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  • Galapagos Week: Marine Iguanas

    Galapagos Week: Marine Iguanas

    One of the most unique inhabitants of the Galapagos Islands is the marine iguana. These reptiles live in colonies of thousands and subsist entirely on marine algae. Smaller iguanas are intertidal feeders, grazing on green and red algae when it is exposed near low tide. But the largest iguanas feed near midday by swimming out and diving to feed on richer pastures. 

    The iguanas are surprisingly good swimmers, even though marine iguanas exhibit little extra specialization for it compared to other iguana species. They swim both at the surface and underwater with an undulatory motion driven by their tails. The iguana also streamlines its body somewhat by tucking its legs along its sides. Although the marine iguana is a much slower and less efficient swimmer than a bony fish of equal size, swimming is still a good choice for getting around. The marine iguana expends only 75% as much energy per distance swimming as it does walking. The big challenge is staying warm in the cold Galapagos waters. Small iguanas are both less efficient swimmers and lose body heat faster. This is why you’ll only see the biggest iguanas feeding underwater. (Image credits: N. Sharp; research credit: K. Trillmich and F. Trillmich; J. Videler and B. Nolet; G. Bartholomew)

    This is the first post of Galapagos Week here on FYFD. Check back every day for new Galapagos-themed posts!

  • Galapagos Week: Introduction

    Galapagos Week: Introduction

    One hundred and eighty-two years ago today, the H.M.S. Beagle reached the Galapagos archipelago carrying, among others, naturalist Charles Darwin. The ship would spend the next month exploring the islands, and Darwin’s experiences during that time, and the specimens he collected, would ultimately lead him to propose the concept of evolution.

    I had the incredible opportunity to visit the Galapagos Islands last October, and, like so many before me, I was fascinated by the islands and their remarkable ecosystems. The Galapagos Islands are located at the equator, but they owe much of their rich biodiversity to sitting at the confluence of several ocean currents, both warm and cold. In particular, the cold Cromwell Current’s upwelling on the western side of the archipelago carries valuable nutrients up from the deep and helps support vibrant marine life from bioluminescent plankton to leaping mobula rays. (And, yes, I geeked out over both.)

    Over the next week, FYFD will be exploring some of the fluid dynamics of the Galapagos Islands and their denizens on land, sea, and air. Be sure to check back every day for a new post! (Image credit: N. Sharp and J. Shoer)

  • Farewell, Cassini!

    Farewell, Cassini!

  • Featured Video Play Icon

    Lift Over Wings

    One of the most vexing topics for fluid dynamicists and their audiences is the subject of how wings generate lift. As discussed in the video above, there are a number of common but flawed explanations for this. Perhaps the most common one argues that the shape of the wing requires air moving over the top to move farther in the same amount of time, therefore moving faster. The flaw here, as my advisor used to say, is that there is no Conservation of Who-You-Were-Sitting-Next-To-When-You-Started. Nothing requires that air moving over the top and bottom of a wing meet up again. In fact, the air moving over the top of the wing outpaces air moving underneath it.

    In the Sixty Symbols video, the conclusion presented is that any complete explanation requires use of three conservation principles: mass, momentum, and energy. In essence, though, this is like saying that airplanes fly because the Navier-Stokes equations say they do. It’s not a terribly satisfying answer to someone uninterested in the mathematics.

    Part of the reason that so many explanations exist – here’s one the video didn’t touch on using circulation – is that no one has presented a simple, intuitive, and complete explanation. This is not to say that we don’t understand lift on fixed wings – we do! It’s just tough to simplify without oversimplifying.

    Here’s the bottom line, though: the shape of the wing forces air moving around it to change direction and move downward. By Newton’s 3rd law (equal and opposite reactions), that means the air pushes the wing up, thereby creating lift. (Video credit: Sixty Symbols)

  • Elastic Bounces

    Elastic Bounces

    A rigid ball accelerated by a moving surface can only ever move as fast as the surface propelling it. But that’s not true for squishy objects like a water droplet. The composite image above shows the trajectory of a water droplet launched from a moving superhydrophobic surface. As the surface starts rising, it squishes the droplet like a pancake, triggering a deformation cycle where the droplet will squish and extend repeatedly. How quickly the drop changes shape depends on factors like its size and surface tension. The researchers found that a droplet’s launch was strongly affected by the ratio of the droplet’s shape-changing frequency and the frequency of the plate’s motion. When the drop’s shape changed three times faster than the surface’s motion, it would catapult off the surface with 250% of the kinetic energy of a rigid ball!

    Launching elastic balls works the exact same way as droplets, indicating that the phenomenon depends on the way the projectiles deform. The process is similar to jumping on a trampoline. If a trampolinist times her jump just right, she’ll get more energy from the trampoline and fly higher. The droplet does the same when its deformation is properly tuned to its catapult. (Image credit: C. Raufaste et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Flying Fish Aerodynamics

    Flying Fish Aerodynamics

    Flying fish, strange as it sounds, have aerodynamic prowess comparable to hawks. The fish aren’t true fliers, but they do glide for hundreds of meters using their large pectoral and pelvic fins as wings. Wind tunnel research shows the fish have their maximum lift at an angle of attack around 30-35 degrees, matching their typical take-off angle (top). Their best gliding performance occurs when they’re roughly parallel to the water (middle). The researchers even found that the fish use ground effect to enhance their lift. Although their aerodynamics allow flying fish to get out of reach of their aquatic predators, the fish must be wary of flying too high, as this makes them a target for frigatebirds (bottom). These acrobatic seabirds can’t get wet, but they have some impressive aerodynamics of their own to help make up for it.  (Image credit: BBC Earth, source; research credit: H. Park and H. Choi; see also SciAm)

  • Adaptive Meshing

    Adaptive Meshing

    The use of numerical simulations in fluid dynamics has exploded over the past half century with new computational techniques being developed constantly. Most methods involve solving the equations of motion (or an approximation thereof) on a grid of points known as a mesh. To accurately capture the physics, meshes must often be quite closely packed in areas where detail is needed, but they can be more widely spaced in areas where the flow is not changing quickly. An increasingly common technique is adaptive meshing in which the mesh of grid points shifts between time steps; this places more grid points where the flow requires them and removes them from less important areas in order to reduce computational time.

    An example of adaptive meshing is shown above. On the left particles are falling into salt water. The colors show the concentration of particles. The right side shows the solid particles and the fluid mesh around them. Notice how the grid shifts as the particles fall. (Image credit: C. Jacobs et al., source)

  • Wriggling Threads

    Wriggling Threads

    A thread of mineral oil laid across a pool of water twists and turns like a river run wild. Because the oil has a lower surface tension than the water, Marangoni forces spread it outward (far left). Small variations in the thread make the areas of highest oil concentration start to bend just a bit. Inside the bends, the gradient of surface tension – the difference between the lowest and highest surface tensions – is very high, which pulls at these regions more than others. So bends beget more bends, causing the entire thread to wrinkle. Although the behavior is driven by a completely different process than the one that causes rivers to meander, the end result looks remarkably similar; this is because, in both cases, forces act to make each bend increasingly sinuous. (Image credit: B. Néel et al., source)

    Editor’s note: Starting tomorrow I’ll be on a trip that takes me out of range of the Internet until next week. Regular posts are queued up and should post as usual, but we’ll all have to trust Tumblr to handle everything because I won’t be able to check. Thanks!

  • Soaring Pelicans

    Soaring Pelicans

    Earlier this summer, I looked up on a bright, sunny day and saw a quartet of black and white figures soaring overhead. Initially, I thought it might be a formation of kites or unmanned aerial vehicles (UAVs) because I saw no flapping as the group wheeled about. With the help of the Cornell Lab of Ornithology’s awesome Merlin app, I was able to identify the soarers as American white pelicans – not a species I’d expected to find flying along the Front Range of the Rocky Mountains! (Turns out, they breed on lakes around here.)

    The reason I saw so little flapping is that the birds were riding thermals. As the sun heats the ground, air near the surface warms up and begins to rise due to its buoyancy. Pelicans interested in flying between breeding and foraging grounds will start testing the thermals early in the day, as soon as they begin to form. As the heating continues, the intensity of thermals strengthens and they extend higher into the atmosphere. This is where the birds can really excel at using atmospheric energy for their flight. Pelicans will circle within a thermal until they reach roughly the middle of its height. Then they will glide, gradually losing altitude until they reach another thermal where they can climb without expending their own energy. With a 2.7 meter wingspan and a relatively low drag coefficient, the pelicans can glide and soar remarkably well. Researchers have even suggested using them as a sort of biological UAV for studying atmospheric dynamics! (Image credits: D. Henise, M. Stratmoen; research credit: H. Shannon et al., pdfs – 1, 2)

  • Review: “ABCs From Space”

    Review: “ABCs From Space”

    For me, one of the most fun aspects of studying science is seeking out examples of it in the world around us. Adam Voiland – who writes for NASA Earth Observatory, one of FYFD’s favorite sources for excellent fluids in action – takes this a step further with his children’s book “ABCs From Space: A Discovered Alphabet”. Voiland has sought out satellite imagery from around the world to illustrate all twenty-six letters, creating a lovely book for budding scientists of all sorts.

    Each letter has its own full-page image with no added text, like the G and H shown above. Younger children will have fun identifying and tracing out each letter. The back of the book provides more detail for older kids and adults, including brief descriptions of where and what each image shows, a map of all image locations, and some FAQs about satellite imagery and the geology, meteorology, and earth science on display. There are enough specifics to satisfy casual interest, but I suspect that science-inclined adults will find the book a fun springboard for more in-depth discussions with curious kids.

    Fluid dynamics itself makes a solid showing in the book. Several letters are formed by vortices (like G above) and various types of clouds, including the ship track clouds (like H) that form when water condenses on aerosols released by ship exhaust. There are also meandering rivers, creeping glaciers, and erosion features among the letters.

    I’m often asked about resources for teaching kids about fluid dynamics, and Voiland’s book is a great option for introducing that subject, as well as many other fields of science. (Image credits: A. Voiland/Simon & Schuster)

    Disclosure: I received a review copy of this book but was not otherwise compensated by the author or publisher. All opinions are my own. Additionally, this post contains affiliate links. Purchases made using these links do not cost you anything extra but may provide FYFD with a commission. Thanks!

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