Turbulent free-surface flow through an idealised nature-like fish pass

Results of large-eddy simulations (LES) of turbulent free-surface flow through an idealised nature-like fish pass are presented. The selected flow conditions correspond to Reynolds numbers of Re=56,350 and Re=39,675, defined based on the diameter of obstacles and the gap velocity. The Froude numbers varies from Fr = 0.42 to Fr = 0.84. The geometry of the fish-pass and the flow conditions of one case are chosen to match a previous experiment, the data of which are used to validate the chosen LES approach. Three additional cases are studied, and the LES results are then analysed to investigate the effect of Fr on the hydrodynamics, turbulence characteristics, free surface deformation, and drag forces on the obstacles comprising the fish pass. It is found that the Reynolds number has little effect on the time-averaged and fluctuating flow field or the spatial distribution of the turbulent kinetic energy. However, significant changes are observed when the Froude number increases and approaches unity. In all cases, significant water surface deformation is observed and the severity of the water surface dynamics is, again, governed by Fr. The instantaneous flow is affected greatly by shear layer vortices originating near the leading edge of the obstacles for all cases, which results in unsteadiness in the obstacles' wake and periodic deformation of the water surface. Velocity spectra provide evidence of the strong link between vortex shedding and water surface dynamics, further demonstrating that the Froude number is the governing parameter in this type of flow.