Tag: biology

Why Fishing with Dynamite is So Harmful
In some countries, there are still people using dynamite to catch fish. This practice is incredibly destructive, not just to adult fish but to the entire marine ecosystem. A blast wave traveling through air loses some its energy to the compression of the gas. Water, on the other hand, is incompressible, so the blast wave’s energy just keeps going, expanding its destructive radius. Many fish contain swim bladders, gas-filled organs the fish use to regulate their depth. When a shock wave passes through the fish, the gas in the swim bladder will expand and contract violently, much like the balloons shown underwater in the animation below. This typically ruptures the swim bladder and surrounding tissues.
Fish without swim bladders will often hemorrhage after being struck by a blast wave. The sudden changes in pressure create bubbles in the dissolved gases collected in their gills. Those bubbles tear apart the fish’s blood vessels.
Blasting is effective but entirely indiscriminate. It kills adults and juveniles of all species, not just the ones a fisherman can sell. Simultaneously, it destroys the slow-growing coral reefs that are key habitats for these populations. It’s an incredibly short-sighted practice that guarantees there will be no fish to catch in years to come. (Video credit: National Geographic; image credit: M. Rober, source; research credit: K. Dunlap, pdf)
June 7, 2016
Flying with Large Ears
Evolution often requires compromise between competing effects. Large-eared bats, for example, rely on the size of their ears to aid their echolocation, but such large ears can hurt them aerodynamically, thus limiting their flight. Results from a recent experiment, however, suggest that large ears are not a total loss aerodynamically speaking. Researchers used particle image velocimetry to study the wakes behind free-flying, large-eared bats and found significant downward flow behind the bats’ bodies. This indicates that the bats generate some lift with their ears, body, and/or tail. The position and tilt of the ears in flight is similar to forward swept wings, which the authors suggest could help contract the wake behind the ears and reduce its negative influence on flow over the wings. Although the evidence is not yet conclusive, the study does suggest that large ears may be more aerodynamically beneficial than they appear. (Image credit: L. Johansson et al./Lund University, source; via Jalopnik)
The next FYFD webcast will be this Saturday, May 21st at 1pm EDT. My guests will be Professor Jean Hertzberg of the University of Colorado at Boulder and Professor Kate Goodman of the University of Colorado at Denver. Dr. Hertzberg is the creator of the course Flow Visualization, an interdisciplinary course combining engineering, art, and fluid dynamics. It’s a class (and website) that’s been an inspiration for me and FYFD since the early days! Dr. Goodman, an expert in engineering education, earned her PhD studying the Flow Viz course and its impact. This will be wide-ranging discussion – with everything from experimental fluid dynamics and engineering education to art, photography, and hopefully even cardiac fluid dynamics!
(Original images: P. Davis et al.; B. Moore; L. Swift et al.)
May 16, 2016
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)
May 12, 2016
Fluttering Feathers
Birds do not always vocalize in order to make their songs. The male African broadbill, shown in the top video above, makes a very distinctive brreeeet in its flight displays, but as newly published research shows, the sound comes from its wings, not its voice. During the display, the broadbill spreads its primary feathers and sound is produced on the downstroke, when wingtip speeds reach about 16 m/s. By filming a broadbill wing with a high-speed camera in a wind tunnel at comparable air speeds, researchers could localize the sound production to the 6th and 7th primary feathers.
In the second video above, you can see these feathers twisting and fluttering in the breeze. This is an example of aeroelastic flutter, a phenomenon in which aerodynamic and structural forces couple to induce oscillations. The same phenomenon famously caused the collapse of the Tacoma Narrows Bridge in 1940. In the birds, however, the flutter is non-destructive and the vibration produces audible sound which the other feathers modulate into the calls we hear. Broadbills aren’t the only birds to use this trick; some species of hummingbirds use flutter in their tail feathers during mating displays. (Video, image, and research credits: C. Clark et al.; additional videos here)
April 6, 2016
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)
April 5, 2016
Mushrooms Make Their Own Breeze
Plants and other non-motile organisms have developed some clever methods to disperse their seeds and spores for reproduction. Some plants use vortex rings for dispersal; others make their seeds aerodynamic. Low ground-dwellers like mushrooms must contend with a lack of wind to lift their spores and carry them away. Instead, they use evaporative cooling to generate their own air currents.
Mushroom caps contain a lot of water and, as that water evaporates, it cools air near the mushroom, just as sweat evaporating off your skin cools you. That cooler, denser air tends to spread, carrying the spores outward. At the same time, the freshly evaporated water vapor is less dense than the surrounding air, so it rises. This combination of rising and spreading is capable of carrying spores tens of centimeters into the air, where the wind is stronger and able to carry spores further. (Image credit: New Atlantis, source; research credit: E. Dressaire et al.)
March 31, 2016
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)
March 30, 2016
Filter-Feeding Mantas
Large filter-feeders like the manta ray face the interesting challenge of obtaining enough small particulates like plankton to sustain an animal the size of a car. They do this through what is known as ram filter-feeding, essentially swimming open-mouthed through food-laden waters, filtering out the food, and releasing the water through their gills. Their internal filtration doesn’t simply catch particles like a colander does, though – it would be too easy for the ray’s filters to clog. Instead, the animals use several alternative methods to catch and redirect particles toward their esophagus. One, known as crossflow filtration, causes water to turn sharply through the filters. Heavier particles cannot accelerate that quickly, so they are carried onward. Another method, vortex filtration, works like a tiny centrifuge, spinning the water and ejecting the heavier particles back toward the esophagus. (Video credit: Science Friday; research credit: E. Paig-Tran, thesis)
March 23, 2016
Review: “Life in Moving Fluids”
If you liked the prairie dog post earlier this week and you’re interested in more examples of biological fluid dynamics, you may enjoy Steven Vogel’s “Life in Moving Fluids”. I’m often asked for suggestions of readable textbooks for those who want an introduction to fluid dynamics, and this book is a great option. It addresses a wide variety of basic fluids concepts without getting as bogged down mathematically as many of the engineering texts do. It is written as an introduction to fluid dynamics for working biologists, though, so it contains plenty of technical detail – including relevant equations, discussions of basic flow measurement techniques, and overviews of the early academic literature.
It is also chock full of interesting biological applications of fluid dynamics with examples ranging from the growth patterns of barnacles to the shape-shifting drag capabilities of trees. Vogel keeps a light-hearted tone and dry humor throughout and doesn’t shy away from puns.
I read a first edition of the book (copyright 1981). The second edition, from the mid ‘90s, has updated coverage of the research literature, but I dare say the the topic has exploded within the last 20 years, so your mileage may vary with regard to the literature review. However, age in no way impacts the quality of Vogel’s coverage of the basics of fluid dynamics, and I feel confident in recommending this as an introductory text for those who’d like to pursue fluids in more depth. (Images: S. Vogel/Princeton U. Press; h/t to Chris R.)
March 18, 2016
Prairie Dog Physics
One challenge facing burrowing mammals is ensuring sufficient oxygen within their den. Prairie dogs achieve this with a clever use of Bernoulli’s principle. They build multiple entrances to their tunnels. One of them, labeled as Entrance A above, is built with a raised mound of dirt, while the other, Entrance B, is not. The raised mound creates an obstacle for the wind to move around, which increases the wind velocity at Entrance A compared to the normal wind speed at Entrance B. From Bernoulli’s principle, we know that a higher velocity means a lower pressure, so there is a decreasing pressure gradient through the tunnel from Entrance B to Entrance A. That favorable pressure gradient pulls fresh air through the prairie dog tunnels, allowing the colony to breathe easy. (Image credits: N. Sharp; original prairie dog photos by jinterwas and J. Kubina; final images shared under Creative Commons; research credit: S. Vogel et al.; h/t to Chris R.)
March 15, 2016