As they move, sand dunes interact with and repel their downstream neighbors, according to a new study published in the journal Physical Review Letters. Using an experimental dune’ racetrack,’ the study authors observed that two identical dunes start out close together, but over time they get further and further apart; this interaction is controlled by turbulent swirls from the upstream dune, which push the downstream dune away.
Sand dunes. Image credit: Nicola Giordano.
Sand dunes rarely occur in isolation, but usually form vast dune fields.
The large scale dynamics of these fields is hitherto poorly understood, not least due to the lack of longtime observations.
“There are different theories on dune interaction: one is that dunes of different sizes will collide, and keep colliding, until they form one giant dune, although this phenomenon has not yet been observed in nature,” said study first author Karol Bacik, a PhD candidate at the University of Cambridge.
“Another theory is that dunes might collide and exchange mass — sort of like billiard balls bouncing off one another — until they are the same size and move at the same speed, but we need to validate these theories experimentally.”
“We’ve discovered physics that hasn’t been part of the model before,” said study senior author Dr. Nathalie Vriend, also from the University of Cambridge.
Most of the work in modeling the behavior of sand dunes is done numerically, but Dr. Vriend and colleagues designed and constructed a unique experimental facility which enables them to observe their long-term behavior.
Water-filled flumes are common tools for studying the movement of sand dunes in a lab setting, but the dunes can only be observed until they reach the end of the tank.
Instead, the team built a circular flume so that the dunes can be observed for hours as the flume rotates, while high-speed cameras allow them to track the flow of individual particles in the dunes.
“We hadn’t originally meant to study the interaction between two dunes,” Bacik said.
“Originally, we put multiple dunes in the tank just to speed up data collection, but we didn’t expect to see how they started to interact with each other.”
The two dunes started with the same volume and in the same shape. As the flow began to move across the two dunes, they started moving.
“Since we know that the speed of a dune is related to its height, we expected that the two dunes would move at the same speed. However, this is not what we observed,” Dr. Vriend said.
Initially, the front dune moved faster than the back dune, but as the experiment continued, the front dune began to slow down, until the two dunes were moving at almost the same speed.
Crucially, the pattern of flow across the two dunes was observed to be different: the flow is deflected by the front dune, generating ‘swirls’ on the back dune and pushing it away.
“The front dune generates the turbulence pattern which we see on the back dune,” Dr. Vriend explained.
“The flow structure behind the front dune is like a wake behind a boat, and affects the properties of the next dune.”
As the experiment continued, the dunes got further and further apart, until they form an equilibrium on opposite sides of the circular flume, remaining 180 degrees apart.
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Karol A. Bacik et al. 2020. Wake Induced Long Range Repulsion of Aqueous Dunes. Phys. Rev. Lett 124 (5): 054501; doi: 10.1103/PhysRevLett.124.054501
