Civil Engineering Reference
In-Depth Information
Figure 4.6
Drag coefficients for two square
plates in series.
The drag force on two flat plates separated by small distances normal to the flow is also a
relevant situation in wind engineering, with applications for clusters of lights or antennas
together on a frame, for example. Experiments by Marchman and Werme (1982) found
increases in drag of up to 15% when square, rectangular or circular plates were within
half a width (or diameter) from each other.
If uniform porosity is introduced, the drag on a normal flat plate or wall reduces, as
some air is allowed to flow through the plate and reduce the pressure difference between
front and rear faces. The reduction in drag coefficient can be represented by the
introduction of a porosity factor,
K
p
,
which is dependent on the solidity of the plate, δ,
being the ratio of the 'solid' area of the plate to the total elevation area as indicated in
Equation (4.9):
C
D,δ
=
C
D
·
K
p
(4.9)
K
p
is not linearly related to the solidity. An approximate expression for
K
p
,
which fits the
data quite well for plates and walls with ratios of height to breadth between about 0.2 and
5, is given by:
(4.10)
