A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Engineering and Technology

Thesis (M.Eng.) - Asian Institute of Technology, 1992

Flow energy dissipation due to an abrupt bed drop was previously studied extensively on a macroscopic scale, either by a one-dimensional (1-D) analysis or by experimental measurement. The macroscopic aspect of the flow phenomena and energy dissipation is well understood while the microscopic aspect has not been revealed clearly. A two-dimensional (2-D) hydrodynamic model in the vertical plane with the standard k - e turbulence model closure (VEST model) is used to compute the temporal mean flow parameters and turbulence characteristics of the flow through an abrupt bed drop in a microscopic scale. The numerical results are compared with the available experimental measurements. It is found that the model can predict the velocities accurately and can predict the turbulence characteristics reasonably well. The spatial distribution of the flow energy dissipation is computed along the longitudinal distance and a good agreement between the computed results and the previous experimental data is obtained. The effects of the inlet Froude number, the relative step height and the downstream end water level on the energy dissipation are examined. The computed energy dissipation is compared with those obtained by the classical 1-D analysis. The deviations of 1-D results from the 2-D model results are quantitatively evaluated. Sensitivity analysis are performed to investigate the effects of the k - e model constants, the upstream boundary conditions of the inlet velocity and the k and e turbulence characteristics, and the effects of the water surface boundary condition on the computed flow conditions and on the flow energy dissipation.