Turbulent Structure in a Closed Density Stratified Water Body with Floating Vegetation

Abstract

In a closed water body, fluid is generally stratified by its vertical density difference (temperature, nutrient salts, etc.). These density differences arise due to the limited inflow and outflow in the water body. In the absence of disturbance in the water body, the vertical density stratification is stabilized. The behavior of a substance in the water body is influenced by the mechanical disturbance (wind–induced flow) and the thermal disturbance (convective flow). With the action of the wind on the surface of such the water body, the surface layer becomes turbulent due to the wind–induced flow. Such wind action on the water body with density stratification gives rise to the entrainment phenomenon at the density interface. This phenomenon, which results from the mixture between the upper and lower water layers, affects the water quality in the water body. When there are little disturbances in the water body, the driving force of flow in the water body is absent and water quality deteriorates. Therefore, when we address the water quality problem in a closed water body, it is very important to clarify the characteristics of fluid flow based on disturbances. By the way, water quality problems associated with closed water bodies, including eutrophication, have been studied, and water purification with floating vegetation has been proposed for several water bodies (Masushima 2001, Oki 2001, Tai 2001). The purification ability of floating vegetation is due to a filter effect with regard to suspended solids and an absorption effect with regard to nutrient salts. These abilities are attained when the floating vegetation is eliminated outside the water body (Fujita et al., 2000, Menya et al., 2001). Therefore, in order to enable water purification with floating vegetation, the suitable management of floating vegetation is required. In addition, floating vegetation does not absorb nutrient salts directly, it absorbs inorganic nutrients such as phosphorus that are mineralized by microbes at the bottom of the water body (Michioku et al., 1999, Fujita et al., 2000). Therefore, in order to utilize floating vegetation for purifying the water body, a certain amount of flow is required. However, as mentioned above, in the water body, the flow tends to stagnate and the fluids are generally stratified due to vertical density differences; thus, maintaining water quality becomes problematic. In the water body, wind–induced flow and convective flow may be induced by mechanical and thermal disturbances, respectively. When floating vegetation grows on the water surface, the flow is affected by its luxuriant growth; therefore, it is very important to clarify the relationship between its luxuriant growth and fluid movement in the water body. With regard to the relation between flow in a closed water body and luxuriant growth of a floating vegeTurbulent Structure in a Closed Density Stratified Water Body with Floating Vegetation