Vortex induced vibration (VIV) is a phenomenon observed for bluff
bodies in a free stream. The shedding of asymmetric vortices from the
bluff body results in oscillating hydrodynamic forces acting on the body
and thus leading to its vibration. The vibrations cause severe fatigue
damage to structures and thereby reduce the operation life of the
structures. The study of VIV of long flexible cylindrical structures and
the development of VIV suppression methods is therefore an area of
active research interest.
The vortex shedding pattern observed near (left) the Guadalupe
Island off Baja California and (right) the Juan Fernandez Islands near
Chile.
Source : The NASA Earth Observatory website.
Flow passing bluff bodies in nature displays vortex shedding
phenomenon such as the wake pattern observed as air flows past an island
as shown in Fig.1 above. The asymmetric nature of the vortex shedding
apply oscillating forces on the body and thus induce vortex induced
vibration (VIV).
VIV causes severe fatigue damage to long cylindrical structures,
which are widely employed in various ocean engineering applications,
such as marine risers, oil platforms and mooring lines.
Our research in VIV aims to explain the various physical processes
involved and how such knowledge may be used to predict the extent of
fatigue damage to structures or how such vibrations can be suppressed.
One commonly used suppression method is the use of strakes to break down the vortex shedding.
Spring-dashpot model of a cylinder in a flow and the von Kármán
vortex street: top view (left) and side view (right). Left figure
adapted from An Album of Fluid Motion, Van Dyke (1982)
Source : https://anakkelautan.wordpress.com/2014/01/30/advances-in-vortex-induced-vibrations/
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:
Bibliography:
https://nonerieska.wordpress.com/2013/02/01/vortex-induced-vibration-viv-on-offshore-pipeline-2/
https://anakkelautan.wordpress.com/2014/01/30/advances-in-vortex-induced-vibrations/
Dega Damara Aditramulyadi
Student ID : 15512046
Course : KL4220 Subsea Pipeline
Lecturer : Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D.
Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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