Time domain simulation of ship motions

As a matter of fact, strip methods for seakeeping still represent a convenient method for predicting the motion of ships with arbitrary geometry and speed. Three dimensional panel codes for solving the ship motion problem in frequency domain are becoming more and mature to be used as a design tool and even nonlinearities related to the steady basis flow can be incorporated in the formulation. Unfortunately, further inclusions of unsteady nonlinear effects seem impractical and difficulties related to the enforcement of the radiation condition presently limit Rankine panel methods in frequency domain to $\tau=U\omega/g$ high enough. These drawbacks motivate the development of time domain approaches for ships of arbitrary geometry and speed. One possible approach, renewed by King et al. 1988 , 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 allows for the inclusion of nonlinearities. Fully nonlinear potential flow and viscous flow codes are too much time consuming for being of practical use in the near future. Here we present preliminary results concerning our ongoing effort to develop a linear time domain code of general purpose. Having in mind the possible inclusion of nonlinear effects, the algorithm is based on Rankine sources. This requires the discretization of the entire boundary domain (hull and free surface) and to reduce the computational effort a numerical transient test technique is adopted to evaluate with a single run the whole behaviour of the ship. Results concerning two mathematical catamarans and a fast container vessel are presented. In the latter case satisfactory agreement with experimental data is obtained for head and following waves.