Kunihiko Hamagami

Structure of thermal convection development based on inhomogeneous water surface cooling

The generation and development of thermal convection based on inhomogeneous water surface cooling were examined by hydraulic and numerical experiments to examine the characteristics of thermal convection in a closed water body with aquatic plants. A visualization experiment revealed the structural characteristics of a whirlpool when thermal convection was generated quantitatively by using PIV analysis. Then, a water temperature measurement experiment demonstrated that a steady cold water mass generated based on the heat flux transport from the water surface increases. This explained each of the three stages in the convection development process. Moreover, aquatic plants, which grow thickly on the water surface, cause not only vertical but also horizontal flows based on the density difference with the water surface that is not covered by plants, and thus change the development process of the convection cell.

The impact of floating water plants on wind-induced flow and wind wave in a closed water area

In some closed water bodies, the fluids are generally stratified based on vertical density differences. The causes of density these differences can be categorized as being due to temperature, salt levels, and turbidity. In addition, the dynamics of water environmental substances are induced by wind-induced turbulence, which is based on mechanical disturbances, and convective flow, which is based on thermal disturbances. Wind-induced flow and wind waves are produced when mechanical disturbances act on the water surface. These effects in turn generate a lowering of the density interface and increases in the upper layer density based on mixing between upper- and lower-layer fluids, which is commonly referred to as an entrainment phenomenon. This entrainment phenomenon greatly affects the dynamics of water quality in a closed water body. The production of turbulent energy at the water surface and its transport towards the interface play an important role in the dynamics of water environmental substances. When the surface of a closed water body is partially covered with floating water plants, it is considered that wind waves and wind current based on wind action body affected by the plants to some extent, especially in small reservoirs and creeks, which have less surface area for the wind to acts on. We have previously reported that the value of entrainment velocity decreases as the coverage rate of floating water plants increases. It is considered that the covered water surface cause a reduction in the turbulent energy produced by wind shear. It is therefore important to clarify the effects of floating water plants on the wind-induced flow and wind waves generated by wind shear. Accordingly we experimentally considered the turbulent characteristics of wind-induced flow and wind waves in a closed water body with floating water plants. The results indicate that the energy of wind-induced flow is affected by the existence of floating water plants and that the energy decrease as the covereage rate of floating water plants increases.

Impact of luxuriant aquatic plants on thermal convection in a closed water body

The attenuation of heat flux at the water surface with aquatic plants was examined by hydraulic experiments to consider the influence of aquatic plants on the magnitude of thermal disturbance in the closed water body. First of all, the experiment measuring temperatures on surface of leaf revealed the thermal conductivity of the leaf which is peculiar physicality of each aquatic plant. Then, the hydraulic experiment examined attenuation of the underwater illuminance and the heat flux in the covered part which depending on the kind and the luxuriant density of the aquatic plants. As the result, it is clarified that the attenuation rate of the illuminance is large in the plant with a thick leaf, and that the heat flux at the water surface and the developmental velocity of the mixed layer decreased when the luxuriant density of the aquatic plants increased.

Longwave radiation environment of an irrigation reservoir surrounded by mountains

In areas where irrigation facilities are not available or in mountainous areas, reservoirs are used for paddy rice culture in Japan. Water temperature must be adequately maintained because it affects water quality and has a significant impact on the growth and yield of the rice crop. Stratification of water in a reservoir occurs due to heat storage on the water surface by solar radiation, and destratification results from vertical mixing by wind-induced flow and heat convection. This study used a simple topography model to estimate numerically the spatial distribution of longwave radiation on the water surface of an agriculture reservoir surrounded by mountains. A radiation thermometer was used to estimate global longwave radiation. The spatial and temporal variations of longwave radiation on the water surface were calculated using view factors of objects surrounding the reservoir. The simulation showed that longwave radiation absorbed by the water surface increased with distance from the center of the reservoir. The proposed procedure can be used to estimate the longwave radiation environment and heat balance on a water body such as a reservoir and a stream.

Effect of Floating Vegetation on Wind-Wave in a Closed Water Body

In some closed water body the dynamics of substances in the water are affected by windinduced turbulence based on mechanical disturbances and convective flow based on thermal disturbances. Wind-induced flow and waves are produced when mechanical disturbances act on the water surface. The production of turbulent energy at the water surface and its transit through the density interface play an important role in the dynamics of the water environmental substances contained in the water. When the surface in the closed water body is partially covered with floating vegetation, it is considered that wind waves and wind-induced flow should be to some extent affected by the vegetation, especially in small reservoirs, ponds and creeks that have less surface area upon which the wind can act. Therefore, it is important to clarify the effect of floating vegetation on windinduced flow and wind waves generated by wind shear. We experimentally considered the turbulence characteristics of wind-induced flow and wind waves in the closed water body with floating vegetation using a test tank. This presentation includes a discussion of the impact of floating vegetation on wind-induced flow and wind wave characteristics caused by mechanical disturbance in a closed water body. The result indicate that the energy of wind-induced flow is affected by the existence of floating vegetation and that the wind wave characteristics changed as the floating pattern of the vegetation changed.