Author(s): Sylvain Raynal; Ludovic Chatellier; Laurent David; Dominique Courret; Michel Larinier
Linked Author(s): Laurent David
Keywords: Fish-friendly trashracks; Head loss; Numerical Modeling; RANS; Velocity distributions
Abstract: Several amphihaline species, such as silver eels, suffer high mortality rates during their downstream migration, due to their passage through turbines. The combination of adapted trashracks (inclined or angled screen, lower bar spacing, …) with bypasses can efficiently prevent these mortalities. A numerical study has been carried out with such angled trashracks. Numerical results with model scale racks were validated against previous experimental results on model trashracks, resulting from head loss and velocity distribution measurements (Raynal et al., 2013). Real scale racks were then computed in order to evaluate the influence of both the bar spacing and the channel width on velocity distributions in real dimensions. The mesh generation and the numerical simulations were performed by the open source CFD software suite OpenFOAM. The trashrack solid comprised basic elements, individually created using CAD software and directly inserted in OpenFOAM’s mesh generation utility. For small scale configurations, grid refinement was applied at the flume walls, at trashrack bars and downstream of the trashrack, whereas only bars were refined for real scale racks. The resulting number of cells ranged between 100, 000 and 1, 500, 000. Steady state results were obtained by solving the Reynolds-averaged Navier-Stokes (RANS) equations for an incompressible and monophasic flow. The k-ε, k-ε-based RNG, k-ω, k-ω-based shear stress transport (SST) and Spalart-Allmaras models were examined to select the most appropriate one in terms of computation time and result accuracy. Results show that the k-ε-RNG is the model best agreeing with experimental results. Two-dimensional calculations seem to provide quite satisfactory results although both the head losses and the size of the recirculation zone downstream of the trashrack are slightly under-estimated. Real scale results confirm experimental ones and show that the bar spacing slightly effects upstream velocity profiles. Downstream of the rack, simulations with different flume width demonstrated that the size of the recirculation zone is proportional to the channel width.
Year: 2013