In fluid dynamics, vortex-induced vibrations are motions induced on bodies interacting with an external fluid flow, produced by – or the motion producing – periodical irregularities on this flow. A classical example is the VIV of an underwater cylinder. You can see how this happens by putting a cylinder into the water 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 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, 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. VIV manifests itself on many different branches of engineering, from cables to heat exchanger tube arrays. It is also a major consideration in the design of ocean structures. Thus study of VIV is a part of a number of disciplines, incorporating fluid mechanics, structural mechanics, vibrations, computational fluid dynamics, acoustics, statistics, and smart materials.
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 1000 m or more. See also and. Vortex-induced vibration is an important source of fatigue damage of offshore oil exploration drilling, export, production risers, including steel catenary risers and tension leg platform tendons or teathers. These slender structures experience both current flow and top-end vessel motions, which both give rise to the flow-structure relative motions and cause VIVs. 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 Reynolds numbers the streamlines of the resulting flow is perfectly symmetric as expected from potential theory. However, as the Reynolds number is increased the flow becomes asymmetric and the so-called Kármán vortex street occurs. The motion of the cylinder thus generated due to the vortex shedding can be harnessed to generate electrical power. The Strouhal number relates the frequency of shedding to the velocity of the flow and a characteristic dimension of the body. It is defined as and is named after Čeněk Strouhal. In the equation fst is the vortex shedding frequency of a body at rest, D is the diameter of the circular cylinder, and U is the velocity of the ambient flow.
Lock-in range
The Strouhal number for a cylinder is 0.2 over a wide range of flow velocities. The phenomenon of lock-in happens when the vortex shedding frequency becomes close to a natural frequency of vibration of the structure. When this happens large and damaging vibrations can result.
Much progress has been made during the past decade, both numerically and experimentally, toward the understanding of the kinematics 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, 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, 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 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 whose degrees of freedom have been reduced from six to often one with a three-dimensional separated flow, dominated by large-scale vortical structures.
