Takumi Ohyama

Development of a numerical wave tank for analysis of nonlinear and irregular wave field

Abstract A numerical wave-absorption filter has been developed for an open boundary condition in the analysis of nonlinear and irregular wave evolution. The filter is composed of a simulated sponge layer and Sommerfelds radiation condition at the outer edge of the layer. The wave-absorption characteristics of the filter have been investigated by applying the linear potential theory and a two-dimensional nonlinear boundary element model. In both cases, the filter is found to he applicable for a wide range of wave parameters. In order to realize an idealized “numerical wave tank”, the present model also incorporates a nonreflective wave generator in the computational domain composed of a series of vertically aligned point sources. Numerous numerical experiments demonstrate that the present approach is effective in generating an arbitrary wave profile without reflection not only at the open boundaries but also at the wave generator.

Expanded mild-slope equations for the analysis of wave-induced ship motion in a harbor

Abstract Expanded mild-slope equations incorporating evanescent modes are derived for the analyses of wave-induced ship motion in a harbor. The fluid motion in the whole harbor domain comprising the vicinity of a floating body is represented in vertically integrated form. This achieves much higher computational efficiency as compared to conventional “partially three-dimensional” approaches. The results of fundamental examinations using one- and two-dimensional FEM models indicate both the satisfactory basic performance and sufficient reliability of the present method. Comparison with a partially three-dimensional model shows significant reductions in the computer resources required.

A NUMERICAL METHOD OF SOLITARY WAVE FORCES ACTING ON A LARGE VERTICAL CYLINDER

Tanah Lot Temple is situated in Tabanan Regency - Bali, on the coast of the Indonesian Ocean. Due to continuous wave attack, wind force, and weathering of the rock bank where the Temple stands, abrasion has occured which is more and more threatening the existence of the Temple. Considering that Tanah Lot Temple is a sacred place for the Hindu Balinese people and a place of high cultural value, and also an important tourism, steps to save the Temple are imperative. The Central as well as the Regional Authorities, and also the Bali nese community are very much interested in the effort to keep the Temple intact. Measures have been undertaken to protect both the seaside and land-side banks of the Temple rock bank. This paper only discusses counter measures of the sea —side bank of the Temple.

Modeling the floating-body motion in nonlinear random waves based on vertically integrated wave equations

An efficient numerical model, based on vertically integrated nonlinear-wave equations, is proposed for the analysis of wave-induced floating-body motion in nonlinear random waves. Wave equations adopted here are derived by using a multi-term coupling technique, which favorably represents the vertical structure of the fluid motion with plural vertical-dependence functions. A nonlinear wave-body interaction model, given also in vertically integrated form, is incorporated into the wave equations. The result of fundamental examinations, conducted via comparisons between the present FEM-based numerical model and other analytical and numerical models, demonstrates sufficient reliability and desirable performance of the model proposed.

Structural Optimization of a Semi-Submersible Floating Structure for Offshore Wind Power Generating System

The authors previously proposed a semi-submersible floating structure for offshore wind power generation system, which is mainly composed of three vertical base floaters and horizontal connecting beams. The present study attempts a structural optimization of the draft depth of the connecting beams by a cumulative fatigue damage analysis. Although the wave induced motion was varied according to the draft depth, it does not interfere the operation of wind turbines, however, fatigue damage was much more influenced by the draft depth. The optimum depth of 30 m has been obtained from the viewpoints of structural safety.