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)
Tag: science
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)
Self-Siphoning Stream of Beads
Pull a bit of a long chain out of a container, and you’ll quickly find the beads take on a life of their own, siphoning out of the jar while leaping and looping in the air. Some of the dynamics are clear – the ever-growing free end of the chain has weight enough to pull the rest of the chain out, much like the pressure difference that drives a siphon. But a lot of the rest of the dynamics are unclear and have generated a lot of discussion. It turns out that the same behavior is observed for chain laid out on a horizontal surface (video links on the right of that page) and even the dynamics of that simpler version of the problem are complex. All part of the beauty inherent in Newton’s second law. (Video credit: Steve Mould/Earth Unplugged; Research credit: J. A. Hanna et al.; submitted by Elin R)
Dancing Jets
Vibrating a gas-liquid interface produces some exciting instability behaviors. The photo above shows air and silicone oil vibrated vertically within a prism. For the right frequencies and amplitudes, the vibrations produce liquid jets that shoot up and eject droplets as well as gas cavities and bubble transport below the interface. To see a similar experiment in action, check out this post. (Photo credit: T. J. O’Hern et al./Sandia National Laboratories)
