LES-based insights into the instability mechanism of the wall-normal vortex in a vertical hydraulic intake system

Vortices near hydraulic intakes degrade hydraulic machinery performance, adversely affecting the operational efficiency and stability of tidal power plants or pumping stations. In this study, large-eddy simulations of a vertical intake system based on a source-sink model are performed. The formation process of the wall-normal vortex and its velocity distribution characteristics are elucidated. The evolution process can be classified into the vortex-pairing and single-vortex stages, and an empirical model that more accurately characterizes its velocity distribution is proposed based on canonical vortex model. Velocity perturbations in the streamwise and spanwise directions dominate the turbulent kinetic energy due to the intense streamwise-spanwise momentum exchange triggered by vortex interactions during the vortex-pairing stage and vortex meandering during the single-vortex stage. Turbulent kinetic energy production in the vortex-pairing stage is driven by the coupling u^′u^′¯ and ∂u¯/∂x, while in the single-vortex stage, it is sustained by the coupling of w^′w^′¯ and u^′w^′¯ with ∂w¯/∂z and ∂w¯/∂x, respectively. The meandering phenomenon of vortex occurs in single-vortex stage, and the meandering spatial scope conforms to a joint Gaussian probability density function. Further proper orthogonal decomposition analyses of the velocity and vorticity fields reveal that the in-plane meandering of wall-normal vortex is driven by large-scale coherent structures and that these coherent structures originate from streamwise and spanwise turbulent velocity fluctuations. Moreover, the helical structures extracted from the decomposed vertical turbulent velocity fluctuations reveal the spatial structural characteristics of the wall-normal vortex. These are manifested as the three-dimensional distortion induced by axial stretching effects and as the spiral topology associated with the twisting and writhing of the vortex lines.