24
STRUCTURE-EN VIRONMENTAL FORCE INTERACTIONS
as équation (2.10), where CL dépends on cylinder roughness, Reynolds number,
and the proximity of objects nearby.
In a uniform flow field, both séparation and periodic wakes or vortices may
form behind the stationary cylinder. This phenomenon, discussed extensively by
Blevins (1977), is depicted in Figure 2.6. The vortices behind the cylinder detach
alternately. Accompanying this is a periodic pressure fluctuation, top to bottom,
at a characteristic frequency of f3, typically expressed in units of Hz (cycles per
second). Periodic vortices or vortex sheets occur for Reynolds numbers in the
range of 60 to 10,000 and sometimes even higher. (Swimmers can observe this
phenomenon, for instance, by moving a hand downward through the water with
Angers spread and feeling a tendency for the Angers to vibrate horizontally
or side to side).The nondimensional parameter that correlates vortex-shedding
data for the flow of Figure 2.6 is the Strouhal number, deflned by
S = —
(2.12)
u
pattern times: / - l/fs. 2/fs,
(n - 1, 2,...)
pattern times: t = 1.5/fs, 2.5/fs, ...(n+0.5)/fs
(n = 1. 2, ...)
Figure 2.6 Periodic vortices trailing behind a rigid, stationary cylinder.
Generally, S correlates well with the Reynolds number. For instance, corrélations of S with Re showing the apparent effects on vortex-shedding frequency
of a cylinder in proximity to a twin cylinder and a ground plane are reported by
\\ ilson and Caldwell (1971). In some instances, vortex-induced pressure forces
on a cylindrical structure could be large enough to destroy the structure. For
instance, periodic vortices behind the cylindrical piles supporting an offshore
pier lead to a complété destruction of that pier in a tidal current of two knots.
Since vortex-induced loading is unpredictable, one generally makes provisions to
a\oid periodicity of the vortices. Although many methods hâve been proposed
to suppress vortices (Hafen et al., 1976), a particularly practical one is the addition of helical strakes around the cylinder, as shown in Figure 2.7. Optimal
strake geometries for the least vortex-induced loads on circular cylinders were
studied by Wilson and Tinsley (1989).
STRUCTURE-EN VIRONMENTAL FORCE INTERACTIONS
as équation (2.10), where CL dépends on cylinder roughness, Reynolds number,
and the proximity of objects nearby.
In a uniform flow field, both séparation and periodic wakes or vortices may
form behind the stationary cylinder. This phenomenon, discussed extensively by
Blevins (1977), is depicted in Figure 2.6. The vortices behind the cylinder detach
alternately. Accompanying this is a periodic pressure fluctuation, top to bottom,
at a characteristic frequency of f3, typically expressed in units of Hz (cycles per
second). Periodic vortices or vortex sheets occur for Reynolds numbers in the
range of 60 to 10,000 and sometimes even higher. (Swimmers can observe this
phenomenon, for instance, by moving a hand downward through the water with
Angers spread and feeling a tendency for the Angers to vibrate horizontally
or side to side).The nondimensional parameter that correlates vortex-shedding
data for the flow of Figure 2.6 is the Strouhal number, deflned by
S = —
(2.12)
u
pattern times: / - l/fs. 2/fs,
(n - 1, 2,...)
pattern times: t = 1.5/fs, 2.5/fs, ...(n+0.5)/fs
(n = 1. 2, ...)
Figure 2.6 Periodic vortices trailing behind a rigid, stationary cylinder.
Generally, S correlates well with the Reynolds number. For instance, corrélations of S with Re showing the apparent effects on vortex-shedding frequency
of a cylinder in proximity to a twin cylinder and a ground plane are reported by
\\ ilson and Caldwell (1971). In some instances, vortex-induced pressure forces
on a cylindrical structure could be large enough to destroy the structure. For
instance, periodic vortices behind the cylindrical piles supporting an offshore
pier lead to a complété destruction of that pier in a tidal current of two knots.
Since vortex-induced loading is unpredictable, one generally makes provisions to
a\oid periodicity of the vortices. Although many methods hâve been proposed
to suppress vortices (Hafen et al., 1976), a particularly practical one is the addition of helical strakes around the cylinder, as shown in Figure 2.7. Optimal
strake geometries for the least vortex-induced loads on circular cylinders were
studied by Wilson and Tinsley (1989).