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 'criticality' 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