Basic bluff-body aerodynamics - Wind Loading of Structures - page 89

In the case of the two-dimensional plate, strong vortices are shed into the wake
alternately from top and bottom, in a similar way to the bluff-body flow shown in Figure
4.1. These contribute greatly to the increased entrainment into the wake of the two-
dimensional plate. Suppression of these vortices by a splitter plate has the effect of
reducing the drag coefficient to a lower value, as shown in Figure 4.4.
This suppression of vortex shedding is nearly complete when a flat plate is attached to
a ground plane and becomes a wall, as shown in the lower sketch in Figure 4.4. In this
case, the approach flow will be of a boundary-layer form with a wind speed increasing
with height as shown. The value of drag coefficient, with U taken as the mean wind speed
at the top of the wall, Ū h , is very similar for the two-dimensional wall and finite wall of
square planform, i.e. a drag coefficient of about 1.2 for an infinitely long wall. The effect
of the finite length of wall is shown in Figure 4.5. Little change in the mean drag
coefficient occurs, although a slightly lower value occurs for an aspect ratio
(length/height) of about 5 (Letchford and Holmes, 1994).
The case of two thin normal plates in series, normal to the flow, as shown in Figure
4.6, is an interesting one. At zero spacing, the two plates act like a single plate with a
combined drag coefficient (based on the frontal area of one plate) of about 1.1, for a
square plate. For spacings in the range of 0 to about 2 h , the combined drag coefficient is
actually lower than that for a single plate, reaching a value of about 0.8 at a spacing of
about 1.5h for two square plates. As the spacing is allowed to increase, the combined
drag coefficient then increases so that, for very high spacings, the plates act like
individual plates with no interference from each other and a combined drag coefficient of
about 2.2. The mechanism that produces the reduced drag at the critical spacing of 1.5 h
has not been studied in detail, but clearly there is a large interference in the wake and
vortex shedding, generated by the downstream plate.
Figure 4.5 Mean drag coefficients on walls in
boundary-layer flow.