The microscopic description of the mixture behaviour of air-water flow remains a challenge. It is not clear how to represent a complex two-phase interaction with the turbulent air-water structure development process. In this study, based on the air-water mixing fluctuation properties in self-aerated chute flows, a prediction model of air concentration distribution related to a theoretical transition depth is developed. The air-water turbulent mixing analysis reveals that the mixture flow depth, at which the local air concentration is 0.5, represents the interior transition boundary. The agreement of the calculated results with the test data confirms that local water and air turbulent mixing through the free surface area should be equally considered. On the basis of the interior transition depth, the development of self-aeration mainly manifests as the air turbulent mixing process in the low aerated region, while the water turbulent mixing in the high aerated region remains mostly unchanged. A series of relationships concerning the development of self-aerated flows is proposed to enable quantitative estimations in practical applications of water engineering.
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