Author(s): Haitham Ghamry; Chris Katopodis
Linked Author(s):
Keywords: Turbulent flow; RANS; Turbulence models; Trashracks; Head loss; Openchannel; Numerical Modeling
Abstract: Trashracks or bar racks consist of an array of vertical bars that are typically fitted at the inlet of hydraulic power generating units to prevent large debris from damaging turbines. Bar racks may also prevent larger fish from being entrained into turbines and may influence their movement behavior. Trashracks produce energy losses which can be partly attributed to the turbulent large scale flow structures generated by the bars. Therefore there is a need, both from the fish protection and head loss perspectives, to more accurately predict the magnitude and patterns of turbulent flow characteristics, and velocity fields around and between the bars. This paper investigates various Reynolds-Averaged Navier–Stokes (RANS) based turbulence closure models to simulate the turbulent flow near the bars. The k-epsilon, k-omega, eddy viscosity transport, k-omega-based shear stress transport (SST) and baseline (BSL), k-epsilon-based RNG, and zero equation models were examined to select the most appropriate one. The commercial 3D CFD code, ANSYS CFX-11 (www. ansys. com) was used. The results from a series of open channel experimental tests conducted at the University of Manitoba Hydraulic Research &Testing Facility were used to assess the adopted turbulence models. Comparisons of the turbulence models were performed by matching the measured water surface and velocity profiles, iso-contours of mean velocity, and turbulent quantities upstream, around, and downstream of the bars. Computational efforts were also compared. Bar rack models with an array of 3 to 18 vertical bars of various leading edges (square and round edges), bar shapes (rectangular and streamlined), bar thickness, bar spacings, bar depths, bar inclinations, and approach flow velocities were used to perform these comparisons. The results showed that the standard k-omega turbulence closure model produced more satisfactory results over the other models in simulating the water surface and velocity profiles as well as the turbulence quantities.
Year: 2009