NUMERICAL SIMULATION OF ARITIFICIAL PURIFICATION SYSTEM BY USING HYDROSTATIC AND FULL-3D COMBINED MODEL

摘要

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.