Numerical study of mechanisms of air-core vortex evolution in an intake flow

The mechanism of the generation and evolution of the air-core vortex formed between a free surface and a horizontal flow intake is investigated using large-eddy simulation of two-phase flows, with the air-water interface captured by a coupled level-set and volume-of-fluid method. The simulation provides a detailed description of the flow features of the air-core vortex formation. Based on the simulation data, the flow patterns and the underlying vorticity dynamics at different stages of the vortex formation are analyzed. It is found that the development of the air-core vortex is associated with the evolution of the vertical component of the vorticity. Analyses of the vorticity evolution dynamics reveal the fundamental mechanism of the air-core vortex generation, which is the amplification of the free-surface vertical vortices due to the vertical stretching effect above the intake associated with the pipe suction. Initially, weak vertical vortices caused by the spanwise disturbance of streamwise flows are randomly distributed near the surface and are enhanced by the stretching effect. Then, these randomly distributed vortices interact with each other and evolve into a dominant vortex, which eventually draws air into the intake and creates a fully-developed air-core vortex. The mechanism responsible for maintaining the fully-developed air-core vortex is still the vertical stretching effect in the core. The tilting from streamwise and spanwise vortices to vertical vortices, albeit large in the presence of the fully-developed vortex, nearly cancel each other and has negligible influence on sustaining the vortex.