Flow characteristics of orifice spillway aerator: numerical model studies

Abstract

Orifice spillways are provided in dams situated in hilly regions for carrying out dual function of passing the flood and flushing of sediments. Aerators are required on these spillways for mitigating cavitation damage for head over spillway exceeding 50 m. The available current literature describes mainly the investigations for aerators on overflow spillways and no systematic work has been reported for aerators on orifice spillways. Traditionally, physical model studies have been used for designing the aerators on overflow spillways. However, extensive experimentation with sophisticated instrumentation is required to understand the flow field comprising pressure and velocity distribution and air entrainment characteristics. Numerical modelling using computational fluid dynamic techniques provides a complete picture on flow field in the vicinity of aerator. This study describes numerical simulation of flow over an aerator on deep-seated orifice spillway. The two-phase three-dimensional (3D) numerical model has been developed with 934,136 hexahedral cells and solved using FLUENT. The numerical model has been validated with data obtained from a 1:25 scale physical model. The pressure, velocity and air concentration profiles along the flow and across the depth at various locations highlights the air entrainment process on an orifice spillway aerator. The global air entrainment coefficient described as the ratio of entrained air and water discharge, average air concentration across depth and the bed air concentration profiles obtained from the numerical model completely describe the characteristics of aerator on orifice spillway and is found to be different than the one for the overflow spillway.