Stratified grain flows form barchan dunes

The surfaces of Earth and Mars are similar enough to host some of the same geomorphological phenomena. For example, crescent-shaped dunes whose tips point downstream of a wind or water flow have been found in many environments on both planets. Known as barchan dunes, they span a wide range of spatial and temporal scales. For example, kilometer-sized ones on Mars have lasted thousands of years, and centimeter-sized underwater ones on Earth can last a few minutes.

Previous experimental studies of barchan dunes have tended to focus on the shorter-lived ones, and researchers have generated simplified versions of them in the lab using grains of uniform size. Field observations, however, indicate that the dunes form from mixtures of various-sized grains.

Now Erick Franklin of the University of Campinas in Brazil, his former PhD student Carlos Alvarez, and current graduate student David Cúñez have created underwater barchan dunes in the lab using various two-grain mixtures. The grains were sorted by size, by density, and by both. The researchers found that three types of grains readily flow across a dune’s surface: less dense, larger, and larger and denser.

The picture below shows the experimental apparatus. Mixtures of round zirconium or glass grains of either 0.15–0.25 mm or 0.40-0.60 mm in diameter and of 2.5 g/cm³ or 4.1 g/cm³ in density were each poured into a water-filled channel. Then a turbulent flow was imposed on the system with centrifugal pumps, and cameras captured the evolution of the grain mixture’s morphology, from a conical pile to a barchan dune.

The observations revealed that the dunes comprise two somewhat counterintuitive horizontal layers. The grains that flow the easiest—those less dense, larger, and larger and denser—accumulate on the downstream side of a dune. Then the denser, smaller, and smaller and less dense grain types spread across the upper half of a forming barchan dune to make its top layer.

In all the experimental cases, the grains flowing across the dune surface initially accumulate at a constant rate that eventually plateaus. By modeling the initial rate as a diffusion of grains across the dune’s surface, the researchers reproduced the observations and can now predict for how long grains will spread across a dune surface. Similarly stratified layers in barchan dunes have been observed at the north pole of Mars. The researchers speculate that those observations could be partly a result of the same size and density segregation they observed in their experiments. (C. A. Alvarez, F. D. Cúñez, E. M. Franklin, Phys. Fluids 33, 051705, 2021 .)