FYFD

Tag: vortex

  • Fire Tornadoes in Action

    Fire Tornadoes in Action

    Commonly called fire tornadoes, these terrifying vortices often occur in large wildfires and have more in common with dust devils or waterspouts than true tornadoes. They form when warm, buoyant air rises due to the fire’s heat. This creates low pressure over the fire source and draws in fresh, cooler air from the surroundings. If there is any small vorticity or rotational motion to that surrounding air, its spin will be amplified as it gets drawn in. This is akin to an ice skater spinning faster when she pulls her arms in – it’s a result of conservation of angular momentum. That intensification of the air’s rotation is what forms the vortex, which we see here due to the flames it draws upward. This footage was captured yesterday by crews fighting fires in Missouri.  (Image credit: Southern Platte Fire Protection District/WCPO 9, source)

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    Perching Physics

    Compared to birds, manmade aircraft tend to be quite limited and inelegant. Fixed-wing aircraft, for example, require long, flat areas for take-off and landing, whereas birds of all sizes are adept at maneuvers like perching. This video examines the perching behaviors of large birds and extends the physics to a small unmanned aerial vehicle (UAV). As a bird approaches a perching location, it pitches its body and wings upward. This places the bird in what’s known as deep stall, where air flowing over the upper surface of the wing separates just after the leading edge. This move dramatically increases drag on the bird, slowing it for landing. At the same time, the speed of the pitch maneuver generates a vortex on the wing that helps the bird maintain lift despite the drop in speed. With the help of both forces, the bird can make a graceful, controlled landing in only a short distance. (Video credit: J. Mitchell et. al.)

  • Hiding in the Sand

    Hiding in the Sand

    Flounders, stingrays, and other flat, bottom-dwelling fish often hide under sand for protection. These fish move by oscillating their fins or the edge of their bodies. They use a similar mechanism to bury themselves–quickly flapping to resuspend a cloud of particles, then hitting the ground so that the sand settles down to cover them. Researchers have been investigating this process by oscillating rigid and flexible plates and observing the resulting flow. When the flapping motion exceeds a critical velocity, the vortex that forms at the plate’s edge is strong enough to pick up sand particles. Understanding and controlling how and when these vortex motions kick up particles is useful beyond the ocean floor, too. Helicopters are often unable to land safely in sandy environments because of the particles their rotors lift up, and this work could help mitigate that problem. (Image credits: TylersAquariums, source; Richmondreefer, source; A. Sauret, source; research credit: A. Sauret et al.)

  • Fire Tornadoes

    Fire tornadoes, despite their name, are more closely related to dust devils or waterspouts than to true tornadoes. Though rarely documented, they are relatively common, especially in wildfires. The heat of the fire creates an updraft of warm, rising air that leaves behind a low-pressure region. Air from outside is drawn toward this low-pressure area, gets heated, and rises. As the outside air gets pulled in, any vorticity or rotation it had gets intensified via conservation of angular momentum–the same way a spinning ice skater speeds up when she pulls her arms in. The result is the tightly-spinning vortex at the heart of a fire tornado. (Video credit: C. Fleur; via NatGeo)

  • The Free Surface of a Typhoon

    The Free Surface of a Typhoon

    Gazing across the top of of Typhoon Maysak highlights the three-dimensionality of the storm. Like a swirling vortex seen in a bathtub, hurricanes are a kind of free surface vortex with a surface indentation near their eye. To understand this shape, imagine spinning a container of water on a rotating plate. Like the vortex, the water’s surface would take on a parabolic shape. The two forces acting on the rotating water are gravity in the downward direction and centrifugal force in the radial direction. By taking on a parabolic shape, the fluid remains perpendicular to the combination of these two forces at every point along the surface, thereby ensuring that pressure is a constant across the free surface of the fluid. (Image credits: S. Cristoferreti/ESA/NASA; T. Virts/NASA)

  • Newtonian and Non-Newtonian Vortices

    Newtonian and Non-Newtonian Vortices

    Not all vortex rings are created equal. Despite identical generation mechanisms and Reynolds numbers, the two vortex rings shown above behave very differently. The donut-shaped one, on the top left in green and in the middle row in blue, was formed in a Newtonian fluid, where viscous stress is linearly proportional to deformation. As one would expect, the vortex travels downward and diffuses some as time passes. The mushroom-like vortex ring, on the other hand, is in a viscoelastic fluid, which reacts nonlinearly to deformation. This vortex ring first furls and expands as it travels downward, then stops, contracts, and travels backward! (Image credit: J. Albagnac et al.; via Gallery of Fluid Motion)

  • Martian Dust Devil

    Martian Dust Devil

    This photo from the Mars Reconnaissance Orbiter stares almost straight down a dust devil on Mars. Like their earthbound brethren, Martian dust devils form when the surface is heated by the sun, causing warm air to rise. The rising air causes a low pressure area that the surrounding air flows into. Any rotational motion of the air intensifies as it is entrained. This is a consequence of conservation of angular momentum. Just as a spinning ice skater spins faster when he pulls his arms in, the vorticity of the inward-flowing air increases, forming a vortex. In addition to dust devils, this same physical mechanism applies to waterspouts and fire tornadoes, although the heating source for those is different.  (Photo credit: NASA; via APOD)

  • Below a Surfer’s Wave

    Below a Surfer’s Wave

    From below a plunging breaking wave–the classic surfer’s wave–looks like a giant vortex tube. Smaller rib vortices, the rings around the main vortex in the photo above, can form where there are variations along the breaking wave. As the wave rolls on, it stretches the vorticity variations along the wave’s span. When stretched, vortices spin up and intensify; this is a result of conservation of angular momentum. Check out more amazing photos of waves in Ray Collins’ portfolio. (Photo credit: R. Collins; via The Inertia)

  • Volcanic Vortex

    Volcanic Vortex

    This infrared image shows a kilometer-high volcanic vortex swirling over the Bardarbunga eruption. The bright red at the bottom is lava escaping the fissure, whereas the yellow and white regions show rising hot gases. Although the vortex looks similar to a tornado, it is actually more like a dust devil or a so-called fire tornado. All three of these vortices are driven by a heat source near the ground that generates buoyant updrafts of air. As the hot gases rise, cooler air flows in to replace them. Any small vorticity in that ambient air gets amplified as it’s drawn to the center, the same way an ice skater spins faster when she pulls her arms in. With the right conditions, a vortex can form. Unlike a harmless dust devil, though, this vortex is likely filled with sulphur dioxide and volcanic ash and would pose a serious hazard to aviation.  (Image credit: Nicarnica Aviation; source video; via io9)

  • Typhoon Neoguri

    Typhoon Neoguri

    Astronaut Reid Wiseman has been posting photos of Typhoon Neoguri in his Twitter feed this week. From our perspective on the ground, it’s easy to forget how three-dimensional the typhoons and hurricanes in our atmosphere are. But Wiseman’s photos capture the depth in the storm, especially the depression of the eye. From the top, the typhoon looks much like a vortex in a bathtub, or what’s more formally known as a free surface vortex. To understand why a vortex dips in the middle, imagine a container of water on a rotating plate. As the water is spun, its interface with the air takes on a paraboloid shape. Two external forces are acting on the fluid: gravity in the downward direction and a centrifugal force in the radial direction. The free surface of the fluid adopts a shape that is always perpendicular to the combination of these two forces. This ensures that the pressure along the free surface is a constant. (Photo credits: R. Wiseman 1,2,3)