Barchans are crescent-shaped sand dunes whose two horns face in the direction of the fluid flow. They appear in different environments, such as inside water pipes or on river beds, where they take the form of ten-centimeter ripples, and deserts, where they can exceed 100 meters, and the surface of Mars, where they can be a kilometer in length or more. If their size varies greatly, so does the time they take to form and interact. The orders of magnitude range from a minute for small barchans in water to a year for large desert formations and a millennium for the giants on Mars.
They are formed by the interaction between the flow of a fluid, such as gas or liquid, and granular matter, typically sand, under predominantly unidirectional flow conditions.
"What's interesting is the similarity of their formation and interaction dynamics, regardless of size. As a result, we can study aquatic barchans in the laboratory to make predictions about the evolution of the dunes in Lençóis Maranhenses [a coastal ecosystem in the Northeast of Brazil] or to investigate the origins of the topography in the Hellespontus region on Mars," said Erick Franklin, a researcher and professor at the University of Campinas's School of Mechanical Engineering (FEM-UNICAMP) in the state of São Paulo, Brazil.
Working with his Ph.D. student Willian Righi Assis, Franklin performed more than 120 experiments and identified five basic types of interaction between barchans.
The study, conducted entirely at UNICAMP, is reported in an article published in the journal Geophysical Research Letters. It was supported by FAPESP via a Phase 2 Young Investigator Grant awarded to Franklin and a direct doctorate scholarship awarded to Assis.
A striking aspect of the topic is that as well as having a robust shape that appears in many different environments, barchans typically form corridors in which their sizes are approximately the same. Analysis of individual dunes suggests they should grow indefinitely, becoming steadily larger, but this is not the case. One explanation for their characteristic size in a given environment is that binary interactions, especially collisions, redistribute the mass of sand, and instead of growing continuously they subdivide into smaller dunes.
"This has been…
José Tadeu Arantes