Vortex Induced Vibration
06.18
Vortex-Induced Vibration, abbreviated as VIV, are
motions induced on bodies facing an external flow by periodical irregularities
on the flow. A simple example of VIV is an underwater cylinder,
offshore pipelines.
VIV happens when the vortices are not formed
symmetrically around the body (with respect to its mid plane), different lift
forces develop on each side of the body, and leading to motion transverse to
the flow.
This motion changes the nature of the vortex formation leading to a limited
motion amplitude.
VIV TYPES
There
are two types of VIV, self-excited oscillations and forced oscillation.
1. Self-excited oscillations
This
type of VIV occurs naturally. For instance, when the vortex-shedding frequency
and the natural frequency are approximately the same. This is the real VIV,
vortex-induced vibration.
2. Forced oscillations
This
VIV occurs at velocities and amplitudes which are preset and can be controled
indepedently of fluid velocity. This is not the “real” VIV, this is
vibration-induced vortices.
In order to prevent VIV phenomenon, some offshore
structures are design with strakes to suppress VIV. Strakes can be seen
in the following figure:
Sumber
http://en.wikipedia.org/wiki/Vortex-induced_vibration
http://web.mit.edu/towtank/www/viv.html
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