Air-water interfacial coherent structures in wind-induced open-channel flows and its effect on gas transfer

风致明渠流中气水界面相干结构及其对气体传输的影响

• 批准号: 12450200
• 负责人: NEZU Iehisa
• 金额: $ 6.27万
• 依托单位: KYOTO UNIVERSITY
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (B)
• 财政年份: 2000
• 资助国家: 日本
• 起止时间: 2000 至 2002
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-12450200/
• 关键词: Open-Channel Turbulence; Interfacial Coherent Structure; Wind-Induced Flows; Bursting Phenomena; PIV Measurements; Air-Water Interface; Gas Transfer; LDA Measurements; 気体輸送係数; 閉鎖性水域; 風波; バースト現象

As for the scalar transport phenomena of the issued gases such as CO_2, O_2 and NO_x across an air-water interface, it is essential to clarify the turbulent features in the free-surface region of water flows. Previous studies of wind-induced water-wave flows have been conduced in the coastal engineering and marine sciences. It is especially important to clarify gas exchange mechanism across the air and water. In previous studies, the conditional time-space correlation analysis and the u-v quadrant method have been intensively conducted to investigate coherent structures in sheared flows such as open-channel flows and non-wavy air sheared flows.The present investigation describes an experimental study on turbulent bursting phenomena across air-water interface in wind-induced water waves by using Particle-Image Velocimetry (PIV) in the water layer. The purpose of this study is to clarify coherent structures near the air-water interface in a wind-driven shear layer, and to evaluate an effect on momentum transport due to the busting events from the interfacial water to water-body flow. As the results of experiments, it is shown that the water flow is a combined flow with a wall shear flow and orbital motions, and that the wave-induced fluctuations have major power in the whole velocity fluctuations. In particular, when the bursting events are detected near the air-water interface by human eyes in the time-series of PIV raw images, only the low-frequency fluctuations are enhanced obviously at that time in wavelet-filter analysis. Therefore, it is inferred that the mechanism of water momentum transport is organized by the wave-induced fluctuations from the air-water interface to the water bulk, not by the bursting events with small fluctuations. It is also recognized by PIV measurements that the busting events are closely related to the unsteady wave-induced velocity fluctuations near the interface.

对于CO_2、O_2和NO_x等气体在气-水界面上的标量输运现象，有必要弄清水流自由表面区的湍流特征。以往的研究主要集中在海岸工程和海洋科学领域。因此，弄清气水交换机理尤为重要。在前人的研究中，条件时空相关分析和u-v象限方法被广泛应用于明渠流和无波浪空气剪切流等剪切流中的相干结构研究，本文利用粒子图像测速技术（PIV）对风激水波中气-水界面湍流猝发现象进行了实验研究。本研究的目的是澄清相干结构附近的空气-水界面在风驱动的剪切层，并评估动量传输的影响，由于从界面水到水体流的破裂事件。实验结果表明，水流是壁面剪切流和轨道运动的混合流，波浪引起的脉动在整个流速脉动中占主导地位。特别是当PIV原始图像的时间序列中的爆发事件被人眼检测到的空气-水界面附近时，只有低频波动在当时明显增强，在小波滤波分析。因此，可以推断，水的动量传输机制是由波浪引起的波动从空气-水界面到水体，而不是由突发事件与小波动。PIV测量结果还表明，破裂事件与界面附近的非定常波致速度脉动密切相关。