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.