Yusaku Kyozuka

Slow drift damping due to drag forces acting on mooring lines

Abstract Hydrodynamic forces acting on mooring lines are essential in the case of a slow drift oscillation of a floating structure, because drag forces acting on mooring lines act as the damping force and have a great effect on the slow drift oscillation. In this paper a time domain Modified Approximate Method for calculating the dynamic tension of mooring chain is presented. The calculated results of the slow drift oscillation of floating structure and dynamic tensions of mooring lines obtained by using this method are compared with the experimental ones. The calculated results are in good agreement with experimental ones.

Low-Frequency Temperature Variability at Fukue Island Located Southwest of the Tsushima Straits

Sea surface temperature (SST) and sea surface salinity variations at Fukue Island (located southwest of the Tsushima Straits) were investigated. In spring, low-frequency SST fluctuations with periods of 10–20 days predominate. Synthetic analysis of in situ observation and satellite infrared image reveals that these SST fluctuations are caused by movement of mixed warm water masses which have a temperature intermediate between those of the Kuroshio and the East China Sea (ECS) shelf waters. Since these fluctuations do not correspond with those in the Tsushima Straits, it is indicated that these water masses can hardly pass the Tsushima Straits while retaining their original water properties. In July, SST fluctuations with a period of several days are also found at Fukue Island. Since these SST fluctuations show an opposite correspondence with its salinity fluctuations and a good correspondence with the SST fluctuations at Okinoshima in the Tsushima Straits, it is inferred that warm and low-salinity water originated from the ECS shelf water causes these fluctuations and intrudes into the Tsushima Straits.

Tidal Current Power Generation Making Use of a Bridge Pier

There are several advantages to make use of a bridge pier for the tidal power generation. Current velocity increases near the pier, therefore the tidal power generation becomes more efficient because the power is proportional to the cubic of the current velocity. The pier is convenient for access and maintenance of the hydraulic turbine and the power unit. The project is now underway at the Ikitsuki bridge in Tatsuno-Seto strait of Nagasaki Prefecture, where the tidal current was measured by the bottom mount ADCP for three months. The numerical simulations for the tidal current were conducted by the MEC ocean Model, and the results were compared with the measurements. A long term estimation of the tidal current power is presented and the variation of the power by the lunar period is discussed. Model experiments for a Darrieus water turbine with three vertical blades were carried out in the water circulating channel, in which the power coefficients of the turbine are studied as a function of blade section and the mounting angle. Those experimental results are discussed to obtain an optimum Darrieus turbine for tidal power generation.

A numerical study on environmental impact assessment of Mega-Float of Japan

A numerical simulation of the tidal flow and ecosystem in the sea around a very large floating structure or a Mega-Float, developed in the first phase of the Mega-Float project in Japan is presented. The model consists of two parts, a hydrodynamic model and a marine ecosystem. Tidal currents, water temperature, salinity and water density are calculated in a bay with/without a Mega-Float in the hydrodynamic model. A marine ecosystem model including nutrients, phytoplankton, zooplankton and organic matters has been developed in detail. Simulation results with/without a Mega-Float of 4.75 km length, 1.5 km breadth and 1.2 m draft in Tokyo Bay are described. Changes of marine environmental factors are studied due to the installation of certain configurations of the Mega-Float and breakwater. Lastly, some potential technologies for environmental preservation and restoration are evaluated.

Numerical and experimental study on a floating platform for offshore renewable energy

This paper presents recent experimental and numerical work on dynamic analysis and load prediction of a floating platform. A new offshore renewable energy platform is designed for the second stage on-sea experiment of Kyushu University. An experiment is carried out in the towing tank with a 1/50 scale model, to verify the hydrodynamic performance of the platform and to prepare a benchmark database for validation of the numerical simulation method. The in-house CFD code, RIAM-CMEN, is extended for numerical simulation of the platform in harsh sea conditions. Numerical simulation is carried out and validated against the experiment.Copyright

A Numerical Simulation of Physical Environments in the Ariake Sea Including the Effects of the Tidal Flat

The Ariake Sea surrounded by Nagasaki, Kumamoto, Saga and Fukuoka prefecture, Japan has a huge tidal flat area and very high productivity. However, human activities such as reclamation of this tidal flat and loading from the rivers have polluted its environments. As a result, the number of red tides and amount of oxygen deficient water in the Ariake Sea and the area where those phenomena occur has increased from 1980s. The red tide by Rhizosolenia imbricata that occurred November, 2000 caused particularly severe damage to the laver cultivation. The real cause for these phenomena has not clearly been identified, But resolution of these problems requires a physical and biochemical study of the environments. As the first step in such an approach, the authors developed a method to reproduce the physical environments with high accuracy by using numerical simulation which considers the effects on tidal flat and compares them with the observed physical environments. The simulation shows good agreement with the observations made in the Ariake Sea itself.Copyright

A feasibility study on the water quality improvement by making use of underground water in Hakata Bay

Hakata Bay is an enclosed bay adjacent to Fukuoka City and the eutrophication of the bay water has been getting serious by the sewage contamination in recent years. In this study, we propose a plan to improve the water quality by making use of the underground water upwelling by plunged pipes into the bottom of the bay. The density of the underground water is smaller than sea water that the vertical circulation and the upwelling of the bottom water by the entrainment effects will be generated in the bay, which may improve the water quality of the bay. Field measurement results of the water quality and the quantity of the underground water together with the laboratory experiments and numerical simulations of the diffusion of the underground water in the bay are presented in this paper.

An Ocean Model Using Cut-Cell Method

An ocean model using cut-cell method has been developed to simulate the flow field around the complicated topographic feature in the three dimensions. The finite-volume method is applied for the accuracy of the mass-and momentum-conservations. Formulations for the finite difference are given for the advection, pressure and viscous terms, including the continuity equation. As the first application, the present model is applied for the analysis of the tidal sea-surface elevation in a bay connected with the ocean by a very narrow and oblique channel. Although results by a Cartesian-coordinate model strongly depend on the direction of the channel, the results by the present model show good agreement with the theoretical values. As the second application, the present model is applied for the analysis of the flow around a tall seamount in steady currents. Those results are discussed in comparison with the results by an existing z-coordinate model.

A discontinuous Galerkin finite element shallow water model in simulating tidal flows

The tidal flows are simulated by a discontinuous Galerkin (DG) finite element method (FEM), which can capture shock waves near the discontinuities and conserve the system fluxes during a long term calculation. A nonconforming linear triangular element is used to simplify the discretization of the boundary integration of horizontal diffusion terms, though there is no difference in treating convective terms for any other linear element. The nature of the diagonal mass matrix couples with the special element makes the time integral explicit so that no mass lumping is needed, whereas it does in the usual continuous FEM. The numerical flux between the discontinuous element interfaces is obtained by the HLL approximate Riemann solver, together with an efficient multi-dimensional limiter on a triangular mesh. Numerical results show the robustness of the present model

The structure of flow field at Yokogan-sone seamount

The structure of flow field around a tall seamount, called Yokogan-sone, which is located to the southwest of the Nansei Islands in the East China Sea was investigated with a combination of observations and a numerical model simulation. The flow field around the seamount was preliminarily measured using an acoustic Doppler current profiler (ADCP) in May 2003. It was found that a distinct westward current flowed above the south flank of the seamount at a depth below 200 m while a eastward current attributed to the Kuroshio dominated over the seamount. Current meter moorings around the seamount were deployed for about one month in May 2004 to discriminate contributions of oceanic and tidal currents to the flow field. The results indicated that oceanic currents flowing into Yokogan-sone changed as the Kuroshio flow path dynamically moved around, and tidal currents were variable both temporally and spatially around the seamount. The Princeton ocean model (POM) was used to simulate the flow field around the seamount. The simulation was driven by steady inflow and oscillatory elevation of the sea surface, corresponding to oceanic currents and tidal elevations, respectively. The simulation results successfully reproduced the westward flow above the south flank of the seamount in spite of the dominant eastward steady flow over the structure, whereas they did not show the considerable temporal and spatial variation of strength of the tidal components