Turbulence Numerical Simulation And Model Test Of The Stepped Spillway Overflow

In order to study the characteristics of the stepped spillway overflow comprehensively and seek for the measure to increase the energy dissipation of the stepped spillway at large unit discharge, the method of turbulence numerical simulation and model test are used to study the stepped spillway overflow. In view of the curved free water surface, complicated boundary condition and turbulence flow, it very difficult to simulate the stepped spillway overflow. In this paper, the fractional volume of fluid (VOF) model of the air-water two phase flow is introduced to the k-E turbulence model and combined with the geometry reconstruction scheme to solve the free water surface. The unstructured grid is used to treat the irregular boundary. By these methods, the turbulence flow field of stepped spillway is simulated successfully. The distribution of velocity, pressure, turbulence kinetic energy and turbulence dissipation rate are obtained by simulation. The typical velocity field and the pressure on the step surface of the stepped spillway are measured using the Laser Doppler Anemometer and the piezometer tube respectively. The numerical simulation results are analyzed and compared with the test results, and they are agreed well with each other.According to the distribution of the turbulence kinetic energy and turbulence dissipation rate in the whole flow field, the mechanism of energy dissipation of stepped spillway is analyzed. One of the main reasons of the step spillway having greater energy dissipation is that the potential energy of the flow continuously transforms to the kinetic energy along the spillway and the kinetic energy partially transforms to the turbulence kinetic energy, and the flow energy is finally dissipated by means of turbulence dissipation. On every step, the maximum of the turbulencedissipation rate is at the step tip. Because the interaction between the skimming flow and the step tips causes the energy to dissipate continuously, so that the step can have better energy dissipation results.By the three dimensional numerical simulation of a project case, the distribution of pressures, turbulence kinetic energy and turbulence dissipation rate in the gentle slope spillway overflow are obtained. The results of the three dimensional simulation emphatically show flow characteristics change along the spillway width and furthermore verify that the combination of k- E turbulence model with the VOF model and unstructured grid can successfully simulate complex three dimensional turbulent flow on the stepped spillway.By the turbulence numerical simulation, the optimization calculation of some key factors that influence the stepped spillway overflow, such as the spillway slope, the step size, the transition steps and so on, are carried out. Through the analysis and discussion for the simulation results, the energy dissipation mechanism of the stepped spillway is preliminarily made clear and some engineering measures for increasing the energy dissipation ratio are proposed, and some useful and practical conclusions are obtained.In the light of the study result that the energy dissipation ratio is higher as the spillway slope is gentle, to applied stepped spillway to overflow earth-rock dams with a certain height is proposed. Taken the bank spillway at Yubeishan reservoir as an example, the water loads acted on the spillway, such as static water pressure, dynamic water pressure, shear stress on the surface of the step and so on, are calculated, which can provide evidence to the stability analysis of the dam slope and slope protection. By the calculation, it is verified that even if the unit discharge is large, the energy dissipation ratio of the step on the gentle slope is higher.