In fluid dynamics, vortex-induced vibrations (VIV) are
motions induced on bodies interacting with an external fluid flow, produced by
– or the motion producing – periodical irregularities on this flow.
Vortex induced vibration is a major cause of fatigue failure
in submarine oil and gas pipelines and steel catenary risers. Even moderate
currents can induce vortex shedding, alternately at the top and bottom of the
pipeline, at a rate determined by the flow velocity. Each time a vortex sheds,
a force is generated in both the in-line and cross-flow direction, causing an oscillatory
multi-mode vibration. This vortex induced vibration can give rise to fatigue
damage of submarine pipeline spans, especially in the vicinity of the girth
welds.
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| Flow Pattern Around Submarine Pipeline |
A classical example is the VIV of an underwater cylinder.
You can see how this happens by putting a cylinder into the water (a
swimming-pool or even a bucket) and moving it through the water in the
direction perpendicular to its axis. Since real fluids always present some
viscosity, the flow around the cylinder will be slowed down while in contact
with its surface, forming the so-called boundary layer. At some point, however,
this boundary layer can separate from the body because of its excessive
curvature. Vortices are then formed changing the pressure distribution along
the surface. When the vortices are not formed symmetrically around the body
(with respect to its midplane), different lift forces develop on each side of
the body, thus leading to motion transverse to the flow. This motion changes
the nature of the vortex formation in such a way as to lead to a limited motion
amplitude (differently, then, from what would be expected in a typical case of
resonance).
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