Hydrodynamics of rolling cylinders with and without bilge keels.

摘要

In the field of ocean engineering and ship dynamics, roll motion has always been the most difficult problem to model theoretically because viscous effects cannot be ignored. When determining the extreme response of ships, roll damping is a critical factor which is directly related to the extent of vortex shedding occurring around the hull and is not predicted well by most potential-flow methods.; Until the recent emergence of viscous-flow solvers, roll motions could not be solved numerically with accuracy. A number of empirical methods were used for engineering purposes. In the past few years, new numerical models developed at the University of California have been applied to problems with simple geometry and have yielded impressive results.; In this work, the motion of a two-dimensional rectangular cylinder floating in a free surface is investigated both experimentally and analytically, the latter by using the Free-Surface Random-Vortex Method (FSRVM), a viscous-flow solver. The inviscid-fluid solution of a cylinder with rectangular corners has been studied extensively in the past and provides baseline results. The effect of viscosity can be observed by comparing inviscid and viscous-flow solutions. Since viscous flow patterns are greatly affected by the shape of the bilge in rolling ships, the corner geometry is modified to document the importance of bilge keels on roll.; This work is presented in two major sections. First, the forced rolling motion of the cylinder, hinged at the water level, is investigated for various frequencies yielding hydrodynamic coefficients for roll. Second, the same cylinder, now freely floating, is subjected to incoming beam waves and its response is investigated.; The first set of experiments show that viscosity causes higher damping, and lowers the added-moment of inertia. Similarly, adding bilge keels around the corners increases mostly the damping. In the second case, the effect of viscosity and of the keels is to decrease the roll motion. Drift motion observed in experiments is underpredicted by inviscid calculations, while successfully predicted by the FSRVM model. The second set of experiments demonstrates the importance of the roll-sway coupling when investigating the drift of floating bodies. In both cases, the agreement between experimental and numerical results is excellent.