Numerical Simulation of Tidal Flow and Density Field around a Mega-Float in a Bay
Yusaku KYOZUKA1 and Changhong HU
1Dept. of Earth System Science and Technology, Kyushu University
2Dept. of Naval Architecture and Marine System, Kyushu University
Keywords Mega-Float, Density Current, Tidal Flow, Numerical Simulation, Multi-Level Model
ABSTRACT
A multi-level model is developed for the simulation of tidal flow and density current around a very large floating structure or a Mega-Float in a bay. Density current is driven by the density stratification and plays an important role in the mass-transport in the ocean. Density field is determined by the distribution of water temperature and salinity, which are calculated from convection and diffusion equations. Effects on the thermal and salinity conditions at the sea surface with/without the Mega-Float are considered in the diffusion equations. Some numerical calculations including a Mega-Float and a breakwater in a simplified bay are demonstrated and the results are presented graphically.
1 INTRODUCTION
A feasibility research project on large-scale floating structures for ocean space utilization, the so called "Mega-Float Project" is now being conducted in Japan. Floating structures are superior to a reclaimed island in some aspects such as having less impact on the ocean environment. Because the scale is huge compared with those which now exist, however, environmental assessment of such a structure in a bay is necessary before construction.
A few numerical studies are found on the effect of a very large floating structure(VLFS) on tidal currents in a bay. Kyozuka1) developed a two-dimensional model which could take a floating structure into account in the tidal calculation and the diffusion of COD(chemical oxi-gen demand) in a bay. The two-dimensional model deals with vertically averaged values and is often used to obtain horizontal distribution of tidal velocities or diffusion of materials. A three-dimensional model is required, however, to investigate the vertical structures of flows and diffusion.
Inoue et al.2) used a multi-level model to study the change of tidal current caused by a pontoon-type floating airport. In their calculation the floating structure is assumed to be fixed, and the pressure underneath it is determined by iterative adjustment of pressure and velocities so that the vertical velocity at the bottom of the structure is zero. Hu and Kyozuka3) extended the multilevel model to simulate tidal current of a bay with a large structure free floating in a vertical direction. In their method, the pressure beneath the structure is obtained by solving the 2-D Poisson equation which is derived by integrating momentum equations vertically.
From those numerical studies, effects of a VLFS on tidal currents and residual currents are expected to be small. However, steady currents are primarily composed of three components: tidal residual currents, wind driven currents and density currents. Density currents are driven by the density stratification of water, which is important in the vertical circulation and transport of materials in the ocean.
Density field is calculated by water temperature and salinity, so that the shadowed area covered by a Mega-Float on the sea surface could cause environmental damage, and this should be clarified in advance.
In this study, the tidal and density currents around a Mega-Float and a breakwater surrounding it are determined considering the effect of the shadowed area on temperature diffusion and the effect of fresh water from rivers on salinity diffusion. The numerical results in a rectangular bay are presented graphically and discussed.
2 NUMERICAL MODEL
2.1 Formulation of the problem
We consider the tidal flow around a Mega-Float in a bay in a rectangular system of coordinates, where the usual assumptions are made for f-plane, Boussinesq and hydrostatic approximations.
