FYFD

Category: Research

  • Cavitation Through Acceleration

    Cavitation Through Acceleration

    Cavitation refers to the formation of destructive bubbles of vapor within a liquid. Traditionally, we think of it as occurring when the velocity in a flow becomes high enough for the pressure to drop below the local vapor pressure, causing bubbles to form. This is what we see around turbine blades and ship propellers.

    But cavitation also occurs in situations where the overall velocity is relatively low, provided there’s a sudden acceleration. That’s the situation we see above. The impact — either of a mallet off-screen or of the tube striking the floor — causes the liquid inside suddenly accelerate upward. Notice in the second image how the liquid interface moves upward as the first bubbles form.

    Each of these cavitation bubbles has such a low pressure that they’re basically a vacuum, and their collapse can cause shock waves that reverberate through the container, causing it to break. Check out that test tube in the last image. Notice that there’s no sign of cracking when the test tube hits the floor; in fact, the researchers demonstrate in their paper that an empty test tube dropped from the same height doesn’t break. Fractures only form after the cavitation bubbles do. (Image and research credit: Z. Pan et al.; submitted by A.J.F.)

  • 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.

  • Watching a Droplet Freeze

    Watching a Droplet Freeze

    Whether it’s rain hitting an airplane wing or droplet-based 3D printing, the dynamics of a droplet impacting and solidifying on a surface are important. This new study observes the process from below, tracking the progress of freezing on a scale of hundreds of nanoseconds.

    All three of the drops you see above are liquid hexadecane. Each droplet was the same size and impacted at the same velocity. What differs in each image is how much colder the surface was than hexadecane’s melting point. The leftmost image shows a droplet on a surface only a few degrees cooler than the melting point. The initial expanding ring shows the droplet’s contact line expanding as it impacts. Then frozen crystals appear and grow inside the drop until the entire thing freezes.

    With a slightly colder surface (middle image), frozen crystals form while the contact line is still expanding, and rather than form in distinctive spots, they form as a cloud that quickly expands throughout the drop.

    But with an even colder surface (right image), something entirely new happens. As the drop freezes, we see multiple dark rings expand through the drop. Each of these rings is made up of frozen crystals. The researchers argue that we’re seeing a combination of freezing and hydrodynamics here. Essentially, whenever the frozen crystals get large enough, the outward flow of the impacting drop sweeps them toward the contact line. As new crystals grow near the center of the drop, they’re dragged out in a subsequent wave. (Image, research, and submission credit: P. Kant et al.)

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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.)

  • Bouncing Off Hydrophilic Surfaces

    Bouncing Off Hydrophilic Surfaces

    Droplets typically bounce off hydrophobic surfaces due to air trapped beneath the liquid that prevents contact between the drop and surface. But even extremely smooth, hydrophilic surfaces can elicit a bounce under the right circumstances, as shown in a new study.

    The key is that the droplet must bounce at exactly the right speed. If the bounce has too much momentum, it will squeeze the nanometer-sized air cushion too thin, allowing contact. Too slow and the Van der Waals attraction between the droplet molecules and wall molecules will have time to act. But between those lies a sweet spot where the dimple and cushion of air beneath the drop keep it from impacting. (Image credit: droplets – klickblick, drop bounce – J. Kolinski, bounce sim – J. Sprittles et al.; research credit: M. Chubynsky et al.; submitted by James S.)

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    The Birth of a Liquor

    A water droplet immersed in a mixture of anise oil and ethanol displays some pretty complicated dynamics. Its behavior is driven, in part, by the variable miscibility of the three liquids. Water and ethanol are fully miscible, anise oil and ethanol are only partially miscible, and anise oil and water are completely immiscible. These varying levels of miscibility set up a lot of variations in surface tension along and around the droplet, which drives its stretching and eventual jump.

    Once detached, the droplet takes on a flattened, lens-like shape that continues to spread. That spreading is driven by the mixing of ethanol and water, which generates heat and, thus, convection around the drop. This not only spreads the droplet, it causes turbulent behavior along the drop’s interface. (Image and video credit: S. Yamanidouzisorkhabi 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.)

  • Dunes Avoid Collisions

    Dunes Avoid Collisions

    The speed at which a dune migrates depends on its size; smaller dunes move faster than larger ones. That speed differential implies that small dunes should frequently collide into and merge with larger dunes, eventually forming one giant dune rather than a field of smaller separate ones. But that’s not what we observe in nature.

    To figure out why dunes aren’t colliding that often, researchers built a dune field of their own in the form of a rotating water tank. Inside the tank, their two artificial dunes can chase one another indefinitely while the researchers observe their interactions. What they found is that the dunes “communicate” with one another through the flow.

    As flow moves over the upstream dune, it generates turbulence in its wake, which the downstream dune then encounters. All that extra turbulence affects how sediment is picked up and transported for the downstream dune, ultimately changing its migration speed. For two dunes of initially equal size and close spacing, these interactions push the downstream dune further away until the separation between the dunes is large enough that they both migrate at the same speed. Even between dunes of unequal sizes, the researchers found that these repulsive interactions force the dunes away from collision and into migration at the same speed. (Image credit: dune field – G. Montani, others – K. Bacik et al.; research credit: K. Bacik et al.; via Cosmos; submitted by Kam-Yung Soh)

  • Kicking Droplets

    Kicking Droplets

    Moving the surface a droplet sits on creates some interesting dynamics, especially if the surface is hydrophobic. That’s what we see here with these droplets launched off an impulsively-moved plate.

    On the left, the drop has some limited contact with the plate and it takes time for the droplet to completely detach. When accelerated, the droplet first flattens into a pancake, the rim of which quickly leaves the plate. The center of the droplet is slower to detach, stretching the drop into a vase-like shape. When the drop does finally lose contact, it creates a fast-moving jet that shoots upward at several meters per second!

    In contrast the image on the left shows a levitating Leidenfrost droplet. Since this drop has no physical contact with the plate, the kick makes it leave the surface all at once, launching a pancake-like drop that quickly forms unstable lobes. (Image and research credit: M. Coux et al.)

  • Listening to a Bubble’s Pop

    Listening to a Bubble’s Pop

    Sound is an important aspect of many flows, from the scream of a rocket engine to the hum of electrical wires vibrating in the wind. Critically, those sounds carry important information about the flow. A new study extends these acoustic diagnostics to the popping of soap bubbles.

    When a hole opens in a soap bubble, it throws the surface-tension-driven capillary forces of the bubble into disarray. The rim around the hole retracts, pushing fluid away from the expanding hole. At the same time, air is pushed out of the collapsing bubble. Using microphone arrays, the researchers found they could measure and distinguish sound from both sources — the escaping air and the expanding hole.

    From the sound, they developed a model that predicts the rupture location, bubble thickness profile, and other properties of the bubble. They confirmed the model’s results by comparing with high-speed photography. The authors hope their new acoustic technique will shed light on bubble bursting events that are hard to observe visually, like the bubbling of magma. (Image and research credit: A. Bussonnière et al.; via Science News; submitted by Kam-Yung Soh)