Wave-free motions of isolated bodies and the existence of motion trapped modes

DV Evans, R Porter

Research output: Contribution to journalArticle (Academic Journal)peer-review

24 Citations (Scopus)


A motion trapping structure can be defined as a freely floating structure under natural or externally applied restoring forces, on or below the free surface of a heavy fluid extending to infinity in at least one direction, which generates a persistent local time-harmonic oscillation of the fluid of finite energy at a particular frequency, due to its own motion at that frequency. Such an oscillation is termed a motion-trapped mode. In this paper it is shown, using accurate numerical computations, that a submerged circular cylinder making forced time-harmonic two-dimensional heave or sway motions of small amplitude in a fluid of either finite or infinite depth, can create a local flow field in which no waves radiate to infinity at particular frequencies and depths of submergence of the cylinder. By tethering such a buoyant cylinder to the bottom of a fluid of finite depth, using a vertical inelastic mooring line, it is shown, by suitable choice of buoyancy and length of tether, how the cylinder, moving freely under its mooring forces, can operate as a motion trapping structure. Such a cylinder would, if displaced from its equilibrium position and released, ultimately oscillate indefinitely at the trapped mode frequency. This simple geometry is the first example of a submerged isolated motion trapping structure free to move under its natural mooring forces.
Translated title of the contributionWave-free motions of isolated bodies and the existence of motion trapped modes
Original languageEnglish
Pages (from-to)225 - 234
Number of pages10
JournalJournal of Fluid Mechanics
Publication statusPublished - Aug 2007

Bibliographical note

Publisher: Cambridge University Press


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