Large amplitude forced roll motion in two dimensions experiments and theory

Abstract

The results of a series of two-dimensional large amplitude forced roll motion experiments are presented. Special emphasis has been placed on the effect of bilge comer emergence, deck comer submergence and the effect of vortex-shedding in these experiments. A detailed description of the design of a new forced-roll motion generator is given. With this set-up, experiments were carried out on four basic models: a circular section, circular section with a single fm and rectangular sections with B/T=2.5 and B/T=2.0. For the rectangular sections, the bilge comer radius to draught ratios were varied from 0% to a maximum of about 25 %, which provided different degree of reduction in vortex shedding. The non-dimensional frequency ranged from 0.25 to 1.5, whilst experiments with a maximum roll amplitude in excess of 1.2 radians were performed near the natural frequency of the test sections. Similar tests were carried out under heeled conditions to investigate the rolling of asymmetric forms. From the in-phase and quadrature moment components, the added inertia and damping were derived. Wave damping was calculated from the measured wave amplitude. Higher order moments were also presented. The theoretical investigation, which has concentrated on the wave radiation, was based on both linear and non-linear potential theories. Both theories compared well with other published work at small amplitudes. With the non-linear theory, a maximum amplitude of 0.6 radians was achieved. Good comparisons in wave damping were found between the theories and the experiments when the basic assumptions of the theories were broadly met. This study has significantly increased the amount of hydrodynamic data for two dimensional forced roll motion, which can be readily used to assist the developments and validation of theoretical models. It was found that the hydrodynamic data were greatly affected by the bilge comers and deck comers oscillating near the free-surface, and that roll-damping was very sensitive to the comer radius.

More information

Identifiers

Catalogue record

Export record