Influence of viscosity on radiation forces: A comparison between monohull, catamaran and swath

The behavior of the ship in waves should be a matter of hull optimization in order to maintain required levels of seaworthiness and operability in rough sea. Still nowadays the majority of ship motion prediction codes are based on the inviscid flow solvers; but in case of particular ship types, such as catamarans and SWATHs, the higher computational speed of these methods does not justify their inherent lack of accuracy. The paper aims to demonstrate this through a series of application examples of a new numerical method based on the unsteady solution of the Navier-Stokes equations with a mixture of fluid technique and a free surface capturing method to resolve for non-linear free surface effects. The proposed method overcomes the approximation introduced by empirical corrections factors in potential flow based solvers, allowing for accurate prediction on the real shape of the 2D hull section. The numerical method based on the the open source libraries of OpenFOAM, is used to evaluate the two dimensional radiation forces and the hydrodynamic coefficients of three different cross sections, representative of a monohull, a catamaran and a SWATH and it is validated against available experimental results also in terms of radiated wave amplitudes in the far field. The predicted viscous flow field characteristics are presented in detail in the paper, focusing the attention on the separation effects near the hulls and on the free surface breaking, especially in the inner area between the two hulls. Interesting comparison are finally drawn between the behavior of the three different cross section typologies. Copyright © 2013 by the International Society of Offshore and Polar Engineers (ISOPE).