Geoscience Reference
In-Depth Information
(a)
(c)
0.5
6.6 cm
f
= +90
f
=
−
90
0.3
0.5 cm
f
= +23
f
=
4.5 cm
0.1
−
23
(b)
f
= 0
f
0.3
0.9
Wind speed (m s
−
1
)
1.5
u
Figure 21.2
(a) Model leaf used to determine drag coefficient, (b) Specification of orientation of model leaf relative to
wind direction, (c) resulting values of drag coefficient versus angle and wind speed. (Redrawn from Thom, 1968,
published with permission.)
Molecular
diffusion through
boundary layer
Characteristic
dimension of
vegetation
element
Figure 21.3
Representation
of the Nusselt Number (
Nu
)
for a leaf.
Effective
boundary layer
thickness
resistance to the transfer of sensible heat for an element of vegetation with
characteristic dimension
d
is then given by:
d
(21.9)
R
=
H
DNu
H
Nu
has been expressed in terms of empirical relationships with the Reynolds
number for selected shapes such as plates (which broadly compare with leaves)
and cylinders (which broadly compare with coniferous needles) in wind-tunnel
studies, see Table (21.1).
