2020 Fiscal Year Final Research Report

Development of a technique for preventing red tide by damping wind waves and promoting turbulent mixing below the wavy air-water interface

Research Project

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Project/Area Number	19K21937
Research Category	Grant-in-Aid for Challenging Research (Exploratory)
Allocation Type	Multi-year Fund
Review Section	Medium-sized Section 19:Fluid engineering, thermal engineering, and related fields
Research Institution	Doshisha University
Principal Investigator	KOMORI SATORU 同志社大学, 研究開発推進機構, 嘱託研究員 (60127082)
Co-Investigator(Kenkyū-buntansha)	高垣直尚兵庫県立大学, 工学研究科, 助教 (00554221) 松田景吾国立研究開発法人海洋研究開発機構, 付加価値情報創生部門(地球情報基盤センター), 研究員 (50633880)
Project Period (FY)	2019-06-28 – 2021-03-31
Keywords	風波気液界面 / 乱流混合 / 気液二相流 / 縦渦 / 表面張力
Outline of Final Research Achievements	Direct numerical simulation was applied to controlled wind waves in which decelerated wind waves and undecelerated wind waves were alternately arranged at regular intervals in the spanwise direction. The predictions show that when the ratio of the width of the decelerated wave region to one interval is about 1/8, longitudinal vortices larger than those observed in pure wind waves without the decelerated region are generated below the air-water interface and fluid mixing is more promoted. In addition, the effects of surface tension on pure wind waves were investigated using the same direct numerical simulation. The predicted wave spectra show that surface tension reduction is not useful for promoting turbulent mixing below the interface. These results suggest that it is a promising technique for preventing red tide and oxygen depletion in the ocean surface to decelerate the surface current by means of zonal meshes like mesh fabric.
Free Research Field	環境流体力学
Academic Significance and Societal Importance of the Research Achievements	風波に減速領域と減速しない領域を風向きに直交するスパン方向に交互に設定することにより風波気液界面下に縦渦を発生させることができ、その結果、通常の風波の場合よりも混合を促進できることを直接数値計算法を用いて明らかにした。また、表面張力を下げた場合には、表面張力波と有義波のエネルギー輸送の観点から乱流が抑制されることを見出した。これらの学術的成果から、格子や網等を用いて人為的に風波を部分的に減速すれば、海洋表層の混合を促進させ沿岸域や湖沼での赤潮や貧酸素化などの水質汚染を防止できる可能性を示すに至った。