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

对于CO_2、O_2、NO_x等释放气体在气液界面上的标量输运现象，必须弄清水流自由面区域的湍流特征。风致水波流动的研究已在海岸工程和海洋科学中进行过。弄清空气和水之间的气体交换机制尤为重要。在以往的研究中，条件时空相关分析和u-v象限方法已被广泛应用于明渠切变流动和非波状空气切变流动中的拟序结构研究。这项研究的目的是弄清风驱动切变层中空气-水界面附近的相干结构，并评估从界面水到水体流动的破裂事件对动量输送的影响。实验结果表明，水流是壁面剪切流和轨道运动的复合流，波浪引起的脉动在整个流速脉动中起主要作用。特别是，在PIV原始图像的时间序列中，当人眼检测到空气-水界面附近的爆发事件时，小波滤波分析仅对该时刻的低频波动有明显的增强作用。由此推断，水的动量输运机制是由水-气界面的波动引起的，而不是由小波动的突发性事件构成的。PIV测量还发现，破裂事件与界面附近的非定常波速起伏密切相关。

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.