In the ocean, many forces compete in driving convection, including the temperature and salinity of the water. In the laboratory, it’s possible to mimic these characteristics of oceanic circulation using two different fluids driven by temperature and concentration differences. Recently, researchers were exploring this problem–with the added twist of tilting the fluids ~1 degree–when they discovered a surprising result. After an extended time, the convection self-organized into alternating parallel columns of ascending (dark) and descending (light) fluid. The researchers nicknamed this behavior super-highway convection. Read more about it here or in their paper. (Video credit: F. Croccolo et al; submitted by A. Vailati)
Year: 2013
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)
Fire-Breathing Physics
One of the most dangerous stunts for any fire-eater is breathing fire. Dr. Tim Cockerill explains some of the science behind the feat in this video. Volatility–the tendency of the liquid fuel to vaporize–is actually the enemy of a fire-eater. Use a fuel that is too volatile and it will catch fire too easily when the vaporous fuel mixes with the air. Instead fire-eaters use less volatile fuels and spray a mist of fine droplets to mix the air and fuel. This atomization of the fuel creates a spectacular fireball without endangering the fire-eater (as much). To see a similar fireball in high-speed, check out this post. (Video credit: T. Cockerill/The Ri Channel; via io9)
Flow Around a Complex Airfoil
Flow around an airfoil with a leading-edge slat is visualized above. At this Reynolds number, alternating periodic vortices are shed in its wake. Understanding how multi-element airfoils and control surfaces affect local flow is important in controlling aircraft aerodynamics. When multiple instabilities interact–like those in the wing’s boundary layer interacting with the wake’s–it can generate disturbances that are problematic in flight. Being able to predict and avoid such behavior is important for safe aircraft. (Photo credit: S. Makiya et al.)
Water Entry
In the image above we see two spheres of the same size, shape, and material being dropped into water. The left sphere has almost no splash, whereas the one on the right has a spectacular curtain-like splash. Why the big difference? It all comes down to the surface treatments. The glass sphere on the left is hydrophilic, but the right one has been treated to be hydrophobic. As a result, the water-fearing molecules of that sphere push the water away, allowing air to be entrained below the water’s surface instead. This creates a big splash that’s absent when the water moves smoothly around the hydrophilic sphere. (Photo credit: L. Bocquet et al.)
“Adrift”
Sometimes the time scales of a flow can mask its similarities to other flows. Simon Christen’s “Adrift,” a video of timelapsed fog in the San Francisco Bay area, shows just how these low clouds undulate and flow over the land the way a stream of water flows over and around stones. From the flow of gases in a stellar nursery down to the channels of a lab-on-a-chip, the same physics governs fluids everywhere, and there are always similarities to be found and exploited in our efforts to understand and explain fluid dynamics. (Video credit: S. Christen; via io9)
What is Pressure?
Pressure is a critical concept in fluid dynamics – a driving force behind everything from weather patterns to lift on a wing. But where does pressure come from? Like many macroscopic forces dealt with in fluid dynamics, pressure can be traced to the effects of individual molecules within a fluid. Kinetic theory describes gases as a collection of small particles which are all in constant, random motion. These particles’ collisions with each other and with their container create a multitude of tiny forces, as in the demonstration in the video above. When all of these collisions are summed together, their net effect is expressed as pressure, a force per area. (Video credit: Sixty Symbols)
Beads on a String
Adding just a small amount of polymers to a liquid can drastically change its behavior. The polymers make the liquid viscoelastic, meaning that, under deformation, the liquid shows behaviors that are both viscous (like all fluids) and elastic (i.e. able to resume its original shape, like a rubber band). These properties are particularly identifiable under extensional loading, like in the animation above. Under these loads, the polymers in the fluid stretch and rearrange, creating an internal compressive stress that acts opposite the imposed tensile stress. It’s this balance of forces, along with ever-present surface tension that creates the beads-on-a-string effect seen above. (Image credit: B. Keshavarz)
ETA: As usual, Tumblr gave me issues with an animated GIF. It should be fixed now. Sorry!
H Booms
Holidays involving fireworks deserve high-speed videos of hydrogen explosions. Although Periodic Table of Videos focuses on the chemistry involved in setting hydrogen on fire, there are some lovely fluid dynamics on display, too. There’s turbulence, combustion (obviously), and, if you watch closely, you can even see the initial vorticity caused by the rubber’s burst twisting the growing flames. (Video credit: Periodic Table of Videos)
