Abstract
In hydraulic engineering, intense free surface breakups have been observed to develop in high-speed open channel flows, resulting in a mixed air-water layer near the free surface that grows with the development of self-aeration. This region is characterized by a substantial number of droplets coexisting with an induced air layer above. Little information about this droplet layer is currently available and no practicable approach has been proposed for predicting the parameters of the induced air layer based on the related flow structures in the droplet layer. In this research, laboratory experiments were accordingly conducted to observe the detailed droplet layer development in terms of layer thickness, droplet size, and frequency distributions under comparative flow conditions. Based on the simplified droplet layer roughness determined using the experimentally measured mean droplet size, the classical power-law of boundary layer theory was applied to provide an analytical solution for the air velocity profile inside the air layer. The relationship of air layer growth to droplet layer thickness, which is a key factor when determining the air velocity distribution, was also established, and the analytical results were proven to be in reasonable agreement with air velocity profiles presented in the literature. By determining the relationship between droplet layer properties and air velocity profiles, the study establishes a basis for the improved modeling of high-speed open channel flows.
Highlights
Large spillways and discharge tunnels are commonly set in high dams and other hydraulic engineering projects
Air-water flow measurements were used to quantify the relative contributions of the drag effect on the air layer induced above the highly aerated free surface of open channel flows
In the droplet mixture region above the bulk flow, the air-water mixture was characterized by different quantities and sizes of water-dominated flow structures
Summary
Large spillways and discharge tunnels are commonly set in high dams and other hydraulic engineering projects. Unlike for a single phase flow, two-phase structures of high-speed airwater flows in open channels broadly consist of two features: air bubbles and water droplets. For the nonaeration region in hydraulic facilities, a clear air-water interface forms the boundary of the stratified flow, and different scales and features of geometric waves are generated for nonaerated flow conditions [1, 2]. Because significant air motion results from the turbulence of water-dominated structures, the droplets and water waves induce an air flow above the bulk flow. Basic theoretical analyses have determined the turbulence condition for free surface spraying and discontinuity properties, and the airwater surface layer has been identified as an essential factor when determining an appropriate boundary condition for Mathematical Problems in Engineering open channel flow [8,9,10]. The flow structures in the air-water boundary layer, where both air and water interact and interpenetrate, are less well known than the classical wall boundary structures
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