Hydraulic Regulation of Weir–Orifice Fishways Using Different Cylinder Arrays

Constructing and optimizing fishways is essential for ecosystem conservation. In conventional weir–orifice combined fishways, strong jets form local high-velocity zones and hinder continuous fish migration. To solve this problem, cylinder regulating structures are arranged inside fishway pool chambers. Five schemes with various cylinder types and layouts are proposed for hydraulic comparison. Physical experiments and RNG k–ε simulations are adopted to systematically explore how these structures adjust weir–orifice flow distribution and reconstruct pool flow fields, using indicators including weir overflow ratio, velocity distribution, turbulent kinetic energy, dissipation rate and vortex characteristics. The results reveal that cylinder structures block bottom-orifice jets, strengthen lateral flow diffusion and accelerate momentum dissipation, converting jet-dominated flow into a dispersed field with moderate velocities. When the layout changes from a single cylinder to an array, the rising weir overflow ratio mitigates bottom jets and cuts the maximum velocity by around 24.1%. The three-semi-cylinder array achieves lower peak turbulent kinetic energy and smaller vortex areas, effectively restraining turbulence and vortex evolution. This scheme performs well under the maximum discharge of 1.4Q. These findings provide a potential hydraulic regulation strategy for fishway retrofitting and optimization.