Hollywood, here they come! New computing techniques solve some old problems

Landsat image, February 8, 1974, of linear sand dunes encroaching on Nouakchott,
the capital of Mauritania.

After water, granular materials (such as sand, soil, snow...) are the most common material on earth.  We encounter them not only in nature, but in our engineered world: cereals, pharmaceuticals, road aggregate, gravel...  Many scientists consider them--after solid, liquid, and gas--to be a fourth state of matter (well, the plasma physicists would argue with that).  Granular materials sometimes act like a solid, sometimes like a gas, and other times like a liquid.  Modeling their behavior and creating graphic images showing this behavior has been a huge computational challenge. Thousands to millions of grains interact as they flow, and simulating the motion of each grain responding to contact and frictional forces with other grains is computationally prohibitive.

In a new paper posted on the WWW site of the university of North Carolina GAMMA group, Narain and others have unveiled a novel approach. The paper is to be published in the ACM SIGGRAPH conference proceedings; ACM is the Association for Computing Machinery; SIGGRAPH seems to mean Special Interest Group on Computer Graphics and Interactive Techniques. Narain et al. assume that the grains are so small that the precise motion of individual grains is unimportant, and they treat a granular material like a liquid, a so-called "continuum model". But, to avoid having the grains flow just like water or syrup, they impose other conditions.  The material maintains its volume when at rest, like a pile of sand, but disperses freely when perturbed. In fact, unlike a liquid, there may be no actual surface that defines the material once it starts moving: imagine throwing a ball into a pile of sand. Where does the pile of sand end, and the cloud of ejecta begin?  They choose a friction model that can counteract gravity and allow stable piles of grains in equilibrium.  The material responds to external forces (such as a push) and internal stresses (contact and frictional forces). Computationally they divide the simulation into two parts: they calculate the motion under the forces present and determine the internal stresses at a particular time.  Then, they integrate the motion of the material under these forces. Rather than tracking individual grains, they track moving "clumps" of matter.

"Fungus Among us"

Check out the video of their simulation results--amazing stuff! Unfortunately, it's a big file and may not play over slow connections. I can play it in my office at the University, but not at home. The link is http://gamma.cs.unc.edu/granular/narain-2010-granular.mov if you want to try it some other way.

The Feb. 18 issue of Science has images that won the science visualization contest this year. They are also available through Wired magazine here. Titled Fungus Among Us, by Kandis Elliot, Mo Fayyaz at the University of Wisconsin was the first prize winner in informational graphics.

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Namibia declares its whole coastline as a national park

Landsat 5 image May 26, 1992. From this reference.

The (British) Telegraph and the New York Times reported today that Namibia has designated its whole coastline as a national park. Becoming the first country to do so, the coastline park joins up with angola's Iona National Park in the north and South Africa's Richetersveld National Park in the south.  Namibia has long pioneered the use of tourism to fund conservation and supported its tribal communities to set up conservation areas to promote tourism and keep poaching at bay.

The Namib Desert is believed to be the world's oldest desert.  One park that is now included in the large park, the Namib-Naukluft National Park, originated in 1907. The area is ever changing, with the coastline advancing in the off shore direction.  In 1909 a passenger ship, the Eduard Bohlen, wrecked and crashed ashore.  It is now 1200 feet inland from the coast.   In the photo above, notice the dunes nearest the coast (toward the bottom left of the photo) pointing in a northeasterly direction, evidence that the coast line is expanding according to this reference. Two spits (at Sandwich Harbor and Walvis Bay) are also evident in the photo. These are formed by interaction between the southwesterly wind and the offshore Benguela Current.

Evolution of a linear dune
from Bristow et al. , 2000

Namibia is famous for its linear dunes, dunes that are parallel to each other as in the left side of the Landsat photo here.  Some of the worlds largest are situated here in the Namib sand sea. In 2000, Bristow et al. used ground penetrating radar to investigate the 3-D structure of a linear dune in an effort to figure out how these dunes migrate (Bristow et al., Nature, vol. 406, pp. 56-59, 2000). They recognized five stages in the evolution of the dune, illustrated in the Figure: (1) Formation of ripples on the desert sand; (2) Development of a dune with a relatively straight crest. This dune migrates in response to the dominant wind direction, producing planar set beds, called cross-stratification.  (3) Development of an instability that results in a sinuous, snake-like crest-line of the dune. The dune is now so big that a single wind season cannot re-sculpt the dune, and it develops a shape that is controlled by more than one wind direction.  Now there are two opposing sets of cross-stratification as shown in the illustration. (4) Complex superimposed dunes appear; and (5) Things get really messy!!