Effect of Contraction on Backward-facing Step Flow

The hydraulic jump with abrupt drop is found to be of interest of the hydraulic engineers. These types of jumps are effected on by the height of step significantly. With the change of height, there may be three different flow patterns in stilling basin, such as under-flow, mixed-flow and surface-flow. For those flow, the differences of hydraulic index are obvious. Many experiments predicted that contraction may solve the problems effectively from the height of step. In this paper, backward-facing step flow is numerically simulated by solving the Reynolds averaged Navier-Stokes equations. A kappa-epsilon and RNG numerical simulation is utilized to describe turbulence, which combined with volume of fluid method for treating complicated free boundary configuration. A series of numerical results were conducted in backward-facing step high speed flow with contract value between 0m and 0.5m for different step height, with the contract gradient 0, 1: 15 and 1: 30. It is shown the variation of flow pattern and characters for different contraction and step height. A comparison indicated that, the step results in remarkable decrease of flow velocity and pulsating pressure acting on the floor, but at the same time, it affects the stabilization of flow pattern in stilling basin. It is also found that contraction could optimize flow pattern in some extent, as well as enhance the energy dissipation efficiency.