Time domain analysis of ship motions by a Rankine panel method

Seakeeping analysis in frequency domain by 3D Rankine panel codes are continuously improved and presently available results (see e.g. Bertram 1998) are closer and closer to the limits of the underlying theory (small time--harmonic perturbations of the steady base--flow). Physical insight and finer experimental reference data are necessary as a guidance for further enhancements of codes which in the future will include nonlinear inviscid effects and, eventually, viscous effects. In both cases a time domain approach to the problem is required and here we present some linear computations aimed to develop the basic tools for future steps in time-domain analysis. We are trying to take advantage from the theory of dynamical systems. By analysing the response of a fluid dynamic system excited by a suitable input some general properties of the system itself can be evaluated even if the phenomenon is nonlinear. In the linear framework, the Fourier analysis of the ship response to transient captive forced motions leads to the classical added and damping coefficients in a broad frequency range. The advantage in performing a transient test is a shorter simulation time with respect to the one required for a set of (time domain) harmonic forced test. Additionally, we experienced reduced difficulties in damping the signals outgoing the truncated physical domain. In the following some linear results for a Wigley hull are obtained by a numerical transient test technique and frequency domain results are recovered together with experimental confirmation. The interaction of an advancing ship with a wave-packet for evaluating the exciting forces and/or the response amplitude operator is under development and will be presented at the Symposium.