Characteristics of secondary flow and separation zone with different junction angle and flow ratio at river confluences

Abstract

Confluences are widespread in natural rivers where pollutants, sediment and fish tend to accumulate (especially separation zone), and studying their three-dimensional (3D) structure can provide guidance for improving river environmental health. In the current study, a flume experiment was conducted, and a 3D confluence model was built. Three turbulent models were compared based on the experimental results, which confirmed the applicability of the RNG k-ε model. The effects of junction angles and flow ratios on the formation and evolution of secondary flow and separation zone were investigated by the established model. The results indicated that the separation zone with low flow velocity, high turbulence kinetic energy and low pressure is formed downstream of the junction while two secondary flows form on the sides of the shear plane downstream. The shapes of the separation zone and secondary flow at the confluence were defined in a new way. The secondary flow formation process is reversed within the separation zone and highly complicated when the junction angle and flow ratio change. This study can provide a scientific basis for the utilization of confluences to improve water quality and aquatic animal ecology at natural river confluences.