For a little more than century, mankind has taken flight in fixed-wing aircraft. But other species have flown for much longer using flapping techniques, the details of which humans are still unraveling. To really appreciate flapping flight, it helps to have high-speed video, like this beautiful footage of a goshawk attacking a water balloon. The motion of the hawk’s wings is far more complex than the simple up and down flapping we imitate as children. On the downstroke, the wings and tail stretch to their fullest, providing as large an area as possible for lift. During steady flight, the bird flaps while almost horizontal for minimal drag, but as it approaches its target, it rears back, allowing the downstroke to both lift and slow the bird. In the upstroke, the bird needs to avoid generating negative lift by pushing air upward. To do this, it pulls its wings in and simultaneously rotates them back and up. Its tail feathers are also pulled in but to a lesser extent. Leaving them partially spread probably maintains some positive lift and provides stability. At the end of the upstroke, the hawk’s wings are ready to stretch again, and so the cycle continues. (Video credit: Earth Unplugged/BBC; h/t to io9)
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“Aurora”
This bulbous, ethereal shape is a spreading flame front captured by artist Fabian Oefner in his new “Aurora” series. Oefner used a few alcohol droplets in a glass vessel and ignited the volatile vapors, capturing the propagating flame. Take a look at it in action. Because the air inside the vessel is mostly still, the chemical reactions in the combustion occur much faster than the air’s motion. As a result, the flame spreads as a thin sheet instead of a uniform, widespread flame. The wrinkled and corrugated look of the flame front is due local turbulence distorting the flame. (Photo credit: F. Oefner)
When Turbulence Is Desirable
One of the common themes in aerodynamics, especially in sports applications, is that tripping the flow to turbulence can decrease drag compared to maintaining laminar flow. This seems counterintuitive, but only because part of the story is missing. When a fluid flows around a complex shape, there are actually three options: laminar, turbulent, or separated flow. An object’s shape creates pressure forces on the surrounding fluid flow, in some cases causing an increasing, or unfavorable, pressure gradient. When this occurs, fluid, especially the slower-moving fluid near a surface, can struggle to continue flowing in the streamwise flow direction. Consider the video above, in which the flow moves from left to right. Near the surface a turbulent boundary layer is visible, where fluid motion is significantly slower and more random. Occasionally the flow even reverses direction and billows up off the surface. This is separation. Unlike laminar boundary layers, turbulent boundary layers can better resist and recover from flow separation. This is ultimately what makes them preferable when dealing with the aerodynamics of complex objects. (Video credit: A. Hoque)
Cylinder Wakes
A simple cylinder in a steady flow creates a beautiful wake pattern known as a von Karman vortex street. The image above shows several examples of this pattern. Flow is from bottom to top, and the Reynolds number is increasing from left to right. In the experiment, this increasing Reynolds number corresponds to increasing the flow velocity because the cylinder size, fluid, and temperature were all fixed. As the Reynolds number first increases, the cylinder begins to shed vortices. The vortices alternate the side of the cylinder from which they are shed as well as alternating in their sense of rotation (clockwise or counterclockwise). Further increasing the Reynolds number increases the complexity of the wake, with more and more vortices being shed. The vortex street is a beautiful example of how fluid behavior is similar across a range of scales from the laboratory to our planet’s atmosphere. (Image credit: Z. Trávníček et. al)
Soil Liquefaction
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)
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.)
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.
