Animals often move in ways engineers find counter-intuitive. Take, for example, the glass knifefish, an undulatory swimmer that controls its motion through wavelike oscillations of its fin. One might expect the knifefish to move its fin so that a single continuous wave moves from one end to the other. Instead two opposing waves move down the knifefish’s fins, one travelling from head to tail and the other travelling from the tail forward. The intersection of these waves is the nodal point, and, by shifting the nodal point fore or aft, the knifefish can hover in place, move forward or swim backward. At first glance, this seems like a wasteful system since a significant portion of each wave cancels the other, but, through mathematical modeling and experiments with a biomimetic robot, the researchers found that the dual-wave locomotion increases both the stability and maneuverability of the fish. (Video credit: N. Cowan et al.; via phys.org)
Search results for: “waves”
Schlieren in Flight
Schlieren photography is a common method of visualizing shock waves in wind tunnel experiments, but it’s much harder to pull off for aircraft in the sky. This video from NASA shows off some stunning work out of NASA Dryden capturing schlieren video of shock waves from a F-15B aircraft at Mach 1.38. You’ll notice that shock waves extend off the nose, wings, tail, and other parts of the airplane and extend well beyond the camera’s field of view. It’s these shock waves hitting the ground level that causes distinctive sonic booms. These tests are part of NASA’s on-going research into minimizing the effects of sonic boom so that civilian supersonic flight over land is feasible in the future. When the U.S. government shutdown ends, you’ll be able to learn more about this work at NASA Dryden’s GASPS page. (Video credit: NASA Dryden)
Shock Trains
In compressible flows, shock waves are singularities, a tiny distance across which the density, temperature, and pressure of a fluid change suddenly and discontinuously. In this video, there is a wedge at the top and bottom of the frame and a Pitot probe roughly in the center. Flow is left to right and is initially subsonic. Once Mach 6 flow is established in the wind tunnel, a series of shock waves and expansion fans appear as light and dark lines in this schlieren video. Oblique shocks extend from the sharp tip of each wedge and interfere to create a normal shock in front of the Pitot probe. The air that passes through the normal shock is subsonic to the right of the shock, whereas air that goes through the oblique shocks remains supersonic. The fainter lines further to the right are weaker shock waves and expansion fans that reflect off the walls and probe. They exist to continue turning the airflow around the probe and to equalize conditions between different regions. (Video credit: C. Mai et al.)
Flame Feedback
When a flame is enclosed in a combustion chamber, it can create violent oscillations in the pressure field. Flames have a natural unsteadiness in their heat release. These temperature fluctuations create pressure waves in the chamber. In the right enclosure, those pressure waves resonate and feed energy back into the initial perturbation. This creates a self-exciting oscillation, not dissimilar from aeroelastic flutter. This combustion instability is known as a thermoacoustic instability because of the coupling between temperature and pressure (acoustic) waves. The quick demo above lets you see and hear such an instability; here’s the same setup in high-speed, which makes the oscillating flame even clearer. The violence of this instability can be great enough to destroy engines. Famously, the F1 engine used in the Saturn V rocket had a history of instability issues before the fuel-injector was redesigned. For another great demo of this effect, check out this video from T. Poinsot. (Video credit: V. Anandan)
Bouncing in Lockstep
Droplets of silicone oil bounce on a pool of the same thanks to the vibration provided by a loudspeaker. Each droplet’s bounce causes ripples in the pool and the interference between these ripples fixes the droplets in lockstep with one another. As long as the vibration continues to feed the thin layer of air that separates the droplets from the pool during each bounce and no impurities break the surface tension at the interface, the droplets will bounce indefinitely on their liquid trampoline. Such systems can be used to observe quantum-mechanical behavior like wave-particle duality on a macro-scale. (Photo credit: A. Labuda and J. Belina)
Fluids Round-up – 27 July 2013
Fluids round-up time! Here are our latest fluidsy links from around the web:
- Science@NASA explains how to use capillary action to drink one’s coffee in microgravity. (via io9)
- Nature is not exactly a quiet place. Here are a couple of things you probably haven’t heard: icebergs breaking up and running aground and the “seismic scream” preceding a volcanic eruption.
- Mars Curiosity’s work indicates that Mars once had a thick atmosphere but lost it about 4 billion years ago, possibly to the solar wind after losing its magnetic field.
- Check out this great looped surfing footage for a different perspective on waves (submitted by joteefox)
- io9 offers a primer on the Mach number. It’s worth noting that, for a(n ideal) gas, the speed of sound depends only absolute temperature and composition.
- Disney has designed a device called Aireal that uses vortex rings to provide haptic feedback. (submitted by vincent)
- Ever come across mammatus clouds before? Their distinctive shape is a result of forming from sinking air rather than rising air like most other clouds. (via io9)
(Photo credit: T. Thai)
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Rocket Sonic Boom
Originally posted: 22 July 2010 This video of the NASA Solar Dynamics Observatory’s launch is such a favorite of mine that it was part of the original inspiration for FYFD and was the very first video I posted. Watch closely as the Atlas V rocket climbs. At 1:51 you’ll see a rainbow-like cloud in upper right corner of the screen. This effect is created by sunlight shining through ice crystals of the cloud. A couple seconds later you see pressure waves from the rocket propagate outward and destroy the rainbow effect by re-aligning the ice crystals. Just after that comes the announcement that the vehicle has gone supersonic. The atmospheric conditions of the launch happened to be just right to make those pressure waves coming off the rocket visible just before they coalesced into a leading shockwave. (Video credit: B. Tomlinson)
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Levitation By Sound
Levitation is an effect usually associated with electromagnetic forces, but it’s possible with sound as well. This acoustic levitation is achieved by using the pressure from sound waves to balance gravity’s effect. By manipulating the sound, it’s possible to bring separate objects together while continuing to levitate them. The behavior is demonstrated in the video above by combining solid sodium with a drop of water for what any high school chemist will tell you is a spectacular reaction. (Though, if that’s too small-scale for you, there’s also this video.) (Video credit: D. Foresti et al; via SciAm)
Fluids Round-up – 13 July 2013
Prepare yourselves for lots of links in today’s fluids round-up!
- Longtime FYFD favorite Mark Stock (see here, here, and here) and his collaborator James Susinno have unveiled a new interactive art piece, “Everything is Made of Atoms” that utilizes some impressive real-time fluids simulation. NVIDIA’s blog has some details on the computing.
- ScarbsF1 takes a detailed look at the F-duct used to stall an F1 car’s rear wing to reduce drag. (submitted by Vinnie)
- Just in time for summer fun, National Geographic talks about the physics of water slides.
- SpaceX’s reusable Grasshopper rocket has set a new altitude test of over 1000 ft. Check out this feat of aerodynamic control over at io9.
- Stanford engineers are using high-speed video of birds in the wild to study the mechanics of flapping flight. If you check out their video, you’ll notice how the birds rotate their wings as they flap in order to maximize lift throughout the flapping cycle. (via io9)
- Speaking of io9, they highlighted a couple of great examples of meteorological fluid dynamics recently: roll clouds and water spouts.
- New research suggests that thresher sharks may whip their tails quickly enough to produce cavitation-induced shockwaves to stun their prey. If so, they join the pistol and mantis shrimp in utilizing this technique for hunting.
- If you’re looking for some casual games, Liquid Sketch is a fun fluids puzzle game for iOS (submitted by Keri B)
- Finally, congratulations to Toronoto’s AeroVelo for capturing the AHS Sikorsky Prize with their human-powered helicopter. Check out this video from their historic flight (submitted by Chris R).
(Photo credit: AeroVelo)
Reader Question: Non-Coalescing Droplets
Reader ancientavian asks:
I’ve often noticed that, when water splashes (especially as with raindrops or other forms of spray), often it appears that small droplets of water skitter off on top of the larger surface before rejoining the main body. Is this an actual phenomenon, or an optical illusion? What causes it?
That’s a great observation, and it’s a real-world example of some of the physics we’ve talked about before. When a drop hits a pool, it rebounds in a little pillar called a Worthington jet and often ejects a smaller droplet. This droplet, thanks to its lower inertia, can bounce off the surface. If we slow things way down and look closely at that drop, we’ll see that it can even sit briefly on the surface before all the air beneath it drains away and it coalesces with the pool below. But that kind of coalescence cascade typically happens in microseconds, far too fast for the human eye.
But it is possible outside the lab to find instances where this effect lasts long enough for the eye to catch. Take a look at this video. Here Destin of Smarter Every Day captures some great footage of water droplets skittering across a pool. They last long enough to be visible to the naked eye. What’s happening here is the same as the situation we described before, except that the water surface is essentially vibrating! The impacts of all the multitude of droplets create ripples that undulate the water’s surface continuously. As a result, air gets injected beneath the droplets and they skate along above the surface for longer than they would if the water were still. (Video credit: SuperSloMoVideos)