FYFD

Tag: hydrodynamics

  • Whale Feeding

    Whale Feeding

    Whether in groups or as individuals, humpback whales are canny hunters. They herd prey together by encircling them and releasing bubbles that form a “net” that bars escape. Then, the whales lunge through the center with open mouths, gathering prey. Scientists have long wondered whether humpbacks’ unusually long pectoral fins played any role in their hunting. New drone observations of whales feeding (see video below) are beginning to provide some hints.

    The scientific teams observed multiple individual whales feeding under the same circumstances and found that the whales used their fins quite differently. Both used them as additional barriers to prevent prey from escaping, but one whale favored a horizontal fin position that created currents that helped sweep prey into its mouth. The other whale used a more vertical fin position that, while hydrodynamically unfavorable, exposed its bright underside, which seemed to startle prey into fleeing into its darker, more inviting mouth. (Image credit: K. Kosma; video credit: M. Kosma; research credit: M. Kosma et al.; via Science)

  • Floccing Particles

    Floccing Particles

    Adding particles to a viscous fluid can create unexpected complications, thanks to the interplay of fluid and solid interactions. Here we see a dilute mixture of dark spherical particles suspended in a layer of fluid cushioned between the walls of an inner and outer cylinder. Initially, the particles are evenly distributed, but when the inner cylinder begins to rotate, it shears the fluid layer. Hydrodynamic forces assemble the particles together into loose conglomerates known as flocs. Once the particles form these log-like shapes, they remain stable thanks to the balance between viscous drag on particles and the attractive forces that pull particles toward one another. (Image and research credit: Z. Varga et al.; submitted by Thibaut D.)

  • Paddling

    Paddling

    When I lived in New England, I often spent summers paddling around a lake in either a kayak or canoe. Every stroke was an opportunity to stare down into the dark water and watch how the flow curled around my oar. Here you see a bit of what that looks like from underwater.

    The animation above shows a flat plate – twice as tall as it is wide – submerged about 20 mm below the surface and accelerated steadily from rest. As it starts moving, there’s a clear vortex ring formed and shed behind it. You can also see how the plate distorts the free surface into large depressions. Both of these cause extra drag on the plate. Eventually, though, the plate reaches a steady state.

    All together, what you see here is a good representation of what’s going on when a rower first begins to accelerate their boat from rest. Hydrodynamically speaking, the best way to do that isn’t to dig in with a deep stroke. It’s to use a series of short, relatively shallow strokes to get the boat up to speed. This takes advantage of the efficiency of drag generation during acceleration to get the boat to its cruising speed quickly. (Image and research credit: E. Grift et al.)

  • Catching Prey

    Catching Prey

    The skinny, freshwater alligator gar can grow to more than 2 meters in length, giving it a distinct resemblance to its namesake. But this fish’s history traces back more than a hundred million years to the Early Cretaceous. And a new (pre-printed) study, combining live observations and numerical models built from CT-scans, is shedding new light on how the gar and its prehistoric ancestors feed.

    The gar uses a lateral strike (top) to come at its prey from the side. But hydrodynamically speaking, that’s a tough way to catch dinner. As soon as the gar’s snout accelerates toward its prey, it pushes a bow wave ahead of it, like an early warning signal. To counter that disadvantage, the gar has a complex bone structure in its skull (bottom) that helps it generate suction. Note how the gar’s jaw and throat open sequentially from front to back. Each expansion sucks in water, and by timing them just right, the gar produces suction throughout its entire attack. The bow wave warning does its prey no good if both are already getting sucked into the gar’s mouth! (Image and research credit: J. Lemberg et al., bioRxiv pre-print; via Science; submitted by Kam-Yung Soh)

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    Underwater Snakes, Gusty Flying, and Microswimmers

    If you like your fluid dynamics with a healthy dose of biology, this video’s for you! Learn about the hydrodynamics of snake strikes, how birds fly in gusty crosswinds, and the mathematical underpinnings of a microswimmer’s journey. This is the final video in our FYFD/JFM collaboration featuring research from the 2017 APS DFD meeting. If you missed any of the previous videos, you can see them all here. Which one is your favorite? Would you like to see the series continue? Let me know in the comments or on Twitter! (Image and video credit: N. Sharp and T. Crawford)

  • Stone Skipping Physics

    Stone Skipping Physics

    The current record for stone-skipping is about 88 skips. For most of us, that’s an unimaginably high number, but according to physicists, human throwers may top out around 300 or 350 skips. In the video above and the accompanying article, Wired reporter Robbie Gonzalez explores both the technique of a world-record-holding skip and the physics that enable it.

    The perfect skip requires many ingredients: a large, flat rock with good edges; a strong throw to spin the rock and hold it steady at the right angle of attack; and a good first contact with the right entry angle and force to set up the skips’ trajectory. The video is long, but it’s well worth a full watch. It gives you an inside look both at a master skipper and at the experts of skipping science. (Video and image credit: Wired; see also: Splash Lab, C. Clanet et al.; submitted by Kam-Yung Soh)

    ETA: Wired’s embed code is acting up, so if you can’t see the stone skipping video here, just go to the article directly.

    Heads up for those going to the APS DFD meeting! You can catch my talk Monday, Nov. 19th at 5:10PM in Room B206. I’ll be talking about how to use narrative devices to tell scientific stories. I’ll be around for the whole meeting, so feel free to come say hi!

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    Dinosaurs, Propellers, and Hiding Objects

    The latest FYFD/JFM video is out, and it’s all about the interactions between structures and flows! We learn about plesiosaur-inspired underwater robots, how turbulence affects air-water interfaces, and how adding a tail can help hide an object in a flow. If you missed one of the previous episodes in this series, you can find them all here. (Image and video credit: T. Crawford and N. Sharp)

  • The Flutter of Kelp

    The Flutter of Kelp

    Many species of kelp change their blade shape depending on the current they experience. In fast-moving waters, the kelp grows flat blades, but when the water around them is slower, the same plant will grow ruffled edges on its blades. In a slow current, the ruffled version’s extra drag causes it to flutter up and down with a large amplitude. That helps spread the blades out to catch more sunlight and increase photosynthesis, but it comes at the cost of higher drag, which could tear the plant from its holdfast.

    In contrast, the flat-bladed kelp collapses into a more hydrodynamic shape. This clumps the flat blades together, making photosynthesis harder, but it streamlines the kelp, making it easier to resist getting ripped out by fast-moving tides. (Image credit: J. Hildering; research credit: M. Koehl et al.; submission by Marc A.)

  • Leaping Mobulas

    Leaping Mobulas

    Mobula rays are second only to manta rays in size, and, unlike their larger cousins, relatively little is known about them. Like other rays, they propel themselves by flapping their large pectoral fins, and they generate thrust through hydrodynamic lift. They’re quite efficient swimmers, able to generate enough thrust to leap over 2 meters out of the water before flopping back into it. Why the mobula rays jump and why they seem to prefer belly-flopping is unclear. They may be using the slap and splash to communicate with one another. When aggregations of mobulas are observed from overhead, jumping seems to occur along the outside of the group. Maybe this is an effort to attract more mobulas to a group or a method of scaring prey into the midst of the hunting mobulas. In any case, it is spectacular to behold firsthand. (Image credit: BBC; source)

  • Building Smart Swimmers

    Building Smart Swimmers

    Scientists have long wondered whether the schooling of fish is driven by hydrodynamic benefits, but the complexity of their environment makes unraveling this complex motion difficult. A recent study uses a different tactic, combining direct numerical simulation of the fluid dynamics with techniques from artificial intelligence and machine learning to build and train autonomous, smart swimmers.

    The authors use a technique called deep reinforcement learning to train the swimmers. Essentially, the swimmer being trained is able to observe a few variables, like its relative position to the lead swimmer and what its own last several actions have been – similar to the observations a real fish could make. During training, the lead swimmer keeps a steady pace and position, and the follower, through trial and error, learns how to follow the leader in such a way that it maximizes its reward. That reward is set by the researchers; in this case, one set of fish was rewarded for keeping a set distance from their leader, one intended to keep them in a position that was usually beneficial hydrodynamically. Another set of fish was rewarded for finding the most energy-efficient method for following.

    Once trained, the smart swimmers were set loose behind a leader able to make random decisions. Above you can see the efficiency-seeker chasing this leader. Impressively, even though this smart swimmer had the option to go it alone (and had never followed such a dynamic leader), it does an excellent job of keeping to the leader’s wake. Compare it with real swimmers and there’s a definite similarity in their behavior, which suggests the technique may be capturing some of an actual fish’s intuition. (Image and research credit: S. Verma et al., source; thanks to Mark W. for assistance)