Vortex Induced Vibration

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Von Karman Vortex Street behind a cylinder

Vortex-induced vibration (VIV) of ocean structures is a major factor affecting all stages of development of offshore structures (conceptualization, design, analysis, construction and monitoring) and governs the arrangement of risers, details during fabrication, method of installation and instrumentation. Advances to deeper waters in search for crude oil has resulted in multi-billion dollar offshore projects off the Gulf of Mexico, for example: Independence Hub ($2450$m), Atlantis ($2100$m), Nakika ($2000$m), Thunder Horse ($1900$m), Hoover/Diana ($1400$m) to name a few. Under such water depths, long flexible cylinders are increasingly required (umbilics, risers, conductor tubes, pipeline spans), and prediction of VIV response increasingly important. A recent estimate by British Petroleum puts the estimated cost of countering VIV to approximately 10\% of the project cost itself.

Thus study of VIV is a part of a number of disciplines, incorporating fluid mechanics, structural mechanics, vibrations, computational fluid dynamics (CFD), acoustics, wavelet transforms, complex demodulation analysis, statistics, and smart materials.

1 Motivation
2 Current State of Art
3 Major Researchers in VIV
4 External link

[edit] Motivation

They occur in many engineering situations, such as bridges, stacks, transmission lines, aircraft control surfaces, offshore structures, thermowells, engines, heat exchangers, marine cables, towed cables, drilling and production risers in petroleum production, mooring cables, moored structures, tethered structures, buoyancy and spar hulls, pipelines, cable-laying, members of jacketed structures, and other hydrodynamic and hydroacoustic applications. The most recent interest in long cylindrical members in water ensues from the development of hydrocarbon resources in depths of 1000m or more.

Vortex-induced vibration (VIV) is an important source of fatigue damage of offshore oil exploration and production risers. These slender structures experience both current flow and top-end vessel motions, which give rise to the flow-structure relative motion and cause VIV. The top-end vessel motion causes the riser to oscillate and the corresponding flow profile appears unsteady.

One of the classical open-flow problems in fluid mechanics concerns the flow around a circular cylinder, or more generally, a bluff body. At very low Reynold's numbers (based on the diameter of the circular member) the streamlines of the resulting flow is perfectly symmetric as expected from potential theory. However as the Reynold's number is increased the

In spite of most of these complex circumstances, the Strouhal number $S t = f s t D / U$ , after Ce´nek (Vincent) Strouhal (a Czech scientist), where fst is the vortexshedding frequency (or the Strouhal frequency) of a body at rest, D is the diameter of the circular cylinder, and U is the velocity of the ambient flow] emerges as the most robust parameter. When the phenomenon of lock-in happens the Strouhal number reaches a value of 0.2.

[edit] Current State of Art

Much progress has been made during the past decade, both numerically and experimentally, toward the understanding of the kinematics (vice dynamics) of VIV, albeit in the low-Reynolds number regime. The fundamental reason for this is that VIV is not a small perturbation superimposed on a mean steady motion. It is an inherently nonlinear, self-governed or self-regulated, multi-degree-of-freedom phenomenon. It presents unsteady flow characteristics manifested by the existence of two unsteady shear layers and large-scale structures.

There is much that is known and understood and much that remains in the empirical/descriptive realm of knowledge: What is the dominant response frequency, the range of normalized velocity, the variation of the phase angle (by which the force leads the displacement), and the response amplitude in the synchronization range as a function of the controlling and influencing parameters? Industrial applications highlight our inability to predict the dynamic response of fluid–structure interactions. They continue to require the input of the in-phase and out-of-phase components of the lift coefficients (or the transverse force), in-line drag coefficients, correlation lengths, damping coefficients, relative roughness, shear, waves, and currents, among other governing and influencing parameters, and thus also require the input of relatively large safety factors. Fundamental studies as well as large-scale experiments (when these results are disseminated in the open literature) will provide the necessary understanding for the quantification of the relationships between the response of a structure and the governing and influencing parameters.

It cannot be emphasized strongly enough that the current state of the laboratory art concerns the interaction of a rigid body (mostly and most importantly for a circular cylinder) whose degrees of freedom have been reduced from six to often one (i.e., transverse motion) with a three-dimensional separated flow, dominated by large-scale vortical structures.