Civil Engineering Reference
In-Depth Information
representative values of Strouhal number for a variety of cross-sections are shown in
Figure 4.21.
An inclined two-dimensional flat plate with an angle of attack,
α
(Figure 4.7), has a
Strouhal number of about 0.15 based on the breadth,
b,
normal to the flow or (0.15/sin
α
)
based on the chord, c, where
b=c
sin
α
(Chen and Fang, 1996).
The variation with Reynolds number for a circular cylinder is shown in Figure 4.22
(Scruton, 1963; Schewe, 1983). In the sub-critical range, up to a Reynolds number of
2×10
5
, the Strouhal number is quite constant at a value of 0.20. In the critical Reynolds
number range, coinciding with the sharp fall in drag coefficient (see Figure 4.16), the
Strouhal number jumps to 0.3 and then 0.48, although in this range the vortex shedding is
random and not clearly defined. A slightly decreasing Strouhal number to about 0.4, in
the super-critical range, is followed by a fall to about 0.2 again, at a Reynolds number of
2×10
6
. Helical strakes (Figure 4.23) are often used to inhibit vortex shedding and the
resulting cross-wind forces on structures with circular sections such as chimney stacks
(Scruton and Walshe, 1957).
Figure 4.21
Strouhal numbers for vortex
shedding for various cross-sections.
