Abstract: In this research, the flow field of the surface holes, mid-holes and the stilling basin with step-down floor was simulated by 3-D numerical simulation and model test. We modified the shape of the step-down floor by increasing the height of the steps, the depth of the stilling basin, the length of the horizontal platform before the step-down floor, and side contraction at the outlet, and compared their effects on the flow structure and the characteristics of the stilling-basin. Results show that all these measures could reduce the bottom velocity and fluctuating pressure on the floor. In particular, increasing the flat length obviously reduces fluctuating pressure and the velocity at the drop location of jet flow, but has large construction workload; while side contraction at the outlet effectively changes the horizontal and longitudinal distribution of bottom velocity and fluctuating pressure, and in the meantime has less workload. The result could be a reference for engineering design.

Key words: stilling basin, step-down floor, 3-D numerical simulation, velocity near the bottom, fluctuating pressure