Stabilizing bluff structures against oscillation

Method and apparatus for stabilizing an upright structure such as a pile against transverse oscillation due to the relative horizontal motion of the sea past it. Gas is discharged from orifices formed either in a ring duct surrounding the pile, or in the wall of the pile itself. The bubbles tend to rise within the region of low-pressure which forms closely downstream of the pile, so breaking the synchronism of the known vortex-shedding mechanism that can promote the transverse oscillation. A perforated shroud may surround the structure and the bubbles may rise within the gap between structure and shroud. Such a shroud may itself have some stabilizing property even when the gas supply is turned off.

This invention relates to stabilising elongated bluff--typically 
cylindrical--structures against oscillation, and in particular to 
stabilising stationary structures of this kind against being set into 
transverse oscillation due to the phenomenon known as vortex shedding 
which is caused by the relative motion between the structures and the 
fluid environments in which they are located. It is well known for 
instance that tall, lightweight chimneys and stacks are liable to be set 
into such vibration by the relative motion of wind, and that the relative 
motion of the sea can set underwater piles and the supporting legs of oil 
platforms and other marine structures into similar vibration. It should 
also be noted that the phenomenon is not only suffered by 
cantilever-supported structures: comparable oscillations are sometimes 
experienced by structures supported at opposite ends. 
If such transverse oscillations occur at a frequency at which the structure 
resonates, the amplitude of the vibrations may grow and the structure 
eventually fails. 
While it is usually impossible to prevent all transverse vortex shedding 
without modifying the shape of the structure in ways that would otherwise 
be unaccepatable, it has become appreciated that a disastrous resonant 
condition can in practice be avoided by ensuring that the shedding at any 
one point along the length of the structure fails to synchronise with the 
shedding at adjacent points. Many methods of achieving this disturbance of 
synchronism have been proposed, and most of them have involved fitting 
some form of baffle to the structure: for instance various forms of shroud 
or cage, or the spiral "strake" protected by U.S. Pat. No. 3,076,533. Such 
baffles are usually unadjustable once fitted, and have the disadvantages 
firstly that their effect is often far less noticeable at some relative 
velocities of the structure and the surrounding medium than at others, and 
secondly that they almost always increase the normal (as opposed to the 
transverse) forces that the medium exerts upon the structure. 
According to the present invention, means to stabilise a bluff, upright, 
elongated structure against transverse oscillation caused by relative 
motion between the structure and the surrounding fluid medium comprise 
means located on or close to the structure and adapted in use to release a 
second fluid medium of different density from the first, whereby the 
second fluid will be attracted to the low-pressure region that forms 
within the first fluid closely downstream of the structure and then rises 
or falls close to that region, so breaking the synchronism of 
vortex-shedding along the length of the structure. 
Preferably the second fluid medium is less dense than the first and may be 
a gas while the first is a liquid. 
The second medium may be discharged from points spaced around the periphery 
of the structure, for instance from a toroidal or other annular duct 
surrounding the structure. Among other possibilities are that the second 
medium is discharged through orifices formed in the surface of the 
structure itself, and that if the structure is hollow the medium may reach 
these orifices by way of the hollow interior of the structure. 
The structure may also carry another anti-vibration device--for instance a 
shroud of mesh or slatted configuration--which surrounds the structure, 
leaving an annular gap in between, and the device which discharges the 
second fluid may be located closely below this device so that a 
substantial proportion of the gas bubbles that rise from the device enter 
the annular gap.

Tests suggest that the second fluid--air in the examples just 
described--may be said to "decouple" the shear layers of water that would 
otherwise roll up into an alternate vortex street just downstream of the 
structure. Thus the power as well as the synchronism of the 
vortex-shedding would seem to be diminished. With the kind of plain pile 
shown in FIG. 1 tests have shown that if the pile is say two inches in 
diameter and four feet long, and exposed to a water flow rate of up to 
three feet per second, substantial protection against transverse 
oscillation can be obtained by blowing air through two ducts at a supply 
pressure of up to 5 psi gauge. With such test dimensions, it was found 
difficult to obtain useful protection with only a single duct 3. With the 
alternative arrangement as shown in FIG. 2, in which a shroud surrounds 
the structure, without the use of this invention it is common practice to 
surround almost the entire of the under-water length of the structure with 
shroud and to design the latter to match the greatest water velocity which 
the structure may have to withstand. Use of the present invention suggests 
the economy of using a shorter length of shroud, which will be adequate 
for moderate water velocities, and only turning on the air supply to give 
added protection when that velocity rises higher. 
While devices as described to create bubbles of water around fixed, 
underwater structures are an obvious application of this invention, its 
scope is not limited to them. For example it could apply also to devices 
to stabilise the periscopes of submarines moving through still water, and 
the possibility is foreseen of discharging a second fluid medium heavier 
than the first from a point high up the structure so that it falls down it 
instead of rising up.