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. 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.
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 Reynolds 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 Reynolds number is increased the flow becomes asymmetric and the so called Kármán vortex street occurs.Von Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over bluff bodies.
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, 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 from what would be expected in a case of resonance).
Types of VIV:
- Self-excited oscillations
This type of VIV is what occurs naturally, i.e., when the vortex-shedding and the natural frequency are the same. (This is the real VIV – this is vortex-induced vibration)
- Forced oscillations
Occurs at velocities and amplitudes which are preset and can be controlled independently of fluid velocity. (This is not the real VIV – this is vibration-induced vortices)\
Source:
0 comments:
Post a Comment