Analysis on Link Between the Macroscopic and Microscopic Air–Water Properties in Self-Aerated Flows

Self-aeration in high-speed free surface flows occurs commonly and is of interest to ocean engineering, hydraulic engineering, and environmental engineering. For two-phase air–water flows, macroscopic air–water flow properties develop gradually, accompanied by the change of microscopic air–water structures. In this article, representational experimental studies on macroscopic and microscopic characteristics of self-aerated open-channel flows are summarized and compared. The isolated effect of the flow Reynolds number and air quantity on the differences in air count rate and chord size are analyzed and discussed. The results show that the characterized flow depth y, affected by the turbulence transfer, is a specific criterion to distinguish the interior air–water structure development. Two distinct linear trends of self-aeration are found, depending on the y/yvariation with a breaking point at Cmean =0.50. The air count rate and size scale in self-aerated flows are affected by the air quantity of self-aerated flows, even with identical flow Reynolds numbers. Thus, a specific parameter is proposed to assess the air–water structures and a series of self-similarity relationships in self-aeration properties are obtained. The link between macroscopic and microscopic air–water properties results in significant scale effect on air–water structures in self-aerated flows.

A numerical model for air concentration distribution in self-aerated open channel flows

The self-aeration in open channel flows, called white waters, is a phenomenon seen in spillways and steep chutes. The air distribution in the flow is always an important and fundamental issue. The present study develops a numerical model to predict the air concentration distribution in self-aerated open channel flows, by taking the air-water flow as consisting of a low flow region and an upper flow region. On the interface between the two regions, the air concentration is 0.5. In the low flow region where air concentration is lower than 0.5, air bubbles diffuse in the water flow by turbulent transport fluctuations, and in the upper region where air concentration is higher than 0.5, water droplets and free surface roughness diffuse in the air. The air concentration distributions obtained from the diffusion model are in good agreement with measured data both in the uniform equilibrium region and in the self-aerated developing region. It is demonstrated that the numerical model provides a reasonable description of the self-aeration region in open channel flows.

Experimental study of air-water interface properties in self-aerated flows

The microscopic air-water structures and the interface area properties in the self-aerated flows are important research issues in the high-speed self-aerated flows.The present experimental study concerns the mean and medium air chord length distributions in the self-aerated chute flows for different flow Reynolds numbers and air concentration conditions.The relationship between the microscopic and the macroscopic aerated properties in the air-water mixture region is analyzed.The distribution of the microscopic specific air-water interface area with the macroscopic air concentration variation is self-similar in the self-aerated region.In view of the difference of the air-water structure in high and low aerated regions,a new relationship is proposed for predicting the distributions of a specific air-water interface area,and the agreement between the measured and predicted results is satisfactory.

Free-surface air entrainment in open-channel flows

In hydraulic engineering,free-surface aeration is a natural phenomenon occurring in smooth channel flows.In self-aerated flows,a key aspect that has not yet been well understood is the formation mechanism of free-surface air entrainment.In this research,the process of free-surface entrapped deformation is analyzed theoretically and the critical radius of curvature for air entrainment is obtained,affected by flow mean velocity and depth.When the severity of local free-surface deformation exceeds the critical condition,the entrapped free surface encounters closure in the unstable deformation movement process,resulting in air entrainment.This inference agrees well with observed experimental results that are obtained from the processes of surface entrapped deformation and air entrainment captured by a high-speed camera-based data acquisition system.This agreement indicates that self-aeration occurs in low-velocity open-channel flows.It is also confirmed that free-surface turbulent deformation provides a mechanism for air entrainment.

Experimental and CFD investigations of choked cavitation characteristics of the gap flow in the valve lintel of navigation locks

The cavitation is ubiquitous in the water delivery system of high hydraulic head navigation locks.This paper studies the choked cavitation characteristics of the gap flows in the valve lintel of the navigation locks and analyzes the critical self-aeration conditions.The cavitation gap flow in the valve lintel is experimentally and numerically investigated.A visualized 1:1 full-scale slicing model is designed,with a high-speed camera,the details of the cavitation flow is captured without the reduced scale effect.Moreover,the numerical simulations are conducted to reveal the flow structures in the gap.The experimental results show that the flow pattern of the gap flow in the valve lintel could be separated into four models,namely,the incipient(1)the developing,(2),the intensive,(3),and the choked(4)cavitation models.The numerical simulation results are consistent with the experimental data.The choked cavitation conditions are crucial to the gap flow in the valve lintel.When the choked cavitation occurs,the gap is entirely occupied by two cavitation cloud sheets.The gap pressure then decreases sharply to the saturated water vapor pressure at the operating temperature.This water vapor pressure is the ultimate negative pressure in the gap that remains unchanged with the continuous decrease of the downstream pressure.The volumetric flow rate reaches a peak,then remains constant,with the further decrease of the pressure ratio or the cavitation number.At the choking point,the volumetric flow rate is proportional to the root mean square of the difference between the upstream pressure(absolute pressure)and the saturated pressure of the water.Moreover,the pressure ratio is linearly correlated with the downstream cavitation number with a slope of(1+ζc).