Basic bluff-body aerodynamics - Wind Loading of Structures - page 94

on a smaller volume of wake air, causing the base pressure to decrease further and the
drag to increase. However, as (d/b) increases beyond 0.7, the rear or downstream corners
interfere with the shear layers, and if the length d is long enough, the shear layers will
stabilize, or 're-attach', on to the sides of the prisms. Although the attached shear layers
will eventually separate again from the rear corners of the prism, the wake is smaller for
prisms with long after-bodies (high d/b ), and the entrainment is weaker. The result is a
lower drag coefficient, as shown in Figure 4.9.
4.4.2 Effect of aspect ratio
The effect of a finite aspect ratio (height/breadth) is to introduce an additional flow path
around the end of the body and a means of increasing the pressure in the wake cavity.
The reduced air flow normal to the axis results in a lower drag coefficient for finite length
bodies in comparison to two-dimensional bodies of infinite aspect ratio. Figure 4.10
shows the drag coefficient for a square cross-section with one free end exposed to the
flow, which was smooth (Scruton and Rogers, 1972). The aspect ratio in this case is
calculated as 2h/b, where h is the height, as it is assumed that the flow is equivalent to
that around a body with a 'mirror image' added to give an overall height of 2 h , with two
free ends.
Figure 4.10 Effect of aspect ratio on
drag coefficient for a square cross-
section.
4.4.3 Effect of turbulence
Free-stream turbulence containing scales of the prism dimensions, or smaller, can have
significant effects on the mean drag coefficients of rectangular prisms, as well as
producing fluctuating forces. As shown in Figure 4.4, the effect of free-stream turbulence
on a flat plate normal to an air stream is to increase the drag coefficient slightly
(Bearman, 1971). This results from increased mixing and entrainment into the free shear
layers induced by the turbulence. Observations have also shown a reduction in the radius
of curvature of the mean shear layer position (Figure 4.11). As the after-body length
increases, the drag first increases and then decreases, as occurs in smooth flow. However,
because of the decrease in the mean radius of curvature of the shear layers caused by the
free-stream turbulence, the (d/b) ratio for maximum drag will decrease with increasing
turbulence intensity, as shown in Figure 4.12 (Gartshore, 1973; Laneville et al., 1975).