Seakeeping of frigates by numerical transient tests

Ocean going ships have to operate in a wave environment which is frequently uncomfortable, and sometimes hostile. Main consequences are unsteady motions of ships and structural loading on hulls. On this physical ground, the study of hydrodynamics of vessels advancing through waves is of main concern to naval architects community. Three dimensional panel codes for solving ship motions problem in frequency domain are becoming more and more mature to be used as a design tool. Unfortunately, inclusions of unsteady nonlinear effects seem to be impractical and difficulties related to the enforcement of the radiation condition presently limit Rankine panel methods in frequency domain to reduced frequencies $\tau=U\omega/g$ high enough. These drawbacks lead the scientific community to develope time-domain approaches for ships of arbitrary geometry and speed. One possible approach is based on the Green function satisfying the Kelvin free surface condition. Sclavounos and his group are pursuing the use of free space Green function (i.e. Rankine panel) which, in principle, allows inclusion of some nonlinear effects. Fully nonlinear potential flow and viscous flow codes are too much time consuming for being of practical use in the near future. In this framework, we are developing a linear time domain code of general purpose and here we present some results concerning a frigate and a fast container vessel. The algorithm is based on Rankine sources: this requires discretization of the entire boundary domain (hull and free surface) but leaves open the future inclusion of nonlinear effects. To reduce the computational effort a numerical transient test technique is adopted to evaluate with a single run the whole behavior of the ship.