Humans often trigger avalanches purposefully before natural ones can occur. Either way, avalanches begin when external stresses on the snow pack exceed the strength within the snow pack or at the contact between the snow and the ground. Acceleration of the snow is gravity-driven. If the snow mixes with air, powder clouds can form that carry snow even further than the main slab. Although the snow itself is not a fluid, once an avalanche gets moving, its behavior can be better modeled as a fluid than as a solid.
Tag: granular flow
Landslide
Landslides, despite their inclusion of solid materials, function essentially as gravity-driven fluid flows. This timelapse video shows a recent earthflow in Wyoming near the Snake River. Rapid snowmelt and heavy rainfall combined to cause a seven day landslide over the highway and into the river. # (via Bad Astronomy)
Singing Dunes
Some sand dunes can “sing”, but not because of the wind! When loose sand slides down over harder, packed sand, a standing wave is formed, causing the entire surface of the dune to vibrate on a single frequency. We hear this as a musical note – typically an E, F, or G. (via io9)
(Image credit: C. Larson)
Swimming Sandfish Lizards
Sandfish lizards can “swim” through granular flows like sand using an undulating, sinusoidal motion. Having studied this motion, engineers have built a robot that swims similarly through large glass beads and have now created a numerical simulation of the physics that matches the measured forces on the swimmer to within 8%. This type of flow is, in some respects, tougher than actual fluids because individual particles have to followed, while in most of fluid mechanics, we can use the continuum assumption to treat a liquid or gas as a continuous medium. #
Fluidized Sand
What’s shown in this video are some pretty spectacular demonstrations of fluidization, where a gas is introduced at the bottom of a bed of granular particles–like sand. At the critical gas velocity, the aerodynamic forces exerted on each particle by the gas will balance the gravitational force on the particle and it will become suspended. All of a sudden, the macroscopic behavior of the solid particles will be like that of a fluid; you can even make it “boil”!
Avalanche Disk
In the Science Storms section of the Chicago Museum of Science and Industry, you’ll find the mesmerizing sight of an avalanche disk. This 20ft disk spins at a variable rate and angle, and, from the video, you can see that the glass beads simulating an avalanche on the disk move very much like a fluid even though they are not. This is what’s called a granular flow and it’s driven by gravity and friction between particles.
