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  • Rio 2016: Swimming

    Rio 2016: Swimming

    Strange as it seems, elite swimmers are faster when swimming underwater than they are at the surface. So much so, in fact, that they’re restricted to being underwater only 15 m after a dive or turn. To see just how stark a difference this makes, check out this crazy video.  (I know, right?!)

    To understand how this is possible, it helps to look at the three types of drag a swimmer experiences: pressure drag, skin friction, and wave drag. Pressure drag is probably the most familiar; it’s the drag that comes from the swimmer’s shape and how the fluid moves around it. Skin friction is the drag that comes from viscous friction between the swimmer and the water. The final type, wave drag, comes from the energy expended to create waves at the surface of the water. As you might expect, energy that goes into splashing is energy that isn’t going into propulsion.

    When swimming at the surface, swimmers experience a lot of wave drag. At least one experiment showed that wave drag accounted for most of a surface swimmer’s drag. In contrast, at a depth of more than 0.5 m, a swimmer’s wave drag is virtually negligible. The submersion does come at the cost of higher skin friction (since more of the swimmer is in contact with the water), but there is also more opportunity for useful propulsion since both sides of a kick can move water (and not air.) Bonus read for those interested in more: Is the fish kick the fastest stroke yet? (Image credits: AP; B. Esposito)

    Previously: what makes a pool fast?

    Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!

  • Crisscrossing Clouds

    Crisscrossing Clouds

    This natural-color satellite image shows crisscrossing cloud patterns off coastal Africa. These distinctive lines in the sky are gravity waves, and they form when air masses get displaced upward by terrain or other conditions. In this case, dry air cooled overnight on land before moving out over the ocean. That displaced warm, humid air above the water and forced it upward, where it eventually cooled and condensed into clouds. Gravity created the ripple-like waves; as the moist air cooled, gravity again pulled it downward – leaving behind a clear sky. Once the humid air sank, the dry air pushed it up again, creating another line of clouds and continuing the cycle.  (Image credit: NASA; via NASA Earth Observatory)

  • Martian Ripples

    Martian Ripples

    Earth and Mars both feature fields of giant sand dunes. The huge dunes are shaped by the wind and miniature avalanches of sand, and their surface is marked by small ripples less than 30 centimeters apart. These little ripples are formed when sand carried by the wind impacts the dunes. But Martian dunes have a second, larger kind of ripple, too. These sinuous, curvy ripples lie about 3 meters apart and cast the dark shadows seen in the images above. On Earth we see ripples like these underwater, where water drags sand along the surface. On Mars, the same process is thought to play out with the wind, and so scientists have named these wind-drag ripples. (Image credit: NASA/JPL/MSSS; via APOD, full-res; submitted by jshoer)

  • Reversing Time

    Reversing Time

    Waves contain lots of information. They are also time invariant, which means that they will behave the same regardless of whether time moves forward or backward. This isn’t a property we observe often in life since time just moves forward for us. But a new experiment has demonstrated a method of wave control that can, in a sense, roll back the clock.

    To do this, the scientists created a instantaneous time mirror, or ITM. When they create a disturbance on the surface of a pool of water, it sends out capillary waves in the form of ripples. A short time later, they accelerate the pool sharply downward. This universal disturbance is their instantaneous time mirror, which generates backward-propagating ripples. Those new backward-propagating waves travel back toward the source and refocus into the shape of the initial disturbance. This works for both a simple point disturbance (top image) and for a more complicated geometry like a smiley face (bottom image). (Image credit: V. Bacot et al., source; submitted by @g_durey)

    ETA: To be clear, this experiment does not refute causality. It’s more like saying that the information for the initial conditions is still carried on in the later state and that you can do something to extract that information.

  • Daily Fluids, Part 4

    Daily Fluids, Part 4

    Inside or outside, we encounter a lot of fluid dynamics every day. Here are some examples you might have noticed, especially on a rainy day:

    Worthington Jets
    After a drop falls into a pool, there’s a column-like jet that pops up after it and sometimes ejects another small drop. This is known to fluid dynamicists as a Worthington jet, but really it’s something we all see regularly, especially if you watch rain falling onto puddles or look really closely at your carbonated drink.

    Crown Splash
    Like the Worthington jet, crown splashes often follow a drop’s impact into another liquid. But they can also show up when slicing or stomping through puddles!

    Free Surface Dynamics
    Anytime you have a body of water in contact with a body of air, fluid dynamicists call that a free surface. How the interface between the two fluids shifts and transforms is fascinating and complicated. Waterfalls are a great example of this, but so are ocean waves or even the ripples from tossing a rock into a pond.

    Hydrophobic Surfaces
    Water-repellent surfaces are called hydrophobic. Water will bead up on the surface and roll off easily. While many manmade surfaces are hydrophobic, like the teflon in your skillet, so are many natural surfaces. Many leaves are hydrophobic because plants want that water to fall to the ground where their roots can soak it up. Keep an eye out as you wash different vegetables and fruits and see which ones are hydrophobic!

    Check out all of this week’s posts more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)

  • Featured Video Play Icon

    Diffraction

    Wave phenomena can sometimes be a little difficult to wrap one’s head around. In this video, Mike from The Point Studios explains wave diffraction and why opening a window can help you spy on the conversation next door. Diffraction occurs when waves encounter an obstacle. If that obstacle is a slit in a wall, the slit becomes a point source, radiating waves outward spherically. The video focuses on acoustics, but diffraction matters in more than just sound – it’s key to water ripples, light and other electromagnetic waves, and, according to quantum theory, the fundamental building blocks of matter.   (Video credit: The Point Studios)

  • Why Does This Kite Look So Real?

    Why Does This Kite Look So Real?

    A recent viral video features mesmerizing footage of a giant octopus kite flown at a kite festival in Singapore earlier this month. The kite’s arms twist and wave lazily in the breeze. Watching the video, I was struck by how realistic the kite’s motion looks. It really looks like an octopus is just cruising there in mid-air. And that resemblance might not be accidental.

    In fluid dynamics, scientists often use a concept called dynamic similitude to test the physics of a scale model instead of the full-size original. The simplest version of this uses the Reynolds number to compare the model and the original. The Reynolds number is a dimensionless number that depends on the object’s size, the flow’s speed, and the density and viscosity of the fluid. If you match the scale model’s Reynolds number to the original’s Reynolds number, then the physics will be the same – even if you changed the fluid or the size of the object.

    Returning to our kite, one thing the footage doesn’t entirely convey is just how enormous this kite really is. The Straits Times reports the kite is about the length of five buses and requires six people to get aloft. But the kite’s size helps compensate for the fact that it’s flying in air instead of swimming through viscous water like a real octopus. Although I’m left estimating the kite’s size and the wind’s speed, my quick calculations put the Reynolds numbers for the kite and the octopus on the order of 10,000. So, strange as it seems, this giant kite really is acting like a swimming octopus!

    (Image credits: E. Chew, source)

  • Bursting Into Droplets

    Bursting Into Droplets

    Our atmosphere is full of aerosols – extremely tiny particles and droplets of salt, dust, pollutants, and other substances. Wind’s effects alone cannot account for the sizes and quantities of aerosols we measure. Another potential source is the bursting of bubbles; more specifically, the bubbles that form at the oceans’ surface. Frothy, crashing waves often capture pockets of air. When these bubbles burst, the thin film of their surface ruptures into long filaments that break into tiny droplets. Such droplets can be small enough to get carried on the breeze, eventually evaporating and leaving the particulates that were once in the water to ride the winds. (Image credit: H. Lhuissier & E. Villermaux; see also: Y. Couder)

  • Bioluminescent Plankton

    Bioluminescent Plankton

    The blue-outlined dolphins you see above get their glow from microorganisms called dinoflagellates. They are a type of bioluminescent plankton, shown in the lower image, that can be found in oceans around the world. Their glow comes from combining two chemicals: luciferase and luciferin. The dinoflagellates suspended in the ocean do this when they are disturbed–specifically, when the water around them transmits a shear stress above a certain threshold. Typically, this is caused by something larger–a potential predator–moving past, although it can also be stimulated by breaking waves. The higher the shear stress, the more intense the glow, but the dinoflagellates only use their bioluminescence sparingly. If you apply shear stress and keep applying it, their glow fades away without reactivating. After all, they can only produce so much chemical fuel. (Image credit: BBC from Attenborough’s Life That Glows; h/t to Gizmodo; research credit: E. Maldonado and M. Latz)

  • Featured Video Play Icon

    Fluids Round-up

    Time for another look at some of the best fluids content out there. It’s the fluids round-up – with a special focus this week on oceans!

    – Ryan Pernofski spent two years filming the ocean in slow motion with his iPhone to make the short film “Slowmocean” seen above. It’s a gorgeous ode to the beauty of breaking waves.

    – Oceans with higher salinity than Earth’s could drive global circulation that would make exoplanets more hospitable to life.

    – Speaking of alien oceans that could harbor signs of life, there’s discussion afoot of how future missions to icy moons like Europa or Enceladus could collect samples from plumes ejected from beneath the ice.

    – Wind and waves make harsh, erosive environments. This photo essay from SFGate shows how greatly the sands of Pacifica shift over time. (submitted by Richard)

    Bonuses:

    – New research explores how Martian mountains may have been carved out by the wind.

    – Ever listened to an orchestra made from ice? You should! Learn about Tim Linhart, who builds and maintains ice instruments. (submitted by ashketchumm)

    – MIT has demonstrated a new 3D-printing technique that allows for printing liquid and solid parts simultaneously, allowing would-be creators to rapid-prototype hydraulically-driven robotics.

    Even more bonus bonus!

    – ICYMI, the new FYFD video made Gizmodo!

    If you’re a fan of FYFD, please consider becoming a patron. As a bonus, you’ll get access to this weekend’s planetary science webcast!

    (Video credit: R. Pernofski; via Flow Visualization; Pluto image credit: NASA/APL)

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