Kazunori Hosotani

Warm Ocean Water Intrusion into Kagoshima Bay

From the field observations and the buoy robot data, the warm ocean water is known to intrude into Kagoshima Bay intermittently in winter. We first visualize these phenomena using some satellite thermal infrared images. Then the numerical analysis is performed employing a multi-level finite difference method (FDM). It is found from the numerical analysis that the intrusion occurs when the warm water comes into touch with the bay water at the mouth of the bay. The calculated warm water distribution in the bay is similar to the satellite images and the velocity of the intrusion is the same order with the ones estimated by the buoy robot data and the satellite images. The intrusion occurs even without the tidal effect, the role of the tide is only to rectify some warm water distribution in the bay.

Behavior of the Warm Ocean Water at the South Coast of Kyushu

The warm ocean water filaments are emanated periodically in winter from the meandering Kuroshio Current at the East China Sea. They bring the intermittent warm ocean water intrusion into the Kagoshima Bay well known as ‘Kyucho’ phenomenon. From the satellite SST images, the warm water intrusion is also observed at the Fukiage Seashore. In addition to the warm water intrusions, the cold-water mass at the Osumi Strait is observed. In this study, the numerical analysis is performed to understand these phenomena. A density current is calculated by a multi-leveled finite difference method (FDM). Then, particle trajectories are calculated by the Lagrangian particle tracking method by using the previous current result. Particles are dealt with as passive tracers to be a model of plankton and hyponeuston (i.e. larva, eggs or seeds) of an open sea. Calculated results of the warm water mass show similar features of the SST images, i.e. the warm water intrusions into the bay and the cold-water mass generation. The warm water intrusion occurs when it reaches the mouth of the bay or the edge of the coast. The cold-water mass is concerned with a horizontal vortex shed from the edge of the cape. The cold-water mass is produced in a stagnation region by a horizontal vortex rather than upwelling from the deep ocean. The particle trajectories have a tendency to pile up in the bay and a stagnation region behind of the peninsula. These results supply the fundamental knowledge that contributes to biological problems at the south of Kyushu in winter adequately.

Flow visualization of simple pipe and channel flows obtained by MRI time-slip method

Herein, time-resolved magnetic resonance imaging, a noninvasive medical diagnostic imaging technique, was evaluated as a noncontact measurement tool for intuitively understanding fluid machineries. Simple pipe flows and channel flows are investigated by the 2D time–spatial labeling inversion pulse (2D time–SLIP) method, which can track a labeled water mass and visualize it using two-dimensional images. In this article, moving water masses of steady and pulsating pipe flows in a straight single pipe and a double cylindrical pipe (which are often seen in fluid machines and heat exchangers) are described. Then, abruptly contracting and expanding channels were tested and compared with particle image velocimetry (PIV) measurements or numerical simulations to evaluate their validity. In addition, as a feasibility test, a rotating water wheel and a fluidic diode with a strong swirling flow were tested to estimate this method’s applicability to fluid machines. The results suggest that the time-SLIP method of tracking a labeled water mass is sufficiently accurate for use in simple fluid machinery under low Re number conditions.Graphical abstract

Flow Visualization of Pipe Flows Obtained by MR Imaginary

In this study, magnetic resonance imaging (MRI), a noninvasive medical diagnostic imaging technique, was evaluated as a noncontact measurement method for fluid machinery. In this report, various simple flow fields are investigated, and a labeled water mass is tracked and visualized in two-dimensional images by the time–spatial labeling inversion pulse (time-SLIP) method. In this article, steady and pulsating pipe flows in a straight tube and in abruptly contracting and expanding channels were tested and compared with particle image velocimetry measurements or numerical simulations to evaluate their validity. In addition, as feasibility test, a rotating water turbine and a fluidic diode with a strong swirling flow were tested to estimate this method’s applicability to fluid machines. The results indicate that the time-SLIP method of tracking labeled water mass is sufficiently accurate for use in simple fluid machinery with slow flows.Copyright

Oceanic Water Intrusion into Kagoshima Bay Resulting in Thermal Stratification

Kagoshima Bay is an enclosed bay that has the tendency to undergo eutrophication, and a very important site to describe the behavior of open seawater intrusion. According to the results of the satellite SST (Sea Surface Temperature) image analysis or numerical simulation focused on the warm water intrusion into the bay in winter, a warm water mass originates in the meandering Kuroshio Current and reaches the mouth of the bay making the density flow to act like a semi-geostrophic current that is influenced by the Coriolis effect. However, there is still no information on the oceanic intruding flow during the summer, where it is accompanied by thermal stratification. In this research, satellite remote sensing and numerical simulation were employed to investigate the oceanic water intrusion into Kagoshima Bay during the summer season by using satellite chl-a (chlorophyll-a) images instead of SST. As a result, the distribution of the low-concentration chl-a with the oceanic water intrusion was found to be similar to the intrusion that occurs during the winter season. Furthermore, the numerical simulation was performed under the simple assumption that the thickness of the warm water mass that reaches the southern coast of Kyushu also shows the same tendency as in the winter season. Although the characteristics, such as the intruding speed or intruding pattern, are similar in winter, some features such as the generation of the inflow from the middle layer, showed a different tendency.