Self-aeration in the rapidly- and gradually-varying flow regions of steep smooth and stepped spillways

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• 作者：Gangfu Zhang ; Hubert Chanson
• 关键词：Air entrainment ; Self ; aeration ; Smooth chutes ; Stepped spillways ; Rapidly ; varied flow
• 刊名：Environmental Fluid Mechanics
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：17
• 期：1
• 页码：27-46
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth Sciences, general; Environmental Physics; Hydrology/Water Resources; Classical Mechanics; Hydrogeology; Oceanography;
• 出版者：Springer Netherlands
• ISSN：1573-1510
• 卷排序：17

文摘

In high-velocity chute flows, free-surface aeration is often observed. The phenomenon is called self-aeration or white waters. When the turbulent shear stresses next to the free-surface are large enough, air bubbles are entrained throughout the entire air–water column. A rapidly-varied flow region is observed immediately downstream of the inception point of free-surface aeration. An analytical solution of the air diffusion equation is proposed and the results compare well with new experimental data. Both experiments and theory indicate that the flow bulking spans over approximately 3–4 step cavities downstream of the inception point of free-surface aeration on a stepped chute. Further downstream the void fraction distributions follow closely earlier solutions of the air diffusion equation. The application of the diffusion equation solution to prototype and laboratory data shows air bubble diffusivities typically larger than the momentum transfer coefficient. The result highlights however a marked decrease in the ratio of air bubble diffusivity to eddy viscosity with increasing Reynolds number. The finding might indicate some limitation of laboratory investigations of air bubble diffusion process in self-aerated flows and of their extrapolation to full-scale prototype applications.