FYFD

Category: Phenomena

  • Airplanes Creating Snow

    Airplanes Creating Snow

    Scientists now think that that airplanes may be responsible for increasing local snowfall by flash-freezing supercooled water vapor in clouds. Water droplets can persist in the atmosphere to temperatures of -42 degrees Celsius. But when an airplane’s wing passes through moist air, the acceleration of the air passing over the wing causes a pressure decrease that can drop the temperature by as much as 19 C, causing the water droplets to form ice crystals immediately. (The particulate matter in the aircraft exhaust probably also aids this process.) The same behavior can also create holes in clouds and cause ice to form on the wings. # (Related behavior: vapor cones)

    Photo credit: lhoon

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    Water Drops at 10,000 FPS

    We’ve seen water droplets join a larger pool at 2,000 frames per second, but what about 10,000 frames per second? (via Gizmodo)

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    Mach Diamonds

    Joe asks:

    Why does this rocket have that repeating pattern in its exhaust? I’m amazed that it’s so stable for so far as distance from the nozzle.

    Excellent question! The diamond-shaped pattern seen in the rocket’s exhaust is actually a series of reflected shock waves and expansion fans. The rocket’s nozzle is designed to be efficient at high altitudes, which means that, at its nominal design altitude, the shape of the nozzle is such that the exhaust gases will be expanded to the same pressure as the ambient atmosphere. At sea level, the nozzle is overexpanded, meaning that the exhaust gases have been expanded to a lower pressure than the ambient. The supersonic exhaust has to reach ambient pressure, and it does so through an oblique shock right at the exit of the nozzle. However, the oblique shock, in addition to raising the pressure, turns the gases toward the exhaust centerline. To ensure flow symmetry, two additional oblique shocks form. But then the exhaust is at a higher pressure than ambient. Expansion fans form to reduce the pressure, but those, too, affect the direction the exhaust gases flow. The pattern, then, is a series of progressively weaker oblique shocks and expansion fans that raise the exhaust gas pressure to that of the ambient atmosphere.

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    Crown Breakup

    When a droplet falls into a pool of similar fluid, one often observes a crown-like impact effect. This student video shows high-speed footage of different fluids crowning and explores the effects of surface tension on crown breakup.

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    Microgravity Water Films

    In this video astronaut Don Pettit demonstrates some interesting laminar flow effects using a water film in microgravity. By using a film, fluid motion is essentially confined to two dimensions. This is important because it prohibits the development of turbulence, which is a purely three-dimensional phenomenon. Doing the experiment in microgravity allows Pettit to leave the experiment for a long period of time without buoyant effects or similar disturbances. When he first stirs the film, the tracer particles show some signs of what looks like turbulent mixing, but soon the film rotates uniformly with streaks of gray caused by different concentrations of tracer particles. Pettit notes that he allowed the film to rotate overnight and it eventually all turned milky white. This is the effect of molecular diffusion of the tracer particles; without turbulence, the only way for mixing to occur is through the random motion of molecules. See more of Pettit’s Saturday Morning Science videos for additional microgravity fluid mechanics.

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    Seeing the Invisible

    Schlieren photography is a common experimental flow visualization technique, especially in supersonic flows (where it enables one to see shock waves). Here the Science Channel’s “Cool Stuff: How It Works” show explains the technique and shows some examples from everyday life.

  • Vibrating Oobleck

    [original media no longer available]

    This video explores some of the non-Newtonian behaviors of oobleck when shaken. The pattern across the surface once the vibrations start is called Faraday waves, a type of nonlinear standing wave that forms once a critical vibrational frequency is passed and the flat surface of the fluid becomes unstable. Toward the end of the video, the frequency of the vibrations is increased until “finger-like protrusions” form. This is a behavior exhibited by shear-thickening non-Newtonian fluids.

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    Wavy Vortices

    Shown above is the flow between two concentric cylinders (Taylor-Couette flow). In the laminar regime, the velocity profile between the two cylinders is linear. As the rate of rotation of the inner cylinder increases, the flow develops toroidal vortices known as Taylor vortices, seen in the video above after 9 seconds or so. This is a fluid instability exhibited by transitional flow. Increasing the rotational rate further can result in wavy Taylor vortex flow. At high enough speeds, the flow will become completely turbulent.

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    Superfluid Dripping

    This high-speed video shows superfluid helium dripping and breaking up. Although superfluid has no viscosity, this does not prevent the Plateau-Rayleigh instability from breaking the helium into droplets once the mass of the liquid is too great for surface tension to contain.

  • Tears of Wine

    Tears of Wine

    Tears of wine are caused by the Marangoni effect, in which a gradient in surface tension causes mass flow. The water in the wine has a higher surface tension than the alcohol in the wine, causing the wine to be drawn away from regions of higher alcohol concentration. #