Abstract
The efficiency of a fishway is determined by the ability of immigrating fish to follow its attraction flow (i.e., its jet) to locate and enter the fishway entrance. The hydraulic characteristics of fishway entrance jets can be simplified using findings from widely investigated surface jets produced by shaped nozzles. However, the effect of the different boundary conditions of fishway entrance jets (characterized by vertical entrance slots) compared to nozzle jets must be considered. We investigate the downstream propagation of attraction jets from the vertical slot of a fishway entrance into a quiescent tailrace, considering the following boundary conditions not considered for nozzle jets: (1) slot geometry, (2) turbulence characteristics of the approach flow to the slot, and (3) presence of a lateral wall downstream of the slot. We quantify the effect of these boundary conditions using three-dimensional hydrodynamic-numeric flow simulations with DES and RANS turbulence models and a volume-of-fluid method (VoF) to simulate the free water surface. In addition, we compare jet propagation with existing analytical methods for describing jet propagations from nozzles. We show that a turbulent and inhomogeneous approach flow towards a vertical slot reduces the propagation length of the slot jet in the tailrace due to increased lateral spreading compared to that of a jet produced by a shaped nozzle. An additional lateral wall in the tailrace reduces lateral spreading and significantly increases the propagation length. For highly turbulent flows at fishway entrances, the RANS model tends to overestimate the jet propagation compared to the transient DES model.
Highlights
The attraction flow of common vertical slot fishways is a three-dimensional rectangular surface jet formed by discharge through the entrance slot
To point out the differences in approach flow of the ch, sh and si configurations we evaluated the velocity distribution and turbulence intensity at the orifice exit section (x = 0)
Summarizing, the results show that the combination of a vertical slot with an inhomogeneous and turbulent approach flow typical for fishway entrances reduces the length of attraction flow compared to homogeneous approach flows
Summary
One of the primary challenges in designing a fishway is the determination of an effective attraction flow that creates an uninterrupted migration corridor to guide fish through turbulent turbine discharges to the entrance of a fishway [2,3,4]. The effectiveness of attraction flow can be quantified by its streamwise propagation length with regard to threshold minimum flow velocities [5]. The attraction flow of common vertical slot fishways is a three-dimensional rectangular surface jet formed by discharge through the entrance slot (the most downstream slot). Fishway hydraulics have been extensively studied, research on attraction flow propagation in the tailrace is sparse and existing studies do not consider jet theory [3,4]
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