Desert sand dunes rarely occur in isolation, but usually form vast dune fields. The large-scale dynamics of these fields are poorly understood, not least due to the lack of long-time observations. Theoretical models often abstract dunes in an area as self-propelled autonomous agents, exchanging mass, either remotely or as a consequence of collisions.
In contrast to the spirit of these models, a new study by the University of Cambridge offers experimental evidence that sand dunes can ‘communicate’ and repel with each other.
Scientists observed that two similar dunes start close together, however after some time, they get further and further apart. Turbulent swirls constrain this interaction from the upstream dune, which pushes the downstream dune away.
A pile of sand forms a dune shape when exposed to wind to water and then starts flowing with the flow. In deserts or river bottoms or sea beds, the dunes occur in isolation and instead usually appear in large groups, forming striking patterns known as dune fields or corridors.
Karol Bacik, a Ph.D. candidate in Cambridge’s Department of Applied Mathematics and Theoretical Physics, said, “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. 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. Still, we need to validate these theories experimentally.”
In this study, scientists designed and constructed a unique experimental facility that enables them to observe long-term behavior of sand dunes. They built a circular flume filled with water so that dunes can be observed for hours as the flume rotates. Using high-speed cameras, they tracked the flow of individual particles in the dunes.
Bacik said, “Originally, I 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. 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. 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.
Dr. Nathalie Vriend, who led the research, said, “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.”
“The first dune generates the turbulence pattern which we see on the back dune. The flow structure behind the front dune is like a wake behind a boat, and affects the properties of the next dune.”
Scientists are further planning to find out the quantitative evidence of large-scale and complex dune migration in deserts, using observations and satellite images.
The results of the study are reported in the journal Physical Review Letters.