerodynamic loads of medium- and large-scale
HAWTs are presented and compared with theory. The aerodynamic behavior of VAWTs is
examined in a parallel fashion, starting with an analysis of limiting VAWT performance and
then proceeding to a development of the streamtube theory . Comparisons are made between
power output predictions and test results for a medium-scale research VAWT. The effects on
VAWT performance of rotor solidity , blade number , rotor shape , and Reynolds Number are
presented, along with a discussion of starting and stopping. Test data are used to demonstrate
the shape of rotor power curves and the effects of vortex generators .
Translating Aerodynamic Devices
Perhaps the simplest type of wind power
device is one that moves in a straight line
under the action of the wind, like the ice-
boat shown in Figure 5-1. Historically, these
wind-driven translating devices have been
used for propulsion rather than power ex-
traction. Examination of translating lift- and
drag-driven devices can be illuminating for
the aerodynamic analysis of rotary machines,
since a rotating blade element can be consid-
ered as instantaneously translating.
Drag Translator
First, consider a device driven only by
drag forces. Figure 5-2 illustrates the ac-
tion of an elementary drag device in which
the power extracted is the product of the drag
force and the translation velocity. Drag re-
sults from the relative velocity between the
wind and the device, so that
Figure 5-1. An iceboat traveling at a
speed of 27 m/s. (Courtesy of Gougeon
Brothers, Inc., Bay City, MI)
P = Dl v = C D q A p v = [0.5 r ( U - v ) 2 ] C D c l v
(5-1)
where
P = power extracted (W)
D = drag force per unit length of device (N/m)
l = length (spanwise) dimension of device (m)
v = translation velocity (m/s)
C D = drag coefficient ; function of device geometry
q = dynamic pressure = 0.5 r V r 2 (N/m 2 )
r = air density (kg/m 3 )
V r = relative velocity (m/s)
A p = projected area of device (m 2 )
c = width (chordwise) dimension of device (m)
U = steady free-stream wind velocity (m/s)
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