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May 7, 2009 | Using a highly efficient modeling technique, Pablo Ortiz (University of Granada) and Piotr Smolarkiewicz (ESSL/MMM) have simulated the role of winds in forming sand dunes and sandholes. Their study, one of the first to fully couple the atmosphere with dune behavior, appeared in the April issue of the journal Physical Review E.
Pablo and Piotr focused on barchan dunes, whose horseshoe-like figures are easily recognized but not widely studied. “The development of an isolated barchan dune on a nonerodible bed still eludes complete understanding,” says Piotr. Most models neglect the direct two-way interaction between dune shape and air flow; instead, they typically rely on a simplified approach using average wind speeds that are only weakly affected by the dunes themselves.
In order to carry out a more dynamic simulation without putting a strain on computing resources, the authors designed a large-eddy simulation model that accelerates saltation (the small-scale bouncing of windborne sand grains) by 1,000 times. The effect was to speed up the dune formation process, making the full simulation more feasible. Among other results, the model indicated that a flat pile of sand will produce a broader dune than the same amount of sand arranged in a steeper pile, which tends to elongate rather than widen.
The authors also carried out what may be the first-ever numerical simulation of an isolated sandhole. As sandholes grow, secondary dunes can emerge on the windward side. Sandholes may also play an underappreciated role in larger-scale geology, according to Piotr. Major groups of sandholes could be produced by a meteorite shower or by construction projects in sandy desert areas. “The interaction among sandholes could provide a viable hypothesis for the formation of large fields of dunes,” says Piotr. The study was supported by NSF, DOE, and Spain’s Ministry of Science and Innovation.