FYFD

Tag: biology

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    How COVID-19 Affects the Lungs

    One of the best known COVID-19 symptoms of this pandemic is difficulty breathing, and while you’ve likely heard a lot about ventilators used to help patients get oxygen, you may not know much about the processes that cause the breathing problems. This video from Deep Look provides a solid overview of the infection route and how lung damage occurs during infection. Perhaps unsurprisingly — this is FYFD, after all — fluid dynamics plays a major role in this process, both under normal conditions and when air sacs in the lungs get damaged by the body’s immune system responding to the virus. (Image and video credit: Deep Look)

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    Singing in the MRI

    We rarely consider just how complex the process is when we speak or sing. Sound waves produced in our larynx are shifted and amplified by the geometry of our throats, mouths, sinus cavities, tongues, and lips. This video provides a glimpse of that hidden complexity through a trained vocalist singing inside an MRI machine. He sings the same aria in four distinctly different vocal styles, and it’s incredible to watch all the changes his tongue, lips, and soft palette go through to produce those different sounds. (Image and video credit: T. Ross; via Flow Vis)

  • Bristling Sharkskin Fights Separation

    Bristling Sharkskin Fights Separation

    The speedy shortfin mako shark has a secret weapon to fight drag: bristling denticles that line its fins and tail. Denticles are tiny, anvil-shaped enamel scales on the mako’s skin. In the photo above, each one is about 100 microns across. Under normal conditions, with flow moving over the shark from nose to tail, the denticles lie flat, providing no interference.

    But when sudden changes in flow near the shark’s skin cause water to begin moving in the opposite direction, the denticles flare up. Their rise interferes with the reversed flow, trapping it in small eddies beneath each denticle. Since that flow reversal is a precursor to the flow separating from the shark’s body, the bristling effectively cuts off flow separation before it can begin. The result is much less separation and much lower drag. Once the flow stops trying to move upstream, the denticles settle back into their original place. (Image credit: mako shark – jidanchaomian, denticles – J. Oeffner and G. Lauder, illustration – A. Lang, bristling – A. Lang et al.; research credit: A. Lang and A. Lang et al.; submitted by Kam-Yung Soh)

  • Steering as a Boxfish

    Steering as a Boxfish

    Coral reefs are full of odd-looking denizens, but one of the funniest-looking ones must be the boxfish. This family of fish lives up to its name; their bodies feature an angular, bony carapace that helps protect them. But you don’t have to be a fluid dynamicist to wonder how in the world they swim with that kind of shape.

    There’s actually disagreement in scientific circles as to whether the basic shape of a boxfish is stabilizing or destabilizing, in other words, whether the fish’s body shape will try to automatically turn or roll when flow moves past. A new study focuses instead on the role the fish’s tail fin serves. Through experiments (on a fish model) and simulations, the researchers showed that boxfish rely on their tail fins both as rudders and course-stabilizers.

    Living around coral reefs means that boxfish need to be highly maneuverable, and this research indicates that the fish’s body shape, combined with the stabilizing power of its tail, are key to its ability to quickly and easily turn in any direction. (Image credits: boxfish – D. Seddon, simulation – P. Boute et al.; research credit: P. Boute et al.; via NYTimes; submitted by Kam-Yung Soh)

  • COVID-19 and Outdoor Exercise

    COVID-19 and Outdoor Exercise

    By now you’ve probably come across some blog posts and news articles about a new pre-print study looking at the aerodynamics of running and the potential exposure to exhaled droplets. And you may also have seen articles questioning the accuracy and validity of such simulations. I’ve had several readers submit questions about this, so I dug into both the research and the criticisms, and here are my thoughts:

    Is this study scientifically valid?

    I’ve seen a number of complaints that since this paper hasn’t been peer-reviewed, we shouldn’t trust anything about it. That seems like an unreasonable overreaction to me considering how many studies receive press attention prior to their actual peer-reviewed publication. This is not a random CFD simulation produced by someone who just downloaded a copy of ANSYS Fluent. This work comes from a well-established group of engineers specializing in sports aerodynamics, and long-time readers will no doubt recognize some of their previous publications. Over the past decade, Blocken and his colleagues have become well-known for detailed experimental and simulation work that indicates larger aerodynamic effects in slipstreams than what we generally recognize.

    In this paper, they lay out previous (biological) studies related to SARS and droplet exhalation; they use those papers and several wind tunnel studies to validate computational models of droplet evaporation and runner aerodynamics; and then they use those inputs to simulate how a cloud of exhaled droplets from one runner affects someone running alongside, behind, or in a staggered position relative to the first runner.

    In other words, their work includes all the components one would expect of a scientific study, and it makes scientifically justifiable assumptions with regard to its methods. (That’s not, mind you, to say that no one can disagree with some of those choices, but that’s true of plenty of peer-reviewed work as well.) All in all, yes, this is a scientifically valid study, even if it has not yet undergone formal peer-review*.

    Can simulations actually tell us anything about virus transmission?

    One complaint I’ve seen from both biologists and engineers is that simulations like these don’t actually capture the full physics and biology involved in virus transmission. While I agree with that general sentiment, I would point out two important facts:

    1) Blocken et al. acknowledge that this is not a virology study and confine their scientific results to looking at what happens physically to droplets when two people are moving relative to one another. Whether those droplets can transmit disease or not is a question left to biological researchers.

    2) Most medical and biological research also does not account for the physics of droplet transmission and transport. For the past century, this research has focused almost exclusively on droplet sizes, with the assumption that large droplets fall quickly and small droplets persist a little longer. To my knowledge, some of the only work done on the actual physics of the turbulent cloud produced by coughing or sneezing comes from Lydia Bourouiba’s lab at MIT. And, to me, one of the fundamental conclusions from her work is that droplets (especially small ones) can persist a lot longer and farther than previously assumed. Can those droplets facilitate transmission of COVID-19? The general consensus I’ve seen expressed by medical experts is no, but, to my knowledge, that is based on opinion and assumption, not on an actual scientific study.

    The bottom line

    In my opinion, there’s a big disconnect right now between the medical/biological community and the engineering community. To truly capture the physics and biology of COVID-19 transmission requires the expertise and cooperation of both. Right now both sides are making potentially dangerous assertions.

    Honestly, based on what I know about aerodynamics, I am personally skeptical as to whether 6 ft of physical separation is truly enough; whether it is or not seems to depend on how transmissible the novel coronavirus is through small droplets, which, again, to my knowledge, is unestablished.

    Should we leave more distance than 6ft between us when exercising outdoors? Absolutely. Aerodynamically, it makes perfect sense that following in someone’s slipstream would put you inside their droplet cloud, which needs time and space to disperse. Personally, I’ve sidestepped the question entirely by doing all my cycling indoors while quarantined.

    tl;dr: There are a lot of open questions right now about COVID-19 transmission and what qualifies as safe distancing, but it’s smarter to err on the side of more distancing. Don’t hang close to others when running or cycling outdoors.

    (Image and research credit: B. Blocken et al.; submitted by Corky W. and Wendy H.)

    *I will add that, with my training, I have and do occasionally peer-review studies such as this one, and I read the full paper with the same sort of critical eye I would turn to a paper I was asked to review.

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    Why Animals Shake Themselves Dry

    For many animals, letting themselves air-dry is not an option. They would become hypothermic before their wet fur dried completely. This is why dogs and many other furry mammals shake themselves dry. It’s a remarkably efficient process, too, removing the majority of water from fur in a matter of seconds.

    The key is to shake at a frequency such that the centrifugal force of the shake overcomes surface tension’s ability to keep the water attached to fur. The looseness of a dog’s skin (compared to humans!) is a bonus for them; the extra translation as they shake increases the centrifugal force, allowing them to shed more water more quickly. (Image and video credit: BBC Earth; research credit: A. Dickerson et al.)

  • Replacing Injections With Pills

    Replacing Injections With Pills

    In medicine, many medications contain molecules too large to be easily absorbed through the intestinal wall, so these so-called biologics — like the insulin administered to diabetics — are injected into the body. Researchers are studying ways that such injections could eventually be replaced with pills, but there are plenty of challenges involved.

    Some substances, known as transient permeability enhancers, allow the intestines to absorb larger molecules, but they work for only tens of minutes, which means researchers must understand how and when to administer them relative to the medication they help patients absorb. To do so, researchers are building computational fluid dynamics models of the human digestive system so that they can better understand how and when different kinds of pills break down in the body. (Image credit: Macro Room, source; via CU Engineering; submitted by Jenny B.)

  • To Beat Surface Tension, Tadpoles Make Bubbles

    To Beat Surface Tension, Tadpoles Make Bubbles

    For tiny creatures, surface tension is a formidable barrier. Newborn tadpoles are much too small and weak to breach the air-water surface in order to breathe. Researchers found that, instead, the 3 millimeter creatures place their mouths against the surface, expand their mouth to generate suction, and swallow a bubble consisting largely of fresh air.

    When they’re especially small, some of these species are essentially transparent (Image 1), allowing researchers to see the bubble directly. But even as the tadpoles aged (Images 2 and 3) and grew strong enough to breach the surface, they observed many instances in which the tadpoles continued this bubble-sucking method to breathe. (Image and research credit: K. Schwenk and J. Phillips; via Cosmos; submitted by Kam-Yung Soh)

  • Wild Gray Seals Clap Back

    Wild Gray Seals Clap Back

    Here’s a paper that cries out for fluid dynamical/acoustical follow-up: wild gray seals have been observed signaling underwater by clapping their forefins. As you can hear in the video, the sound is quite loud and carries well underwater. The biologists who observed the behavior postulate that it’s used by males during breeding season to ward one another off and to signal strength to nearby females.

    Although many species (including humans) slap against the water surface to generate noise, we don’t know of other species producing such a loud clap entirely underwater. The clap resembles the motions used by seals for propulsion, though the results are obviously quite different. I know plenty of researchers already looking into seal propulsion — here’s your future work! (Image and video credit: B. Burville; research credit: D. Hocking et al.; via Gizmodo)

  • Holding Fast in the Flow

    Holding Fast in the Flow

    Many tiny creatures in the natural world face living in fast flows. The larvae of the net-winged midge, for example, forage their way through fast-flowing Alpine springs with speeds of 3 m/s or more. You or I would find standing in such water a challenge, but these larvae are unbothered, thanks to the clever suction-cup-like appendages that help anchor them to rough rocks.

    The larvae generate their strong attachment with an outer rim flexible enough to conform to uneven surfaces. When they activate the central piston of the suction cup, this creates a seal strong enough to withstand forces up to 600 times the larvae’s body weight. But holding on to one spot forever is hardly useful, so the larvae also have a V-shaped notch in the cup controlled by dedicated muscles. When activated, this quickly breaks the seal, allowing the larvae to relocate. (Image and research credit: V. Kang et al.; via The Engineer; submitted by Marc A.)