This stunning National Geographic photo contest winner shows an F-15 banking at an airshow and a array of great fluid dynamics. A vapor cloud has formed over the wings of the plane due to the acceleration of air over the top of the plane. The acceleration has dropped the local pressure enough that the moisture of the air condenses. Some of this condensation has been caught by the wingtip vortices, highlighting those as well. Finally, the twin exhausts have a wake full of shock diamonds, formed by a series of shock waves and expansion fans that adjust the exhaust’s pressure to match that of the ambient atmosphere. (Photo credit: Darryl Skinner/National Geographic; via In Focus; submitted by jshoer)
Tag: science
Liquid Lenses
Here astronaut Andre Kuipers demonstrates fluid dynamics in microgravity. A roughly spherical droplet of water acts as a lens, refracting the image of his face so that it appears upside down. The air bubble inside the droplet refracts the image back to our normal perspective again. (Photo credit: Andre Kuipers, ESA; via Bad Astronomy)
The Backward-Facing Step
This photo collage shows vortices shed off a backward-facing step. The flow is left to right. Here the flow is visualized using dye released in water. Initially, the vortex forms near the bottom of the step in the recirculation zone. Because flow over the top of the vortex is much faster than the flow beneath the vortex, a low pressure zone forms over the vortex and gradually draws it up toward the top of the step. Eventually the vortex will rise to the point where the upstream flow pushes it downstream and the process begins anew. (Photo credit: Andrew Carter, University of Colorado)
Homemade Hybrid Rocket Engine
In this video, Ben Krasnow details and demos a small hybrid rocket engine he built in his workshop. Hybrid rockets utilize propellants that are two different states of matter, in this case gaseous oxygen as the oxidizer and solid acrylic as the fuel. Krasnow’s verbal explanation of a convergent-divergent nozzle, used to accelerate flow to supersonic speeds is not quite right. In reality, a compressible fluid like air reaches the sonic point (i.e. Mach 1) at the narrowest point of the nozzle, also called the throat. The divergent portion of the nozzle causes the compressible fluid to expand in volume, which drops the temperature and pressure while the velocity increases beyond the speed of sound.
Krasnow says he did no calculations for his rocket, but I decided to have a little fun by doing some myself. Supersonic flow through the nozzle is only achieved if the flow is choked, meaning that the mass flow rate through the nozzle will not increase if the downstream pressure is decreased further relative to the upstream pressure. For Krasnow’s rocket, the downstream pressure is atmospheric pressure (14.7 psi) and the upstream pressure is provided by the oxygen canister, which he notes was at most 80 psi. Fortunately, the upstream pressure necessary to choke the nozzle is only 27.8 psi, so even with the ball valve partially closed, Krasnow’s rocket is definitely capable of supersonic speeds.
The Mach number achievable by any given supersonic nozzle is related to the ratio of the nozzle throat to its exit diameter (#). Krasnow gives the throat diameter as ¼-inch and the exit diameter as 5/8-inch. This means that the Mach number at the exit of the nozzle, assuming choked supersonic flow, is about Mach 3.4. (Video credit: Ben Krasnow; via Universe Today; submitted by jshoer)
Atomizing
High-speed video reveals the complexity of fluid instabilities leading to atomization–the breakup of a liquid sheet into droplets. A thin annular liquid sheet is sandwiched between concentric air streams. As the velocity of the air on either side of the liquid sheet varies, shear forces cause the sheet to develop waves that result in mushroom-like shapes that break down into ligaments and droplets. Quick breakup into droplets is important in many applications, most notably combustion, where the size and dispersal of fuel droplets affects the efficiency of an engine. (Video credit: D. Duke, D. Honnery, and J. Soria)
Shark-Tooth Instability
A viscous fluid inside a horizontally rotating circular cylinder forms a shark-tooth-like pattern along the fluid’s free surface. This is one of several patterns observed depending on the fluid’s viscosity and surface tension and the rotational rate of the cylinder. (Photo credit: S. Thoroddsen and L. Mahadevan; for more, see Thoroddsen and Mahadevan 1996 and 1997)
Formula 1 Aerodynamics
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Computational fluid dynamics (CFD) and the advent of supercomputing have forever changed the way engineers design. Here the use of CFD in the design of Formula 1 racing cars is discussed. Although CFD is used by many companies in place of wind tunnel testing, each method has its advantages. CFD provides information about all flow quantities at all points in the flow but can only do so with an accuracy dependent on the grid and models used. It remains impossible to solve the equations of motion exactly for any problem of practical application because the computational cost is simply too high; instead software packages like FLUENT utilize turbulence models that approximate the physics. Wind tunnel testing, on the other hand, is physically accurate but typically yields only limited data and flow quantities due to the difficulty of instrumentation. (Video credit: BBC News; submitted by carhogg)
(Source: /)Acoustic Levitation
Researchers at Argonne National Laboratory are using acoustic levitation of droplets to further pharmaceuticals. By placing two precisely aligned speakers opposite one another, a standing wave can be created. At nodes along the standing wave, there is no net transfer of energy, but the acoustic pressure is sufficient to cancel the effect of gravity, allowing light objects like droplets to levitate. This is why, in the video, you see the droplets are placed at equally spaced distances and if one is slightly off the node, it vibrates noticeably. The benefit of this levitation to pharmaceutical research comes at the molecular level; drugs formed from solutions kept in a solid container are likely to be crystalline in structure and thus less efficiently absorbed by the body. If the drug can instead be kept in an amorphous state by evaporating the solution without a container, then the resulting drug may be effective at a lower dosage than its crystalline counterpart. (Video credit: Argonne National Laboratory, via Laughing Squid, submitted by @__pj)
Ferrofluid Drop
A drop of ferrofluid is shaped by seven small circular magnets sitting beneath the glass and paper. Ferrofluids are made up of nanoscale ferromagnetic particles suspended in a carrier liquid. Under the influence of magnetic fields, they can take on fantastic shapes, including sharp-tipped droplets and labyrinthine mazes. This image is taken from the National Academy of Science’s book Convergence, focused on the intersection between science and art. (Photo credit: Felice Frankel)
Boiling Without Bubbles
Water droplets sprinkled on a sufficiently hot frying pan will skitter and skate across the surface on a thin layer of vapor due to the Leidenfrost effect. When a solid object is much warmer than a liquid’s boiling temperature, the surface is surrounded by a vapor cloud until the solid cools to the point that the vapor can no longer be sustained. Then the vapor breaks down in an explosive boiling full of bubbles. Unless, as researchers have just published in Nature, the solid is treated with a superhydrophobic coating. The water-repellent surface prevents the bubbling, even as the sphere cools. The technique could be used to reduce drag in applications like the channels of a microfluidic device. (Video credit: I. Vakarelski et al.; see also Nature News; submitted by Bobby E)
