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The Sharp Side of Emulated Erosion

Typically in VR, when we model erosive forces, we think of the smoothing effect of water. We think of smooth stones, smooth edges, and smooth cavities. We don't typically think of sharp edges and triangles. That may be a mistake.

A team of New York University researchers have been working with clays and water to try and understand how the smoothing effect of water is compatible with the more esoteric shapes found in nature: sharp points and edges often abound. Their results were startling, and directly affect our attempts to create natural formations. Water can sharpen just as easily as it smooths.

The researchers' findings, which appear in the latest edition of the journal the Proceedings of the National Academy of Sciences (PNAS), reveal the unexpected ways that erosion can affect both natural and artificial materials.

“The main focus of this study was to understand how and why erosion makes these funny shapes,” explained Leif Ristroph, a post-doctoral researcher at NYU’s Courant Institute of Mathematical Sciences and one of the study’s co-authors.

To explore these questions, the researchers designed an experiment, conducted in the Courant Institute’s Applied Mathematics Laboratory, to replicate natural erosion. In it, the researchers submerged clay—shaped as balls or cylinders—into a 15-ft. long water tunnel. The apparatus was designed to continuously generate a uniform flow of water, which would allow the researchers to observe how erosion shapes an entire object.

What they found was water flow acts as a shearing force—not unlike a nail file—against objects, working them into specific shapes. Starting from a clay ball, the flowing water sheared the sides away, producing a cone with a pointed face. Likewise, the clay cylinder was sculpted into a triangular shape. The researchers then sought to confirm these findings by replicating the experiment using a computer model. These results were consistent with the experimental findings, revealing in a computer simulation how the shape was maintained as the body eroded away.

“Water acts tangentially to the surface of objects and skims off material to create these unique shapes,” explained Ristroph. “In a sense, it works as a sculptor to naturally mould materials into new forms.”

For us, this means we have a better understanding of how water can create sharp edges, as it smooths out both sides of a formation, leaving triangular, sharp points with smooth sides in larger structures.


Erosion Has a Point—and an Edge

Sculpting of an erodible body by flowing water (open Acess PDF)

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