FYFD

Tag: fluid dynamics

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    Testing Flames in Space

    In microgravity, flames behave very differently than on earth due to a lack of buoyant forces. On earth, a flame can continue burning because, as the warm air around it rises, cooler air gets entrained, drawing fresh oxygen to the flame. In microgravity, both the heat from the flame and the oxygen it needs to burn move only by molecular diffusion, the random motion of molecules, or the background environmental flow (air circulation on the ISS, for example). This video shows a test of the Flame Extinguishment Experiment (FLEX) currently flying onboard the ISS. A fuel droplet is ignited, burns in a symmetric sphere and then eventually extinguishes either due to a lack of fuel or a lack of oxygen. Check out this NASA press release for more, including great quotes like this:

    “As a Princeton undergrad, I saw in a graduate course the conservation equations of combustion and realized that those equations were complex enough to occupy me for the rest of my life; they contained so much interesting physics.” – Forman Williams

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    Wave-Particle Duality in Bouncing Droplets

    A droplet atop a vibrating pool is prevented from coalescing by the constant influx of air into a thin lubrication layer between it and the pool. But that is not the strangest aspect of its behavior.  Researchers have found that this system demonstrates some aspects of the mind-bending wave-particle duality at the heart of quantum physics. (Submitted by Dan H.) #

  • Ink Sculptures

    Ink Sculptures

    Dripping ink into water can create fantastic structures as the two fluids mix. In this artwork there are numerous complex mixing phenomena: the eddies and multiple scales of turbulence; the long, thin streams of laminar flow; and the wispy mushrooms and umbrellas of the Rayleigh-Taylor instability. (Photo credit: Mark Mawson; via @thinkgeek)

  • Water Balloon Physics

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    This video explores some of the physics behind the much-loved bursting water balloon. The first sections show some “canonical” cases–dropping water balloons onto a flat rigid surface.  In some cases the balloon will bounce and in others it breaks. The bursting water balloons develop strong capillary waves (like ripples) across the upper surface and have some shear-induced deformation of the water surface as the rubber peals away. Then the authors placed a water balloon underwater and vibrated it before bursting it with a pin. They note that the breakdown of the interface between the balloon water and surrounding water shows evidence of Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The Rayleigh-Taylor instability is the mushroom-like formation observed when stratified fluids of differing densities mix, while the Richtmyer-Meshkov instability is associated with the impulsive acceleration of fluids of differing density.

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    Underwater Plumes

    During 2010’s Deepwater Horizon oil spill there were reports of underwater plumes of oil escaping collection. This video demonstrates how such a plume can form. There are two clips shown here; in both the tank is filled with salt water of varying salinity, with denser saltwater at the bottom. The first jet is a green alcohol/water mixture and the second is a red gauge oil. Both jets have the same density and flow rate, but they vary in their Reynolds number. The first turbulent jet gets trapped at the interface between the denser and lighter saltwater while the less turbulent red jet passes the interface with no difficulty. The researchers suggest that strong turbulence can create an emulsion, a mixture of two normally immiscible fluids–imagine shaking a container of oil and vinegar really well–which can lead to underwater trapping.

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    Cavity Collapse

    When a solid object is driven into a quiescent liquid, a cavity is formed. As the cavity collapses jets–a type of singularity–form.  In this video, researchers explore the effect of the geometry of a disk being driven into water on the shape of the cavity formed and how it collapses. As in this video of droplet impacts on posts of different geometries, there’s a lovely symmetry in the results. (Video credit: O. Enriquez et al)

  • Seed-Ejection via Raindrop

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    We don’t often think of plants as using fluid dynamics aside from capillary action drawing water from their roots, but many plants also use fluid dynamics to disperse reproductive materials.  This high-speed video explores the efficacy of splashing raindrops at ejecting seeds from different blossoms. (Video credit: G. Amador et al)

  • Bow Shock over a Perforated Plate

    Bow Shock over a Perforated Plate

    This schlieren image shows a sphere traveling at Mach 3 over a perforated plate. The bow shock in front of the sphere is clearly visible, as is its reflection off the plate. The pressure caused by the bow shock produces a series of spherical acoustic waves below the plate. A tiny vortex ring moves downward from each hole, followed at the right by a secondary ring moving upward from the holes in the plate. (Photo credit: U.S. Army Ballistic Research Laboratory; reprinted in Van Dyke’s An Album of Fluid Motion)

  • Shark Wakes

    Shark Wakes

    Volumetric imaging of swimming spiny dogfish, a type of shark, shows that their distinctively asymmetric tails produce a set of dual-linked vortex rings with every half beat of their tail. The figure above shows data from the actual shark on the right (b,d,f) and a similarly shaped robotic tail on the left (a,c,e). The second row contains lateral views (c,d) and the bottom row contains dorsal views (e,f) of the vorticity isosurfaces measured. The robotic tail does not demonstrate the same double vortex structure, leading scientists to suspect that the shark may be actively stiffening its tail mid-stroke to control its wake. The finding could help engineers design aquatic robots whose morphing fins help it swim more efficiently. For more, see Wired.

  • Freezing Drops

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    The physics of droplets freezing is important for understanding applications like ice formation on airplane wings. Here we see how a warm droplet deposited on a cold plate freezes. A freezing front advances through the drop, which expands vertically as it freezes. Ultimately, the expansion of the ice and the surface tension of the water create a pointed singular tip.