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Search results for: “lift”

  • The Basilisk Lizard

    The Basilisk Lizard

    One of the most famous water-walking creatures is the common basilisk lizard. These South American reptiles are far too large to be kept aloft by surface tension and other interfacial effects. They generate the vertical force necessary to stay above water by slapping the water hard and fast. There are three phases to a basilisk’s water running gait: the slap, the stroke, and the retraction.

    In the slap phase, the lizard slams its foot flat against the water surface at a peak velocity of about 3.75 m/s. The impact pushes water down and generates an upward force on the lizard that accounts for between 15-30% of the lizard’s body weight, depending on the size of the lizard. The rest of the upward force comes from the stroke phase, where the lizard pushes its foot downward in the water, causing an air cavity to form.

    The air cavity is vital for the last phase of the lizard’s step. The basilisk must pull its foot out and prepare for the next slap, ideally doing so without generating too much drag. The lizard does this by pulling its foot through the air cavity before it seals. Doing so through air is much easier than through water.

    Water-walking this way requires fast reflexes. Basilisks take up to 20 steps per second when running across water and reach speeds of about 1.6 m/s. Although both juvenile and adult basilisks can run on water, the smaller lizards do better because they can generate more than enough impulse to overcome their weight. (Image credit: T. Hsieh/Lauder Laboratory, source; video credit: BBC; research credits: J. Glasheen and T. McMahon, G. Clifton et al.)

    This week FYFD is exploring the physics of walking on water, all leading up a special webcast March 5th with guests from The Splash Lab.

  • Sand Ripples in Tidal Flats

    Sand Ripples in Tidal Flats

    Sand, winds, and waves can interact to form remarkable and complex patterns. These sand ripples from the tidal flats of Cape Cod are a testament to such interactions. When a fluid like air or water flows over a flat bed of sand, it can shear and lift grains of sand, moving them to a new location. Very quickly, turbulence within the flow disturbs the initially smooth surface and begins to form the wavelike crests we see. Because the change in surface shape alters the nearby air or water flow, there is a trend toward self-organization and persistence. In other words, once the ripples form, they’re reinforced by their effect on the wind or water that formed them. Once rippled, the surface does not tend to smooth back out. (Image credit: N. Sharp; research credit: F. Sotiropoulos and  A. Khosronejad)

  • Skipping Squishy Spheres

    Skipping Squishy Spheres

    Skipping a stone on water requires a flat, disk-like stone thrown at a shallow angle, but elastic spheres are remarkable skippers, too, even at higher impact angles. Researchers at the Splash Lab have just published their work on why these balls skip so well. As seen in the top animation, the elastic spheres deform on impact, flattening to a more disk-like shape that rides at an angle of attack relative to the air-water interface. Both features are important to the spheres’ enhanced skipping. By flattening, the sphere comes into greater contact with the water and by orienting at a larger angle of attack, the sphere increases the vertical component of force the water generates on the sphere. It’s this vertical force that lifts the sphere up and lets it keep bouncing.

    Because the ball is soft, it keeps deforming after its impact and bounce (see top animation). For some skips, the timescale of the sphere’s elastic waves is smaller than the length of time the sphere is in contact with the water. When this is the case, the sphere’s elastic waves will affect the impact cavity in the water, forming what the researchers call a

    matryoshka cavity, after the Russian nesting dolls. An example is shown in the second animation. For more, check out the USU press release, the original paper, or the award-winning video they made a few years ago.  (Image credits: J. Belden et al./The Splash Lab)

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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.)

  • Helicopter Tip Vortices

    Helicopter Tip Vortices

    Airplanes and other fixed-wing aircraft produce wingtip vortices as a result of their finite length. Rotor blades, like those on helicopters, produce the effect as well. Both wings and rotors generate lift by trapping low-pressure air on their top surface and high-pressure air below. At their tips, though, the high-pressure air can sneak around the wing or rotor, creating vortices like the ones visualized above. Here smoke from a wire is entrained by the rotors’ inflow and twisted into a tip vortex. The line of vortices drifts downward due to the rotor’s downwash. (Image credit: M. Giuni et al., source)

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    The Tightrope Dancers

    Boiling is a process most of us don’t pay much attention to. But it can be remarkably entertaining and beautiful. This award-winning video shows boiling on and around a heated wire immersed in oil. Depending on the diameter of the wire and the power used to heat it, the researchers observe several different regimes of behavior. In one, vapor bubbles form on the wire and interact with one another: bouncing, merging, and dancing back and forth. When the bubbles become large enough, their buoyancy lifts them upward. In another regime, the wire is hot enough for film boiling. Like the Leidenfrost effect, film boiling occurs when a surface is so hot that it instantly vaporizes any liquid near it. The vapor layer then acts like coating, insulating the remaining liquid from the hot surface. The bubbles formed on the wire in this regime are mesmerizing, rising in periodic patterns or shifting back and forth gobbling up lesser bubbles. (Video credit: A. Duchesne 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.)

  • Re-Entry

    Re-Entry

    Atmospheric re-entry subjects vehicles to extreme conditions. At high Mach numbers, the leading shock wave compresses the air so strongly that it reaches temperatures hotter than the surface of the sun. At these temperatures, oxygen and nitrogen molecules in the air dissociate, bathing a vehicle in a plasma of ionized gas molecules. Often these atoms chemically react with the surface materials of a vehicle causing ablation that removes mass from the vehicle while helping protect the vehicle substructure from re-entry heating. Tests in specialized ground facilities like arc-jet plasma tunnels are necessary to develop thermal protection systems capable of shielding a vehicle during hypersonic flight. (Image credit: D. Ponseggi/NASA)

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    Visualizing Vortices

    Flow visualization can be a valuable tool for understanding fluid dynamics. In this video, we see how it can help elucidate the mechanisms of flapping flight. By dyeing vortices from the leading edge in red rhodamine and vortices from the trailing edge in green fluorescein, it’s possible to distinguish their competing effects for wings of different size. The speed and efficiency of a flapping wing depends on the vortices it sheds–these provide its lift and thrust. On a short wing, the leading edge vortex is significant and spins in a counter-clockwise (positive) direction. When it reaches the trailing edge, it meets a vortex spinning clockwise (negative). The interference of the two vortices weakens the shed vortex, thereby slowing the wing. Lengthening the wing weakens the leading edge vortex, which reduces its interference at the trailing edge and makes the longer wings more efficient. (Video credit: T. Mitchel et al.; via @AlbanSauret)

  • The Challenges of Micro Air Vehicles

    The Challenges of Micro Air Vehicles

    Interest in micro-aerial vehicles (MAVs) has proliferated in the last decade. But making these aircraft fly is more complicated than simply shrinking airplane designs. At smaller sizes and lower speeds, an airplane’s Reynolds number is smaller, too, and it behaves aerodynamically differently. The photo above shows the upper surface of a low Reynolds number airfoil that’s been treated with oil for flow visualization. The flow in the photo is from left to right. On the left side, the air has flowed in a smooth and laminar fashion over the first 35% of the wing, as seen from the long streaks of oil. In the middle, though, the oil is speckled, which indicates that air hasn’t been flowing over it–the flow has separated from the surface, leaving a bubble of slowly recirculating air next to the airfoil. Further to the right, about 65% of the way down the wing, the flow has reattached to the airfoil, driving the oil to either side and creating the dark line seen in the image. Such flow separation and reattachment is common for airfoils at these scales, and the loss of lift (and of control) this sudden change can cause is a major challenge for MAV designers. (Image credit: M. Selig et al.)

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