Environmental Engineering Reference
In-Depth Information
Scale-model blade scattering tests made with microwave signals in an
anechoic
chamber
indicate the relative effects of
airfoil contour, material
,
and
total twist
on back-
scatter efficiency. The results of these laboratory experiments are shown in Figure 9-8 and
lead to the following empirical equations:
h
S
,
H
=
h h
A M
exp( -2.30D )
b
(9-22a)
(9-22b)
h
A
= 0.80
1.00
f or metallic blades
0.41
f or non
-
metallic blades
(9-22c)
h
M
=
where
h
S, H
= signal scattering efficiency of a HAWT blade
h
A
= airfoil contour factor
h
M
= material factor
Db
= total blade twist from root to tip (rad)
If it is necessary to provide lightning protection in the form of metallic strips along a
non-metallic blade, this may increase the scattering efficiency almost to the value of the
corresponding metallic configuration [Sengupta and Senior 1978]. It may be possible to
reduce the scattering efficiency of a metallic blade by coating its surfaces with some radar-
absorbing material or by judicious shaping of the blade itself.
Signal Scattering Efficiency of a Darrieus VAWT Blade
A general scattering model for VAWT rotors has yet to be developed, although some
preliminary results have been obtained for a
Darrieus
(curve-bladed) turbine based on signal
scattering tests around the DOE/Sandia 17-m Darrieus VAWT (Fig. 3-20) and a very simple
hoop model [Sengupta
et al.
1981b], During these tests, it was observed that the strength
of the scattered field was relatively independent of the carrier wave length l
.
Referring to
Equation (9-18a), this suggests that the scatter area
lw
and, therefore, h
S
,
must be
proportional to l
for a Darrieus VAWT. Signal scattering tests in which the receiver was
placed directly between the transmitter and the VAWT at a distance of about 2 rotor
diameters confirmed this suggestion. For these tests, f
RT
=
0 and
B
E
=
1 (because the
second blade is shadowed by the first as well as the central column of the rotor).
Combining Equations (9-16a), (9-18a), and (9-21a), the observed signal scattering
efficiency, h
S,
O
, is
S
,
O
=
Z
O
lz
A
P
h
(9-23)
Figure 9-9 shows observed scattering efficiencies of Darrieus blades as a function of the
wave length normalized by the blade length, which is 24.1 m for the 17-m VAWT. A
linear relationship is found which leads to the following empirical equation:
(9-24)
h
S
,
V
= h
A
h
M
l /
L
where h
S, V
= signal scattering efficiency of a Darrieus VAWT blade
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