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Wikenigma - an Encyclopedia of Unknowns Wikenigma - an Encyclopedia of the Unknown

Sand dune formation

Large swaths of Earth’s surface are covered in loose sediment. The grains that make up this sediment form fascinating bedforms from meandering riverbeds to wavy dunes, whose shapes are constantly changing as water or air currents move the grains. The modeling of sediment transport has significantly advanced in recent years, but researchers lack a unifying framework to describe the transport of grains by different density fluids, like air and water.

Source : APS Physics 13, 62, 2020.

As well as the uncertainty surrounding the formation of dunes, the way in which they disperse and move relies on critcalityplugin-autotooltip__plain plugin-autotooltip_bigSelf-Organized Criticality

The term Self-Organized Criticality (SOC) was introduced in a 1987 paper for Physical Review Letters. The groundbreaking research by Per Bak, Chao Tang and Kurt Wiesenfeld (now known as 'BTW') described how complex systems can feature 'critical' points in their development which can lead to sudden, dramatic changes (phase transitions).
calculations which are as yet poorly defined with regard to sand grains.

[…] exactly how and why dunes form in the way they do still eludes us. Now efforts to get to the bottom of this are taking on a new urgency, and not just because they could solve what Nathalie Vriend at the University of Cambridge explains is a “fundamental physics problem”.

New Scientist, Nov 2020

Also see Squeaky Sandplugin-autotooltip__plain plugin-autotooltip_bigSqueaky Sand

Sand on some beaches and dunes emits a distinct 'squeaking' sound when moved under pressure - e.g. by walking on it Videos here

It only affects certain types of sand - more rounded and drier grains generally don't squeak (as much). The underlying acoustic mechanism is as yet undiscovered.
and Yardangsplugin-autotooltip__plain plugin-autotooltip_bigYardangs

Yardangs are enigmatic wind-parallel ridges found in arid environments. They often form downwind of rocks in sandy environments. They can be unusually stable structures, given their sandy / dusty composition (and the fact that the wind which forms them can also erode them).


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