Simulated tilting salt fingers unlock mysteries of oceanic laminar flow

Salt fingers, an oceanic mixing mechanism that occurs when warm, salty water lies over colder, fresher water, has been a topic of inquiry for decades. But why they often occur on a tilted plane remains poorly understood. A newly presented model of how these complicated interfaces function is poised not only to offer insight into ocean sciences, but also to more broadly inform astrophysics, geology and semiconductor technology.

A team of researchers demonstrated a two-dimensional direct numerical model that simulates the behavior of double-diffusion salt fingers in a two-layer thermohaline (density driven ocean circulation) system with laminar flow. Publishing their work in Physics of Fluids, they used this model to investigate the influence of the Reynolds number, a dimensionless quantity that relates a fluid’s inertial and viscous forces, on vertical mass transport.

Creating salt fingers, even in a simulation, can be tricky. Once the team could optimize such conditions as the buoyancy ratio and the Rayleigh, Lewis, and Reynolds numbers, they were ready to start evolving the simulated fingers. To test the effects of Reynolds numbers, the researchers tested several laminar shear flows with numbers ranging from zero to 900.

With a Reynolds number of zero, the model’s fingers evolved as a pure, nontilted salt finger. As the Reynolds number increased, the shear force increased. At a Reynolds number of 400, they found that the large shear force inclines the salt fingers, strengthening interactions between adjacent fingers and enhancing the interface between the two layers.

Next the researchers intend to expand their model to include such features as salt fingers in turbulent environments, porous media and characterizing the mechanism in three dimensions.

Source: “Numerical study on tilting salt finger in a laminar shear flow,” by Xianfei Zhang, Ling-ling Wang, Cheng Lin, Hai Zhu, and Cheng Zeng, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5017685 .