Sand dunes can ‘communicate’ with each other
the researchers 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.
This downstream movement is normal, the researchers explained, since – when sand is exposed to wind or flowing water – it tends to form a dune shape and gradually move downstream, as in a river. This can happen in deserts, on river bottoms and even sea beds. Typically, many dunes form in a region, creating a dune field.
At the beginning of this experiment, the researchers observed finer detail than has ever been seen before. They observed that two dunes within the flume might have the same shape and volume. As water began flowing over the dunes, they began to move, as expected.
But then something unexpected happened. The researchers thought they’d see dunes moving at the same speed. Instead, the two dunes started moving at different speeds. The dune in front initially moved faster than the dune behind it, but then it gradually slowed down, until both dunes ended up moving at about the same speed.
It’s this variable speed of movement with respect to each other that these researchers are calling “communication.” A key observation in explaining it was the pattern of flow across the dunes. The front dune deflects the flow, creating swirls on the dune behind it. This deflection pushes the back dune farther away from the front dune, effectively slowing down the back dune’s movement relative to the front dune. As Vriend explained:
The front 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.
The longer the experiment ran, the further apart the two dunes became. Eventually, they formed an equilibrium – a state of being balanced – on opposite sides of the flume, where they then remained at 180 degrees apart.
https://earthsky.org/earth/sand-dunes-c ... each-other
Bikermice from Mars! 
dat NRC redactie dit laat plaatsen joh 
