Soil liquefaction is a rather unsettling process in which apparently solid ground begins moving in a fluid-like way after agitation. It occurs in loose sediments when the spaces between individual particles become nearly saturated with water. This can happen, for example, after heavy rains or in a place with inadequate drainage. Such cases are typically very localized, though, and require some significant agitation of the surface, like pressing with heavy machinery or jumping in a single spot. Soil liquefaction becomes a greater danger, however, in an earthquake. Even in a dry area, the earth’s shaking can force groundwater up into the surface sediment and vibrate the soil sufficiently to liquify it, causing whole buildings to sink or tip and wreaking havoc on manmade infrastructure. (Video credit: jokulhlaups)
Month: February 2014
Spinning Polygons
Nature is full of surprising behaviors. If one imagines putting a bucket of water on a rotating plate and spinning it, one would expect the water’s free surface to take on a curved, axially symmetric shape. The photos above are from a similar experiment, but instead of the entire container rotating, only the bottom plate spins. Surprisingly, the water’s surface does not remain symmetric around the axis of rotation. Instead, the water forms stable polygon shapes that rotate slower than the spinning plate. As the plate’s rotation speed increases, the number of corners in the polygon increases. Shapes up to a hexagon were observed in the experiment. Photos of the set-up and more experimental results are available, as is the original research paper. Symmetry breaking and polygons can also be found in hydraulic jumps and bumps, liquid sheets, and planetary polar vortices. (Photo credit: T. Jansson et al.; research paper)
Sand Ripples
Wave motion in a bay or near a beach can cause significant sediment transport. Individual granular particles, like sand, can be lifted by the passage of a single wave, but, over time, complex patterns form as the granular bottom surface shifts due to the waves. This video shows time-lapse footage of the ripples that form and move in submerged sand during many hours of wave motion. A slight imperfection in the surface causes a network of sand ripples to grow and spread. Once formed, those ripples shift and reform depending on changes in the wave conditions. (Video credit: T. Parron et al.)
Knotting Vortices
Knots have long fascinated humans, appearing in art for thousands of years and generating entire fields of study. Until recently, however, the idea of a knotted fluid was purely theoretical. To knot fluids, researchers used 3D printing to create twisted hydrofoil shapes. When towed through water, fluid travels around the shape and spins up at the trailing edge, creating a knotted vortex ring. The knotted vortices were captured with 3D imaging, allowing scientists to observe how they evolve. So far the knots they’ve created have all been unstable, deforming until two vortex lines approach one another. Upon contact, the vortices disconnect and reconnect with one another, unraveling the knot. Intriguingly, these vortex reconnections seem remarkably similar to the vortex reconnections observed between quantized vortices in superfluids. (Video credit: D. Kleckner et al.)
Happy 1000 Posts!
Today is FYFD’s 1000th post! It’s been a wild ride over the last three-and-a-half years and I cannot thank you all enough for coming along. I’m continually amazed by FYFD’s popularity among readers of all ages and backgrounds, and it’s truly a joy to see excitement for fluid dynamics spreading.
The keen-eyed among you may have noticed a subtle change to the main page: I successfully defended my PhD Friday! I’m still working on wrapping my head around the idea of not being a student any more.
Anyway, I just wanted to take a few minutes to celebrate. I encourage you to take a look back at the archives, which are full of amazing science and physics, or read one of the themed series FYFD has featured. And, if you’ve enjoyed the blog, please don’t hesitate to spread the word! Thank you all again for your support. 🙂
Sochi 2014: Link Round-up
I’ve come across a lot of great links over the course of writing the #Sochi2014 series, and I want to highlight some of my favorites here. Be sure to check them out for some great behind-the-scenes looks at Olympic sport science and technology.
- Ski Racing covers the intriguing history behind speed suit development. Of particular interest is the development of Spyder’s Speedwyre suit, which featured a tripwire to induce turbulent flow. The suits were so effective at increasing skiers’ speeds that skiing’s governing body outlawed them ahead of the 1998 Olympics. There are similar restrictions in the speed suits of other sports, but sometimes people get away with it. (h/t @YvesDubief)
- A must-watch: Sir David Attenborough narrates curling.
- Smarter Every Day has had some awesome Olympics-themed infographics during the Games. Some favorites: how clapskates work, how to do an axel jump, an illustration of ski jumping, how curling stones curl, and the basics of curling.
- The National Science Foundation put together a whole series of videos on the science and engineering of the Winter Olympics.
- CBS goes inside the BMW redesign of the US bobsleds, luge, and skeleton sleds.
- Wired took an in-depth look at using science to improve an alpine skier’s performance.
- It’s originally from 2010, but SciAm has a neat podcast on the physics of curling. They also give some background on the granite in the stones, which comes from one particular island off Scotland.
- The distinctive V-style of ski jumping may have developed as a result of an athlete’s mid-air seizure. (via @YvesDubief)
- Inrng compares the aerodynamics of cycling and skiing, wondering if skiers are leaving precious tenths behind on the hill due to bulky equipment.
(Photo credit: A. Bello/Getty Images)
Sochi 2014: Speed Skiing
As FYFD wraps up coverage of #Sochi2014, let’s take a look at a winter sport not currently contested at the Olympics. This year’s Winter Games featured 12 new events. Speed skiing was not among them, though it was a demonstration sport in the 1992 Olympics. Like many of the sports in Sochi, speed skiing is gravity-driven, and friction and drag serve only to slow competitors. Speed skiing is about getting from the top of the course to the bottom, in a straight line, as fast as possible. Athletes reach velocities as high as 250 kph (155 mph), and aerodynamics are of the utmost concern. The skiers’ rubberized speed suits include airfoil-shaped fairings behind their calves that mold the airflow, and athletes wear giant aerodynamic helmets to smooth flow over their heads and shoulders. They spend their entire descent in an aerodynamic tuck, arms extended ahead of them like a cyclist in a time trial. It looks a pretty crazy ride. Would you like to see it added to the Olympics? (Video credit: R. Sill/University of Cambridge)
FYFD is celebrating #Sochi2014 with a look at fluid dynamics in winter sports. Check out the previous poss on why ice is slippery, the aerodynamics of speedskating, and how lugers slide fast.
Sochi 2014: Curling
Curling is rather unique among target-based sports because it allows athletes to alter the trajectory of their projectile after release. Curlers send 19 kg granite stones sliding across a pebbled ice surface at a target 28 meters away. On the way, teammates sweep the ice with natural or synthetic brushes. Sweeping the ice causes frictional heating, which lowers the local coefficient of friction and allows the stone to slide meters further than it would without sweeping. The bottom of the stone is concave, so the rock only contacts the ice along a narrow ring. One explanation for the stone’s tendency to curl in the direction it spins comes from this contact ring. Researchers suggest that the roughness of the leading edge cuts scratches into the ice which the trailing edge attempts to follow, causing the stone to move laterally, as illustrated over at Smarter Every Day. It’s important to note that the sweeping curlers do doesn’t directly guide the stone. In fact, by lowering the coefficient of friction the sweepers prevent the stone’s curling, and thus much of the skill of the sport is in knowing when, how, and how much to sweep. (Photo credit: C. Spencer/Getty Images)
FYFD is celebrating #Sochi2014 by studying the fluid dynamics of the Games. Check out some of our previous posts including how to make artificial snow, the aerodynamics of bobsledding, and how ski jumpers fly further.
Sochi 2014: Bobsledding
Today bobsledding is an sport rife with modern technology and design techniques. In recent years, companies better known for their expertise in automobiles and Formula 1 racing have become players with BMW designing American sleds, McLaren making the UK sleds, and Ferrari providing for the Italian team. Like many winter gravity sports, contenders can be separated by as little as hundredths of a second. This makes aerodynamics a serious concern, but the variability of the sled’s position and orientation over a run makes realistically simulating the aerodynamics, either in a wind tunnel or computationally, extremely difficult. Additionally, the sport’s governing body restricts a sled’s dimensions, weight, shape, and other details; for example, bobsleds are not allowed to use vortex generators that would help maintain attached flow and reduce drag. Instead, designers try to shave drag elsewhere, in the shaping of the sled’s nose or by tweaking the back end of the sled to reduce the drag-inducing wake. Even the shape of the driver’s helmet is aerodynamically significant. (Image credits: Exa Corp, Getty Images, BMW)
FYFD is celebrating #Sochi2014 by looking at fluid dynamics in winter sports. Check out our previous posts on how skiers glide, the US speedskating suit controversy, and why ice is slippery.
Sochi 2014: Downhill Skiing
Like the athletes who compete on ice, skiers rely on a film of liquid beneath their skis to provide the low friction necessary to glide. The moisture results from the friction of the ski’s base and edges cutting into the snow, and, depending on the conditions of the snow, different surface treatments are recommended for the skis to help control and direct this lubricating film. Similarly, skiers uses various waxes on their skis to lower surface tension and provide additional lubrication. Fluid dynamics can also play a role in tactics for various ski-based events. In endurance events like cross-country skiing, drafting behind other skiers can help an athlete avoid drag and save energy. When drafting, cross-country skiers have lower heart rates. Drag and aerodynamics can also play a significant roles in alpine skiing, especially in speed events like the downhill or super G. In these events solo skiers reach speeds of 125 kph, where drag is a major factor in slowing their descent. Between turns smart skiers will tuck, decreasing their frontal area and reducing drag’s effects. Athletes use wind tunnel testing to dial in their tuck position for maximum effect, and, like speedskaters, skiers may also wear special aerodynamic suits. (Photo credits: F. Cofferini/AFP/Getty Images, C. Onerati; h/t to @YvesDubief)
