Large-eddy Simulation of Turbulent Flow in A Vertical Slot Fishway

Fishways are built to restore the upstream passage for migratory fishes and the vertical slot fishway (VSF) is one of the most common types in hydraulic structures. In VSFs, fishes are found migrating upstream in a burst-coast mode. Numerous studies have related their swimming performance with local flow patterns by experimental and/or numerical analysis. This paper presents results of large-eddy simulations (LES) of turbulent flow through a typical VSF, in which extensive field measurements has been taken with a 3D Micro-Acoustic Doppler Velocimeter (ADV). The in-house code Hydro3D is employed to solve the filtered Navier-Stokes equations on a uniform Cartesian grid and the immersed boundary method is used to maintain the no-slip boundaries of the polygonal baffles. To reproduce flow patterns in the VSF, the computational domain is idealized as a box including two pairs of baffles and the periodic boundary condition is applied at the inlet and the outlet. The calculated time-averaged velocity profiles are in good agreement with the ADV measurements, as well as contours of 3D time-averaged velocity distributions indicating the main stream for fish migrating. Fairly good agreement is also found between the predicted and the measured turbulent kinetic energy distribution. Furthermore, 2D turbulence distribution is presented by contours of vertical vorticity in horizontal planes to highlight the K-H instability, while 3D turbulence structures are visualized with isosurfaces of the Q-criterion to track vortex production, propagation and dissipation. Analysis of velocity time signals at several points demonstrates the good spatial and temporal resolution of the simulations. As fish rushes through the slot with the burst swimming speed, its kinematic index is investigated based on the instantaneous flow field. The energy cost is calculated along a typical routine for fish through the computational domain and the lowest burst swimming speed is also evaluated by time series of the instantaneous velocity around the slot.