Optimizing fish friendliness of sudden expansion flow using a cylindrical structure

This study analyzed why fish failed to migrate upstream in a sudden expansion flow and explored how to improve the fish friendliness of the hydraulic conditions using a cylinder. Based on the fish swimming experiments and the computational fluid dynamics, the temporal ratio and spatial ratio of fish friendly positions were defined and calculated as two indicators to quantitatively evaluate the fish friendliness of hydraulic conditions. The influence of the induced cylinder on the fish friendliness of the flow was analyzed systematically and validated using fish swimming trajectory predictions with an Eulerian-Lagrangian-Agent Method. The results indicate that the primary failure cause of fish's migration studied in this paper is the excessively high flow velocity at the sudden expansion section, which significantly exceeds the swimming capacity of the studied fish as well as the unsuitable turbulent kinetic energy or shear rate values in partial regions. Fish friendly hydraulic environments are established in downstream of the cylinder, and the placement of an optimally sized cylinder at a proper position significantly improved the friendliness and enhanced the passage success rates of fish swimming in sudden expansion flow. Specially, the key role of the cylinder lies not only in increasing the proportion of fish friendly regions but also in improving the connectivity of suitable regions for fish to pass through the whole flow field. These findings provide a foundation for constructing fish friendly hydraulic conditions and could serve as a reference for optimizing the design of new advanced fishway geometries.