Author(s): Subhojit Kadia; I. A. Sofia Larsson; Mats Billstein; Leif Lia; Elena Pummer
Keywords: CFD simulations; Experimental study; Sediment bypass tunnel; Supercritical flow; Wave extrem
Abstract: Sediment bypass tunnels (SBTs) are designed as narrow channels flowing under supercritical flow conditions. The design suggestions recommend avoiding any in plane bends to prevent sediment concentration and abrupt invert abrasion pattern. However, bends might be unavoidable, as in the case of some existing SBTs. The flow characteristics in narrow channel bends feature stronger secondary currents induced by centrifugal force and radial pressure gradient. Furthermore, the bend acts as a source of disturbance to the supercritical flows and forms cross waves with a rise in the water level along the outer wall, which can influence the design of the tunnel height. These flow complexities demanded a thorough investigation of the turbulent flow characteristics, sediment movements, and surface undulations, and therefore, a physical scale model was built at Vattenfall's laboratory to represent the Solis SBT in Switzerland. The model channel is 0.2 m wide, has a bend of an angle of deviation of 46.524° and a radius of 6.591 m. This paper focuses on the analysis of wave extrema (required to design the tunnel height) and their angular locations along the bend, obtained for 0.07 and 0.095 m3/s discharges using experimental results, simulations, and analytical & empirical models. The turbulent flow features and sediment transport are also investigated, but they are not the focus of this paper. The simulated wave extrema water depths deviate marginally (within ± 2.9%) from the experimental results. The deviations are slightly higher (within ± 4.7%) for the simulated angular locations of the first wave extrema. However, the angular locations of the first wave extrema values obtained from previously proposed analytical and empirical models deviate significantly from the experimental and numerical results, possibly due to the limitations associated with those models.