Non-linear wave-induced motions of cylindrical-shaped floaters of fish farms

Abstract

This paper addresses the two-dimensional hydrodynamical problem of a floating circular cylinder in waves by means of model tests and numerical simulations. The problem is relevant for floaters of fish farms. Dedicated model tests and computational fluid dynamics (CFD)-simulations, using a presently developed numerical wave tank are presented. Large amplitude sway motion of the cylinder at a wave frequency equal to half the natural sway frequency was observed, both experimentally and numerically. This is argued to be associated with non-linear hydrodynamic effects and instabilities. Further, linear potential flow theory is shown to overpredict the sway motion at resonance of about 500 per cent compared with experiments and simulations. This discrepancy is explained to be mainly attributable to viscous damping caused by flow separation. Higher-order harmonic components of the hydrodynamic forces are significant and should be considered in fatigue life analyses of fish farms.