Effects of discharge-to-submergence ratio on evolution of air-core vortex

An air-core vortex is generated in a tidal power station when the discharge increases or the submergence decreases at the intake. To better understand the complex evolution of the air-core vortex, we conduct high-fidelity coupled level-set and volume-of-fluid and wall-modeled large-eddy simulations for various ratios of discharge and submergence. We characterize the air-core vortex evolution process into two stages: the inception stage and the propagation stage. Our results show that the streamwise and spanwise velocity fluctuations exhibit similar distributions and temporal evolutions and are responsible for generating asymmetrical flow transitioned from a completely symmetrical pattern in the inception stage. The turbulence quadrant decomposition analysis in the confluence region quantifies the velocity fluctuation signal and Reynolds shear stress distribution. Under a specific Reynolds shear stress distribution, eddies converge and evolve into a main vortex. We introduce a vortex burst index to quantitatively characterize the contribution of the Reynolds shear stress to the main vortex evolution. The vortex burst index is positively correlated with the discharge-to-submergence ratio. In the propagation stage, the enstrophy stretching mechanism dominates the air-vortex evolution. The tilting terms nearly cancel each other because of the similar tangential velocity distribution in the axial direction. The results show that the increase in the discharge-to-submergence ratio accelerates the formation of the air-core vortex, owing to the increase in the vertical velocity gradient and the enhancement of the stretching effect.