FYFD

Category: Research

  • Featured Video Play Icon

    Vortex Shedding from a Hot Cylinder

    This numerical simulation shows vortex shedding behind a hot cylinder. The behavior is very similar to what one sees behind an unheated cylinder, until the coefficient of thermal expansion increases and the von Karman vortex street is completely distorted. Describing the particulars of the computation, jessecaps writes (links added):

    I wrote an incompressible flow solver to simulate flow past a heated cylinder. The Navier-Stokes equations are discretized on a Cartesian grid and solved explicitly in time. The pressure-Poisson equation is solved implicitly using a bi-conjugate gradient method. The Boussinesq approximation was used (density is constant everywhere except for the gravity term) to account for buoyancy. Reynolds number = 250, Froude number = 1 (gravity is pointing down). The two simulations show the effect of the coefficient of thermal expansion. Each video shows a plot of velocity and temperature.

    (submitted by jessecaps)

  • Featured Video Play Icon

    Coronal Waves

    NASA’s Solar Dynamics Observatory has found evidence of Kelvin-Helmholtz waves in the sun’s corona. These waves, which occur between two fluids of different densities or moving at different speeds, are similar to the iconic waves surfers ride. Researchers suspect that this turbulent motion may help explain why the corona is 1,000 times hotter than the surface of the sun. #

  • Featured Video Play Icon

    Hot Spheres Sink Faster

    New research shows that the Leidenfrost effect–which causes water droplets to skitter across a hot pan–can drastically reduce the drag on objects moving through a liquid. When raised to a high enough temperature, a sphere falling water will be coated in a protective layer of vapor (see video above) that acts like a lubricant as the sphere moves through the water. If the temperature of the object drops too low, the vapor layer will dissolve into a mess of bubbles (~35 secs into video). One way that this mechanism reduces drag is by keeping flow attached to the sphere for longer as shown in this video. Preventing this flow separation increases the pressure recovered after the point of lowest pressure (the shoulders of the sphere), which reduces overall drag.

    See also:

  • Featured Video Play Icon

    Water-Walking Basilisks

    Some animals, like the common basilisk (a.k.a. the Jesus Christ lizard) are capable of running across water for short distances. The basilisk accomplishes this feat by slapping the water with sufficient force and speed to keep its body above the surface. This slap also creates a pocket of air around its foot. The lizard propels itself forward by kicking its leg back, then lifting its foot out of the water before the air bubble collapses. Water birds like the Western Grebe and tail-walking dolphins rely on similar physics to stay above the water line. # (submitted by Simon H)

  • Microgravity Combustion

    Microgravity Combustion

    Combustion in microgravity is markedly different than that on earth, due to a lack of buoyant convection. The combustion of a droplet of heptane is shown here as a composite image. The bright yellow structure shows the path of the droplet, which gets smaller as it burns. The green structures show the initial development of soot, which eventually streams outward as long streaks. # (submitted by jshoer)

  • Featured Video Play Icon

    How Dogs Drink

    Not long ago, researchers showed that cats use friction to their advantage when drawing liquids into their mouths. New research shows that dogs rely on the same mechanism–they’re just far less efficient with it. The dog touches its backwards-curled tongue to the surface of the water; when it draws the tongue back, friction causes a column of fluid to follow. The dog then closes its jaws around the water. Some water also gets picked up by the back of the tongue, but since dogs have no cheeks, it spills out the sides, creating a mess familiar to dog owners. #

  • Flowing Up a Waterfall

    Flowing Up a Waterfall

    Tea-drinking physicists found that it’s possible for particles to flow up a short (< 1 cm) waterfall to contaminate pure upstream sources. Their apparatus is shown above, along with an inset showing the velocity field on the surface of the channel. The blue arrows indicate flow downstream and the red arrows indicate counterflow that carries particles upstream. The researchers suspect that Marangoni effects may play a role in setting up the counterflow. The finding could have implications for pollution control and manufacturing. # (submitted by Gabe)

  • Featured Video Play Icon

    Feathering on SpaceShipTwo

    Virgin Galactic and Scaled Composites recently performed their first feathered flight with SpaceShipTwo, which is on track to be the first commercial spaceship. Feathering is a re-entry technique devised by Scaled Composites founder Burt Rutan:

    Once out of the atmosphere the entire tail structure of the spaceship can be rotated upwards to about 65º. The feathered configuration allows an automatic control of attitude with the fuselage parallel to the horizon. This creates very high drag as the spacecraft descends through the upper regions of the atmosphere. The feather configuration is also highly stable, effectively giving the pilot a hands-free re-entry capability, something that has not been possible on spacecraft before, without resorting to computer controlled fly-by-wire systems. The combination of high drag and low weight (due to the very light materials used to construct the vehicle) mean that the skin temperature during re-entry stays very low compared to previous manned spacecraft and thermal protection systems such as heat shields or tiles are not needed. During a full sub-orbital spaceflight, at around 70,000ft following re-entry, the feather lowers to its original configuration and the spaceship becomes a glider for the flight back to the spaceport runway. #

    Though it works well for decelerating from sub-orbital speeds, feathering is sadly not useful for orbiting spacecraft due to the much higher kinetic energies that have to be dissipated.

  • White Hole Analogues

    White Hole Analogues

    A white hole–the cosmological opposite of a black hole–is a singularity from which matter emerges but which matter can never enter from beyond the event horizon. Hydraulic jumps, those rings that sometimes appear in the kitchen sink, turn out to be a physical analog of this behavior. The photo above shows a hydraulic jump with a needle placed inside the event horizon. In the wake of a needle, there’s a Mach cone, just like when an object moves faster than the speed of sound. For more, see the Photonist. (via freshphotons)

    Note that we mentioned this item a few months ago, but the full paper has just been published.

  • Venom Properties

    Venom Properties

    Most venomous snakes deliver venom to their prey via grooves in their fangs, rather than through a pressurized bolus through hollow fangs. New research shows that these venoms are shear-thinning non-Newtonian fluids. The surface tension of the venom is such that a drop of venom will tend to flow into and down the groove. Once moving, the shear-thinning properties of the venom decrease the venom’s viscosity, increasing its flow rate down the fang and into the snake’s prey. (via Scientific American; Photo: green mamba, banded snake fang)