Three-dimensional numerical simulation of mixing patterns at open channel confluences

Abstract

Open channel confluences, where two streams or rivers converge, play a crucial role in hydraulic engineering and river dynamics. These confluences are characterized by complex hydrodynamics influenced by the discharge ratios of merging water bodies. This study investigated the mixing structure at open channel confluences using three-dimensional numerical modeling. A comprehensive three-dimensional numerical model was developed and validated against a dataset obtained from controlled laboratory experiments. This dataset incorporated three-dimensional time-averaged velocity measurements. The skew-induced and stress-induced equation systems were adopted as the core governing equations, providing a framework for simulating various scenarios. A total of ten different cases were analyzed. The results highlighted the effect of discharge ratios on turbulence, lateral and vertical vorticities, and the distribution of mixing, which intensified with higher magnitudes of discharge ratios. The mixing structure, driven by velocity gradients and vorticity, revealed the significant role of lateral and vertical vorticities in determining hydrodynamic behaviors and mixing distributions at confluences. Specifically, the momentum ratio of incoming flows governed the spatial evolution of mixing processes. This study revealed that the distribution of mixing served as a key indicator for identifying the formation of mid-channel scours. High normalized velocities induced toward the left bank led to the superelevation of the water surface, enhancing the potential for bed material and the formation of significant scour holes beneath the elevated water surface. This novel approach provides a deeper understanding of the mixing patterns at confluences, particularly in scenarios with equilibrated discharge ratios but in different magnitudes.