Civil Engineering Reference
In-Depth Information
100
C
D
1
10
6
1
Re
FIGURE 4.20a
Typical relationship between the drag coefficient,
C
D
, and Reynold's
number,
Re
.
the effects of bridge cross-sectional shape as well as the wind flow characteristics.
∗
These coefficients are determined from tests and applied to the design process. If the
dynamic pressure,
p
, is integrated over the surface of the bluff body, it will create
a force,
F
, and a moment,
M
, as shown in
Figure 4.19b.
The force is resolved into
horizontal (drag),
F
D
, and vertical (lift),
F
L
, forces. The equations for the forces and
moments can then be expressed in a form similar to Equation 4.35 as
C
D
1
V
u
A
RD
,
F
D
=
2
ρ
(4.36)
C
L
1
V
u
A
RL
,
F
L
=
2
ρ
(4.37)
C
M
1
V
u
A
RM
,
M
=
2
ρ
(4.38)
where
C
D
is the dimensionless drag coefficient that depends on span geometry and
Reynoldsnumber,
Re
.TheReynoldsnumberisindicativeofwindflowpatternsrelated
to inertial effects (
Re
large and
C
D
small) and viscous effects (
Re
small and
C
D
dimensionless lift coefficient,
C
M
is the dimensionless moment coefficient,
=
ρ
V
u
L
D
μ
Re
,
L
D
is a characteristic length of the bridge or object for drag,
A
RD
is a characteristic
area of the bridge or object for drag,
A
RL
is a characteristic area of the bridge or object
for lift,
A
RM
is a characteristic area of the bridge or object for moment, and
μ
is the
dynamic wind viscosity.
∗
Wind flow characteristics are described by the Reynolds number on a characteristic geometry, which is
dependent on wind velocity and viscosity.
