Free surface aeration and development dependence in chute flows

Wangru Wei,Jun Deng

doi:10.1038/s41598-022-05588-y

Abstract

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.

Highlights

The microscopic description of the mixture behaviour of air–water flow remains a challenge
For cavitation erosion protection in high-speed open channel flows and air–water transfer of atmospheric gases in natural streams or conduits, the air bubble concentration distribution is of a higher significance because of the net air–water interface area of thousands of tiny bubbles[4,5]
The mixture of water and air parcels resulted in a profile of air concentration distribution in which the air concentration increased from a minimum value at the channel bottom to the free surface

Summary

OPEN Free surface aeration and development dependence in chute flows

The microscopic description of the mixture behaviour of air–water flow remains a challenge. In this study, based on the air–water mixing fluctuation properties in selfaerated chute flows, a prediction model of air concentration distribution related to a theoretical transition depth is developed. Owing to the high turbulence intensity of free surface flows (Fig. 1), water droplets and air bubbles are key flow structures in self-aerated chute flows, and both air–water mixtures and development processes are important considerations in engineering applications. Based on air concentration measurements and flow pattern observations of self-aerated open channel flows, researchers analysed the cross-sectional mixture flow on the basis of “layers”, such as individual water droplets in air at the upper layer and individual air bubbles in water at the lower layer[12,13]. For air and water diffusion generalizations, the approach of mean and fluctuating decompositions for the air concentration and mixture density is used in turbulent self-aerated chute flows. Detailed descriptions of the two turbulent mixing processes combined with multiple fluctuation properties are shown

Air turbulent mixing

Water turbulent mixing

Air concentration distributions

Findings

Additional information