S.X. Du

A validation study on mathematical models of speed and frequency dependence in seakeeping

Abstract An extensive theoretical validation exercise is presented into the speed and frequency dependent solutions associated with surface piercing vessels travelling in waves. The basis of the study lies in the formulation of the Greens function satisfying the traditionally posed linearized boundary value problem of an oscillating ship with forward speed and the development/implementation of appropriate numerical schemes of study for solution. Two widely different mathematical models in formulation and numerical algorithm, denoted as methods A and B, are discussed and employed to predict hydrodynamic coefficients, wave loads and responses of a Series 60 form and an NPL monohull. Only a selection of results are shown, but great care was taken in the overall investigation to verify and validate intermediate steps within the separate calculation procedures as well as to compare final predicted values. The extensive qualitative and quantitative agreement of comparable results from methods A and B provides a measure of confidence that the presented findings are solutions to the posed seakeeping problem. The influence of speed and frequency dependence within the context of this study are discussed as well as a preliminary study into their influence in the occurrence of irregular frequencies in the numerical schemes.

An improved matching method to solve the diffraction—radiation problem of a ship moving in waves

On the underlying assumption that the disturbance of the far-field velocity potential caused by the motion of a ship is small and may be linearized, an improved matching method is developed. Two arrays of fundamental singularities are placed inside the ship hull, which satisfy the linear free surface condition outside the truncated fluid domain and the far-field radiation condition. The choice of fundamental singularity depends on the problem under investigation (e.g. pulsating, translating or translating and pulsating source). The unknown strength of these singularities and the near-field velocity potential are determined in a coupled manner. It is shown from numerical examples that the present method provides a more efficient and accurate representation of waves in the far field than conventional matching methods.