Near-Field Dynamics and Mixing of Freshwater Plumes*

The discharge of small river plumes in the ocean produces horizontal advection of freshwater near the coast and vertical entrainment and mixing at the surface. However, multiple physical processes governing turbulence interactions in the near-field, close to the river mouth, and throughout the water column at various scales are still not well understood. We investigate the three-dimensional dynamics and energy balance of a freshwater plume in a basin containing denser fluid using direct numerical simulations (DNS). Motivated by the experimental work of Yuan & Horner-Devine (Phys. Fluids 29:10, 106603, 2017), we resolve the dynamics in supercritical and subcritical conditions, focusing on the characteristics of the front, the internal structure, and the thickness of the plume influenced by large-scale vortical structures. We analyze the non-dimensional parameters defining the plume structure and study the development and evolution of turbulent coherent structures, quantifying vertical density fluxes and entrainment, and the evolution of mixing and dissipation. This quantitative description helps identify the fundamental mechanisms of entrainment and transport, and the role of coherent structures in the predominant time-scales and length scales of the near-field, including the potential mechanisms of internal wave formation.