FYFD

Tag: leading edge vortex

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    Meet BILLY

    Many wings in nature are not rigid. Instead they flex and curve with the flow. Here researchers imitate that phenomenon with BILLY (Bio-Inspired Lightweight and Limber wing prototYpe). Using an evolutionary-style algorithm, BILLY determines its own optimal flapping characteristics to maximize performance. Its flexible membrane-style wing actually performs better than a rigid wing! Check out the end of the video for some flow visualization of the leading edge vortex. (Image and video credit: A. Gehrke et al.)

  • Mimicking Insect Flight

    Mimicking Insect Flight

    There’s an oft-repeated tale that science cannot explain how a bumblebee flies. And while that may have been true 80 years ago, when engineers assumed they could apply their knowledge of fixed-wing aircraft to insects, it’s very far from the truth now.

    Being small, insects use aerodynamic tricks that are very different from the physics used by aircraft or even birds. Insects like fruit flies use a forward-and-backward sweeping motion at a very high angle of attack as they flap. This motion creates a vortex at the leading edge of the wing that provides the lift keeping the insect aloft. It still requires fast reflexes — most insects flap their wings hundreds of times a second — but the mechanism is robust enough to keep insects aloft and maneuverable. (Image credits: Robobee – K. Ma and P. Chirarattananon, simulation – F. T. Muijres et al., illustration – G. Lauder; via APS Physics)

  • Stall with Pitching Foils

    Stall with Pitching Foils

    For a fixed-wing aircraft, stall – the point where airflow around the wing separates and lift is lost – is an enemy. It’s the precursor to a stomach-turning freefall for the airplane and its contents. But the story is rather different when the wing is actively pitching through these high angles of attack. In this case, you get what’s known as dynamic stall, illustrated in three consecutive snapshots above.

    In the top image, the flow has clearly separated from the upper surface of the wing, but this isn’t a cause for panic. As the middle image shows, there’s a vortex that’s formed in that separated region and it’s moving backward along the wing as the angle of attack continues to increase. That vortex causes a strong low-pressure region on the upper surface of the wing, allowing it to maintain lift.

    In the final image, the vortex is leaving the wing, taking its low-pressure zone with it. This is the point where the pitching wing loses its lift, but if the vortex’s departure is immediately followed by a pitch down to lower angles of attack, the aircraft will recover lift and carry on. (Image credit: S. Schreck and M. Robinson, source)

  • Flying Backwards

    Flying Backwards

    Spend a summer afternoon floating in a kayak and chances are you’ll see some impressive aerial acrobatics from dragonflies. One of the dragonfly’s superpowers is its ability to fly backwards, which helps it evade predators and take-off from almost any orientation. To do this, the dragonfly rotates its body so that it is nearly vertical, thereby changing the direction it generates lift. In engineering terms, this is “force-vectoring,” similar to the techniques used by helicopters and vertical-take-off jets. 

    Scientists found that backwards-flying dragonflies could generate forces two to three times their body weight, in part due to the strong leading-edge vortices (bottom image) formed on the forewings. They also found that the hind wings are timed so that their lift is enhanced by catching the trailing vortex of the first pair of wings. Engineers hope to use what they’re learning from insect flight to build more capable flying robots. (Image and research credit: A. Bode-Oke et al., source; via Science)

  • Mosquito Flight

    Mosquito Flight

    Mosquitoes are unusual fliers. Their wings are long and skinny, and they beat at around 700 strokes a second – incredibly quickly for their size. Examining how they move has uncovered some interesting mechanics. Despite their short stroke length, the mosquito generates a lot of lift on both its upstroke (when the wing is moving backward) and its downstroke (when the wing moves forward). Some features of the mosquito’s flight are highlighted in the images above. In the animation, blue indicates areas of low pressure and red indicates high pressure.

    Like most flapping fliers, the mosquito generates a leading-edge vortex during its downstroke (and its upstroke). This vortex helps concentrate low pressure on the upward-facing wing surface, thereby creating lift. One of the things that makes the mosquito unique, however, is that it also creates trailing-edge vortices on both half-strokes. To do this, the mosquito rotates its wings precisely to catch the wake of its previous half-stroke. The flow gets trapped near the trailing edge of the wing and forms a vortex and low-pressure region. Like the leading-edge vortex, this low-pressure area on the upward-facing wing surface creates lift. For more secrets of mosquito flight, check out this video from Science or the original paper. (Image credit: R. Bomphrey et al., source)

  • Bumblebees in Turbulence

    Bumblebees in Turbulence

    Bumblebees are small all-weather foragers, capable of flying despite tough conditions. Given the trouble that micro air vehicles have when flying in gusty winds, bumblebees can help engineers to understand how nature successfully deals with turbulence. Under smooth laminar conditions like those shown in the animation above, bumblebees stay aloft by beating their wings forward and backward in a figure-8-like motion. On both the forward downstroke and the backward upstroke, you’ll notice a blue bulge near the front of the bee’s wing. This is a leading-edge vortex, which provides much of the bee’s lift.

    Researchers were curious how adding turbulence would affect their virtual bee’s flight. The still image above shows the bee in moderate freestream turbulence (shown in cyan). Surprisingly, this outside turbulence has very little effect on the flow generated by the bee, shown in pink. In fact, the researchers found that the bees could fly through turbulence without a significant increase in power. Too much turbulence does make it hard for the bee to control its flight, though. The bee’s shape makes it prone to rolling, and the researchers estimated, based on a bee’s 20 ms reaction time, that bumblebees can probably only correct that roll and maintain controlled flight at turbulence intensities less than 63% of the mean wind speed. (Image credits: T. Engels et al., source; via Physics Focus)

  • Hovering Hummingbirds

    Hovering Hummingbirds

    Hummingbirds are incredible flyers, especially when it comes to hovering. To hover stationary and stable enough to feed, the hummingbird’s flapping pattern not only has to generate enough lift, or vertical force, to counteract their weight, but the bird must balance any forward or backward forces generated during flapping.

    As you can see in the animations above, when hovering the hummingbird’s wings move forward and back rather than up and down. When slowed down even further, the figure-8 motion of the wings becomes apparent. This careful motion is key to the hover; it allows the bird to generate about 70% of its lift on the downstroke when the wings move forward and creates the remainder of the lift needed on the upstroke. For much more high-speed footage of hummingbirds, check out the full BBC Earth Unplugged video, but be warned: you may experience a cuteness overdose! (Image credit: BBC Earth Unplugged, source)

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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)

  • Flow Around a Delta Wing

    Flow Around a Delta Wing

    Colorful streaks of dye wrap like ribbons along the leading edge of a delta wing. At an angle of attack, this triangular wing forms a set of vortices that run along its edge, providing much of the low pressure–and therefore lift–on the upper surface of the wing. In contrast, the red streaks of dye in the middle of the wing demonstrate clean, laminar flow. Highly swept delta wings are popular for aircraft traveling at supersonic speeds, but they can also work well subsonically, as shown here. For more incredible and beautiful examples of flow visualizations by Henri Werlé, check out his 1974 film Courants et couleurs. (Photo credit: H. Werlé; via eFluids)

  • Hummingbird Hovering

    Hummingbird Hovering

    The hummingbird has long been admired for its ability to hover in flight. The key to this behavior is the bird’s capability to produce lift on both its downstroke and its upstroke. The animation above shows a simulation of hovering hummingbird. The kinematics of the bird’s flapping–the figure-8 motion and the twist of the wings through each cycle–are based on high-speed video of actual hummingbirds. These data were then used to construct a digital model of a hummingbird, about which scientists simulated airflow. About 70% of the lift each cycle is generated by the downstroke, much of it coming from the leading-edge vortex that develops on the wing. The remainder of the lift is creating during the upstroke as the bird pulls its wings back. During this part of the cycle, the flexible hummingbird twists its wings to a very high angle of attack, which is necessary to generate and maintain a leading-edge vortex on the upstroke. The full-scale animation is here. (Image credit: J. Song et al.; via Wired; submitted by averagegrdy)