Numerical Simulation of Characteristics of Cavitation and Aeration in Free-flow Tunnel

Abstract

Outlet structures were apt to occur cavitation damages with high speed flow, and aerators were usually adopted to reduce and eliminate cavitation damages in hydropower project. In this paper, the Realizable k-e model and the Volume of Fluid method were used to simulate the characteristics of cavitation and aeration of the free-flow tunnel in Baihetan Hydropower Station. The distributions of water surface profile, pressure, flow velocity and cavitation number were gained. The results show that the Realizable k-e model and VOF method were good at simulating the aerated flow of the free-flow tunnel. Aerators position is the main reason for the changes of velocity distribution in a cross section. The design parameters of the free-flow tunnel in Baihetan Hydropower Station are reasonable by the stable aerated cavities and the suitable length. Introduction The free-flow tunnel with high velocity and high water-head is prone to cavitation and lead to cavitation damage on flowing surface. It is a simple, economic and effective engineering measure for eliminating cavitation damages on flowing surface to set aerator devices on the place where is easy to cavitation. In recent years, besides conventional physical model test[1~3], with the development of turbulent numerical simulation technology, the simulation of turbulent flow gradually become an indispensable research way to investigate the characteristics of aerated flow of high water-head tunnel. Standard k-e model and RNG k-e model were induced in most of numerical simulation before[4~9], Standard k-e model is the most simple double equation model in turbulence numerical simulation, it would produce a certain distortion when was used to simulate flow motion in curved streamlines. Then, the RNG k-e model was put forward to solve high strain rate and large streamline curvature motion equation, but the singularity of e equation should be discussed[10]. In the Realizable k-e model, the coefficients C in the e equation was no longer regarded as a constant, but was linked with strain rate and turbulent parameters and set a new e equation to avoid singularity problem. In this paper, the combination of the Realizable k-e model and VOF method was applied to simulate characteristics of aerated flow of the free-flow tunnel in Baihetan Hydropower Station. The results of numerical simulation were comparied with experimental data, and the analysis provided the basis for the further research work of aeration of free-flow tunnel. Calculation Model The Realizable k-e model was applied to solve the two phase flow and the basic control equations were described as follows: Equation of continuity:   0 i i u t x         (1) Equation of momentum:     j i i i i j t i j i j j j i u u u p u u u g t x x x x x x                                          (2) International Conference on Mechanics and Civil Engineering (ICMCE 2014) © 2014. The authors Published by Atlantis Press 116 k Equation:     i t k i j k j k ku k G t x x x                                  (3) e Equation:     2