Masataka Fujino

Theoretical and experimental predictions of the hydroelastic response of a very large floating structure in waves

Abstract A prediction method for the hydroelastic behavior of a very large box-shaped flexible structure in regular waves is proposed. The structure considered is representative of such structures as a floating international airport and thus the horizontal dimensions are expected to be as large as several kilometers in both length and width. In the analysis, the structure is divided into a number of substructures and the continuous deformation is approximated by the succession of a discrete displacement of each substructure. The displacement of each component is determined from the equation of motion of a uniform free-free plate representing the substructure, while taking structural constraints into account as an additional restoring force. The hydrodynamic forces on the substructure are determined by enforcing the normal velocity of the flow to be equal to that of the corresponding body surface. Thus, the fluid motion and the body motion interact with each other, which is termed ‘hydroelastic interaction’, requiring the simultaneous solution of the structure and fluid problems.

On the estimation method of hydrodynamic forces acting on a very large floating structure

The floating structures that may be used for such purposes as an international airport or an offshore city are expected to be as large as several kilometers long and wide. For the estimation of hydrodynamic forces due to waves or motions that will act on such huge structures, a direct application of conventional numerical methods is practically prohibitive, because the required computational burden is enormous. In order to avoid this difficulty, an approximate method is developed in which computational time is drastically reduced without appreciable loss of accuracy. Although a direct application of conventional numerical methods is difficult for the reason that the corresponding structure is so large, the method proposed in this paper exploits the very fact that a structure is very large to simplify the calculation. The effectiveness of the new method is demonstrated in comparisons with results obtained by the direct application of a conventional numerical method.

A study on flow field around full ship forms in maneuvering motion

To estimate the maneuvering ability of a ship, an accurate estimation of the hydrodynamic forces and moment acting on the ships hull is indispensable. For the purpose of developing a numerical method of computing the viscous flow field around a hull and evaluating its validity, the hydrodynamic pressure on the hull and the velocity field were measured. Two full ship models with different hull forms in the aft part were used for the experiment. From the results of pressure measurements, the distribution of hydrodynamic lateral forces was obtained. The simulation method is a numerical solution of the Navier-Stokes equation based on a finitevolume method and applied to the maneuvering motion. The measured and computed results agree qualitatively well, and the method is a valuable tool for estimating the maneuvering ability of a ship. The typical characteristics of the flow field in the steady turning condition are revealed by the numerical simulation, and the mechanism of the relations between hull form, flow field, and hydrodynamic forces are clarified.

Comparison of simulation results and field data on currents and density in Tokyo Bay

A multilevel model was applied to the calculation of permanent current and density variation in Tokyo Bay, and the change of the state of stratification and the accompanying current field was simulated. In the numerical simulation, the observed field data such as wind conditions and atmospheric temperature were used as input to the calculation, and the results were compared with the observed values of currents, salinity, and sea temperature. Comparison of simulation results and observed data revealed that the numerical simulation could describe well the current and density field governed by wind under stratified conditions. In particular, the long-term variations of the vertical structure of salinity and temperature from summer to autumn could be predicted qualitatively, as could the long-term variations of the vertical structure of salinity and temperature from summer to autumn. Additionally, the effects of boundary conditions on the results of numerical simulations were examined. As a result, it was clarified that the simulation results of salinity stratification were strongly affected by the boundary conditions such as river discharge and the vertical structure of salinity at the open boundary adjacent to the outer ocean.

Numerical Simulation of Artificial Purification System by Using Hydrostatic and FULL-3D Combined Model

Ohmura bay is a typical enclosed estuary located in Kyushu, Japan. In the summer season, strong stratification is formed which brings oxygen-dificient water mass in the bottom layer. For the purpose of restoring water quality in the bay, field experiment of an artificial purification system was carried out. In the experiment, a diffusion pump was installed on the bottom of the bay. The instrument draws in the surface water of lower density and rich oxygen, mixes it with the bottom water of higher density and poor oxygen, and diffuses the mixed water upward. The mixed water is expected to spread along the isopycnic as density current, which will cause resolution of anoxic water in the bottom layer and promote the circulation of nutrients. However, it cannot be said the experiment was successful, and detail analysis by numerical simulation should be necessary in order to design more effective purification system. Most of ocean models employ the hydrostatic approximation because the horizontal scale is usually much larger than the vertical scale in oceanic phenomena. In the hydrostatic approximation, dynamic pressure is neglected and momentum equation of vertical direction need not to be solved. But in the present case, around the purification system, hydrodynamic pressure is not negligible and momentum equation of vertical direction must to be solved (called FULL-3D here). In FULL-3D calculation the time of calculation is much longer compared with using hydrostatic approximation. It is almost impossible to calculate the flow of the whole Ohmura bay by FULL-3D approach. The authors developed a new type of ocean model for multi-scale analysis, which conducts hydrostatic analysis for phenomena in wide area and FULL-3D analysis for the detail flow around the interesting object simultaneously. In order to connect the hydrostatic region and FULL-3D region, nested grid system is employed. Using this combined system, the effect of purification system to the whole bay will be investigated accurately.© 2003 ASME

A Numerical Study on Change in the Marine Environment of Tokyo Bay in the Latest 70 Years

A numerical study by means of a marine ecosystem model is conducted to examine the causes of change in the marine environment of Tokyo Bay in the latest 70 years. First, the marine ecosystem model is validated by comparing the predicted water quality such as water temperature, dissolved oxygen (DO), chemical oxygen demand (COD), total phosphorus (T-P) and total nitrogen (T-N) for the year of 1994 with the field data, which has been observed at 48 spots in Tokyo Bay. In addition, topography of Tokyo Bay and the discharge of chemical matters from the rivers are estimated for each year of 1935, 1979, and 1994, and the marine environment in the summer of each year is reproduced by numerical simulation. From comparing the results, it is revealed that the marine environment varies corresponding mainly to change in the discharge of chemical matters, and that the impact of reclamation is about 30% of the impact of change in the discharge of chemical matters.Copyright

A study on real time simulation of oceanophysical environment in Tokyo Bay

A group of university researchers and Technological Research Association of Mega-Float made continuous measurement of temperature and salinity of the sea water around Mega-Float model, moored off Oppama, from August 1996 till December 1998. The purpose of the measurement was to examine the impacts of Mega-Float on surrounding marine environment and it was found that the effects were very small. In the meantime, these observation data is very useful for the verification of the results of numerical simulations which is thought to be one of the useful tools for environmental impact assessment. So we calculated the current flow, water temperature and salinity in the summer of 1996 in Tokyo Bay. In numerical simulation, to compare the results of numerical simulation with observation data directly, we use the observed field data such as the amount of solar radiation, wind velocity and direction, etc. as boundary conditions. Additionally, the effects of boundary conditions on the results of numerical simulations were also examined. It was clarified that the variations of water temperature and salinity were predicted well quantitatively and the simulation results are affected by the boundary conditions such as extinction coefficient of sunlight and wind velocity.

Visualization of the ambient wave field and the elastic responses of a very large floating structure in waves by use of a very small water tank

Very large floating structures of several kilometers long and . wide are now considered as possible alternatives to such land-based big facilities as an airport. Since the principal horizontal dimension is so large that the conventional experimental tanks can not comply with the appropriate similarity law, an attempt is made to carry out experiments in a very small water tank. With the use of a very small tank, the entire wave field around a model or the entire response pattern of the model in waves can be visualized by making use of certain optical techniques, which can not be achieved if the conventional water tanks are used.