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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