Experimental and Analytical Study on Water Quality Transfer Induced by Wind Shear Stress in Stratified Water Areas

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Experimental and analytical works are carried out to examine flow and water quality transport induced by wind shear stress acting on the surface of stratified water areas. Such a situation is found in many enclosed water areas such as lakes, reservoirs, land-locked bays, where density stratification plays an important roles in fluid motions and the wind shear stress is a major driving force of water mass transport. From a view point of hydrosphere environment it is urged to construct an environmental assessment tool based on a theoretical background of hydrodynamics. In order to meet engineering demands, this research project is organized so as to cover the following three themes ; (1) field and numerical works on wind field properties and formation mechanism of wind-driven circulation, (2) experimental and LES simulation on turbulent structure of shear layr generated by wind stress, (3) three dimensional modeling of flow field in bay areas. The major results are followed.(1) A filed m … More easurement on wind velocity and direction is carried out at the top of a dam surrounded by mountains to investigate its properties as a driving force of reservoir fluid motions. It is found the wind direction is profoundly affected by the local terrain around the reservoir and is almost independent on the wind direction observed at a meteorological station located in a nearby flat area, while the wind speed there has a good correlation with data from the station. From this, it is suggested one need to carefully examine wind fields before theoretically reproducing wind-driven fluid motions in water areas by using wind data. A k-epsilon turbulence modeling is performed to examine the vertical circulation and mixing driven by the wind shear. Through the analysis, it is found how amount of bottom water upwelling, entrainment rate across the thermocline and the boundary mixing intensity depend on the density structure as well on the surface shear stress, which would be a first step of construction of water quality prediction tools.(2) It is known that the wind-driven shear layr developing at the thermocline interface is a major mechanism being responsible for vertical water quality exchange in a stratified water body. For formulating a theoretical model being able to describe turbulence in a shear-dominant flow like wind-driven flows, it is required to find its dynamics. A velocity measurement of shear layr is carried out by using a ultrasonic video camera and PIV image processing method. Some hierarchy of organized turbulent structures are found there, which are quite similar to those observed in wall turbulence. LES performance in reproducing flow structure is quite well ; the result are in almost complete agreement with the experiment. It is investigated how the shear-layr turbulence promotes water quality transfer in wind-driven flow fields.(3) Experimental and numerical investigation are carried out to get hydrodynamics in Osaka and Tokyo Bays. Especially in Tokyo Bay, many water quality troubles arise due to upwelling of anoxic bottom water, which brings serious damages to fishery farms along the coast. A laboratory experiment is carried out to examine mechanism of bottom water upwelling. A functional relationship of the upwelling rate on wind velocity and density structure. A three dimensional model is also parameterized in order to reproduce wind-driven currents in the two bays. Model performance is quite satisfactory ; the simulated flow fields are in good agreement with the field data. It is expected that the model could be of practical use in assessing environmental changes after some construction works in bays and coastal areas.The final report is published and delivered to Japanese institutions of hydraulic engineering in May 1996. Less