Comparing simulations to understand how bodies of fresh and salt water mix

In estuarine environments where fresh water and salt water mix, flows that arise from the density differences, called gravity currents, can drive salt transport. Understanding this mixing — though important for maintaining fresh water quality — is computationally challenging.

To reduce this cost, Bingol et al. compared 2D and 3D simulations to understand where 2D simulations reach their applicability limit and 3D simulations become necessary. They aimed to elucidate the dynamics that occur at these estuaries, including Kelvin-Helmholtz instabilities and lobe-cleft instabilities — which affect interfacial mixing and mixing at the front of the salt wedge, respectively.

“Our goal was to identify how these phase-dependent instabilities contribute to mixing between salt water and fresh water, which can improve our understanding of estuarine dynamics and, ultimately, how such processes may influence fresh water availability,” said author Cem Bingol.

They found 2D simulations in the streamwise-vertical plane were sufficient for predicting most long-term effects, like large-scale density distributions and the thickness of the gravity current. However, as the flow becomes more geometrically complex — by approaching curved channels, for example — and in where more localized flow dynamics are needed, 3D simulations become essential.

The researchers hope their detailed comparisons will serve as a benchmark for future work, providing an input for lower-dimensional models to bridge the gap to computationally efficient simulations.

“The numerical setup and the methodology provide a strong foundation for future [large-eddy simulation] studies aimed at investigating mixing over a broader parameter range and more complex geometries and bathymetries, developing improved mixing parameterizations for operational models, and exploring suitable mitigation strategies to reduce salt intrusions,” Bingol said.

Source: “Nonhydrostatic mixing and three-dimensionality of gravity currents under turbulent pulsating flow,” by Cem Bingol, Matias Duran-Matute, Eckart Meiburg, and Herman J. H. Clercx, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0341756 .