D: Y’know Yaël, sand dunes are everywhere. They form in deserts on Earth, Venus, Mars, and Saturn’s moon Titan. They even form underwater on seafloors and riverbeds. They form wherever there is lots of sand, and strong enough wind or water flow to transport the particles. Sand dunes are never alone. They always form as part of dune fields, and the dunes are slowly but constantly migrating. Despite all this, dune fields have stable and well-organized patterns. How can scientists explain this?
Y: This is a problem they’ve only recently started figuring out, Don. In 2020 a team of British physicists reported their discovery that dunes repel each other.
D: How did they find that out?
Y: They studied dune formation in the lab, by making a flow chamber with water flowing over sand. Unlike most flow chambers theirs was circular and contained multiple dunes. At first, they made a chamber that could contain several dunes at once just to collect more data. But then, by accident, they discovered that dunes interact with one another.
D: So what did they actually find?Y: They found that when a dune was downstream of another dune, it moved faster, until the dunes were far apart. This repulsion happens because the upstream dune causes a turbulent wake in the moving fluid, and this wake influences and speeds up the downstream dune. This repulsive effect prevents dunes from colliding. Because the influence of the wake declines with distance, the effect stabilizes the distance between dunes. The scientists found the phenomena happened under a large range of flow conditions. They used satellite images to argue that the same phenomenon can be seen in real deserts.