An experimental study on turbulent coherent structures near a sheared air-water interface |
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Authors: | Wang Shuangfeng Jia Fu Niu Zhennan Wu Zhangzhi |
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Institution: | (1) Institute of Mechanics, Chinese Academy of Sciences, 100080 Beijing, China;(2) State Key Laboratory for Turbulence Research, Peking University, 100871 Beijing, China |
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Abstract: | The turbulence structures near a sheared air-water interface were experimentally investigated with the hydrogen bubble visualization
technique. Surface shear was imposed by an airflow over the water flow which was kept free from surface waves. Results show
that the wind shear has the main influence on coherent structures under air-water interfaces. Low- and high- speed streaks
form in the region close to the interface as a result of the imposed shear stress. When a certain airflow velocity is reached,
“turbulent spots” appear randomly at low-speed streaks with some characteristics of hairpin vortices. At even higher shear
rates, the flow near the interface is dominated primarily by intermittent bursting events. The coherent structures observed
near sheared air-water interfaces show qualitative similarities with those occurring in near-wall turbulence. However, a few
distinctive phenomena were also observed, including the fluctuating thickness of the instantaneous boundary layer and vertical
vortices in bursting processes, which appear to be associated with the characteristics of air-water interfaces.
The project supported by the National Natural Science Foundation of China (Grant No.19672070) |
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Keywords: | air-water interface surface shear coherent structures flow visualization |
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