Archive for the ‘fluid dynamics’ Category

Physicists now have even better models for blood spatter from gunshot wounds

August 29th, 2019
Dexter might be a (fictional) serial killer, but he's also a leading expert in blood spatter pattern analysis. Two new papers could help real forensic scientists better analyze such patterns to solve crimes.

Enlarge / Dexter might be a (fictional) serial killer, but he's also a leading expert in blood spatter pattern analysis. Two new papers could help real forensic scientists better analyze such patterns to solve crimes. (credit: YouTube/Showtime)

Physicists continue to contribute their unique insight into the forensic analysis of blood spatter patterns with two recent papers. The first, published in Physics of Fluids, offers a useful mathematical model of the relationship between the shape and speed of a bullet and the patterns produced by the resulting blood spatter. The second, published in the Journal of Fluid Mechanics, examines how gases propelled from a gun muzzle at firing can affect the distribution of blood spatter and gunshot residue (GSR).

The latter work in particular may have important implications for the famous Phil Spector murder case. The music producer was ultimately convicted in 2009 of shooting 40-year-old Lana Clarkson back in 2003. But his first trial, in 2007, ended with a hung jury in part because the forensic experts called by the prosecution and defense couldn't agree on how to interpret the blood spatter patterns on the clothing worn by Spector and Clarkson. Physics may help shed light on whether he was truly guilty or innocent.

Analysts at a bloody crime scene typically look for patterns relating to the size, shape, location, and distribution of blood stains. They interpret those patterns and use the data to reconstruct what happened as best they can. For example, it's possible to measure the direction and angle of blood stains to determine point of origin. Fans of the Showtime series Dexter—whose fictional central character was a serial killer/forensic technician with expertise in blood-stain analysis—may recall that this is often done with string, making it easier to trace the trajectory of individual drops.

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Posted in blood spatter patterns, fluid dynamics, forensic science, Physics, science | Comments (0)

No more lumps: Let physics be your guide to making the perfect crepe

June 28th, 2019
Batter must be distributed evenly to get uniform thickness in a perfect crepe.

Enlarge / Batter must be distributed evenly to get uniform thickness in a perfect crepe. (credit: Mark Boster/Los Angeles Times/Getty Images)

Order a crepe from a professional vendor and they'll likely cook it on a blade, a flat heated surface that distributes the batter evenly to get just the right consistency in the final crepe: uniform thickness with no unsightly lumps. But home cooks typically make crepes in a frying pan, which can make the process a wee bit trickier.

Still, no worries, all you home-cooking crepe lovers—physics has come to the rescue. According to the latest experiment results of two scientists, the trick is all in the wrist. They described their research in a recent paper published in Physical Review Fluids.

It all started when co-author Mathieu Sellier of the University of Canterbury in New Zealand expressed frustration to his wife that he could never get his homemade crepes to come out just right. "My wife said, 'Being a fluid dynamicist, you should be able to fix this problem,'" he told New Scientist. He found a willing collaborator in Edouard Boujo, a physicist at the Ecole Polytechnique in France. They set out to investigate, conducting multiple experiments making yummy crepes for their respective daughters, who were naturally delighted to participate in the scientific process.

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Posted in Cooking, everyday physics, everyday science, fluid dynamics, Physics, science | Comments (0)

Two new papers explore the complicated physics behind bubbles and foams

June 19th, 2019
Closeup photograph of bubbles floating through the air.

Enlarge / The complex physics of bubbles and foams have fascinated scientists for centuries. (credit: Ian Kington/AFP/Getty Images)

Human beings derive intense pleasure from bubbles and all kinds of foamy products, and scientists have long found them equally fascinating, given the complicated underlying physics. Most recently, a group of Japanese researchers published a paper in Scientific Reports describing two distinct mechanisms by which simple foams collapse. And in a new paper in the Proceedings of the National Academy of Sciences, physicists at MIT and Princeton University demonstrated how to develop spherical bubbles uniformly by confining them in a narrow tube.

Individual bubbles typically form a sphere, because that's the shape with the minimum surface area for any volume and hence is the most energy efficient. Back in the 19th century, Lord Kelvin proposed a bizarre soccer-ball shape called a tetrakaidecahedron (Greek for "fourteen faces" and sometimes translated "tetradecahedron"), with six square and eight hexagonal faces, to describe a bubble's natural geometry. It's known as "Lord Kelvin's cell," and while it was a valiant effort, that exact structure has yet to be observed in real-world bubbles, although physicists from Trinity College Dublin proposed a better solution to the conundrum in a 1993 paper.

Foams are ubiquitous in everyday life, found in foods (whipped cream), beverages (beer, cappuccino), shaving cream and hair-styling mousse, packing peanuts, building insulation, flame-retardant materials, and so forth. All foams are the result of air being beaten into a liquid formula that contains some kind of surfactant (active surface agent), usually fats or proteins in edible foams, or chemical additives in non-edible products. That surfactant strengthens the liquid film walls of the bubbles to keep them from collapsing.

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Posted in bubbles, fluid dynamics, foams, Physics, science | Comments (0)

Dribble no more: Physics can help combat that pesky “teapot effect”

May 17th, 2019
Dribble no more: Physics can help combat that pesky “teapot effect”

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Tea drinkers know all too well that annoying dribble from the kettle spout that so often occurs as one pours a nice refreshing cuppa. It's even known as the "teapot effect," and it usually happens when the tea is poured too slowly. Potters usually design their pots—giving the spout a thin lip, for instance—to reduce the likelihood of dribbling, based on centuries of accrued knowledge derived from trial and error.

Now a group of Dutch physicists has come up with quantitative model to accurately predict the precise flow rate for how much (or how little) a teapot will dribble as it pours, described in a recent paper in Physical Review Letters. The model accurately describes both the simple teapot effect and more complex behavior—notably, the formation of a helix as a water stream swirls around a cylinder. That should be a boon not to just for teapot design, but for 3D printing and similar industrial applications, which are also plagued by inconvenient dribbling.

Physicists have long been fascinated by the phenomenon. The late Stanford engineer and mathematician Joseph B. Keller once recalled attending a lecture by an Israeli scientist who mentioned that he'd posed the question of why teapots dribble to 100 physicists. All opined that it must be due to surface tension, but when the Israeli scientist performed experiments to test that theory, this proved not to be the case.

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Posted in fluid dynamics, Physics, science | Comments (0)

What Starry Night has in common with gassy clouds where stars are born

April 23rd, 2019
The bold blue and yellow swirls of Vincent van Gogh's <em>Starry Night</em> (1889) share turbulent properties with the molecular clouds that give birth to stars.

Enlarge / The bold blue and yellow swirls of Vincent van Gogh's Starry Night (1889) share turbulent properties with the molecular clouds that give birth to stars. (credit: Museum of Modern Art/Public domain)

In 2004, NASA published an image by the Hubble Space Telescope of turbulent eddies of dusty clouds moving around a supergiant star. The agency noted that this "light echo" was reminiscent of Vincent van Gogh's masterpiece, Starry Night. Now, two Australian graduate students have mathematically analyzed the painting and concluded it shares the same turbulent features as molecular clouds (where literal stars are born). They described their work in a paper posted to the physics arXiv.

The notion that van Gogh's often troubled life was reflected in his work is not especially new. In a 2014 TED-Ed talk, Natalya St. Clair, a research associate at the Concord Consortium and coauthor of The Art of Mental Calculation, used Starry Night (1889) to illuminate the concept of turbulence in a flowing fluid. In particular, she talked about how van Gogh's technique allowed him (and other Impressionist painters) to represent the movement of light across water or in the twinkling of stars. We see this as a kind of shimmering effect, because the eye is more sensitive to changes in the intensity of light (a property called luminance) than to changes in color.

In physics, turbulence relates to strong, sudden movements within air or water, usually marked by eddies and vortices. Physicists have struggled for centuries to mathematically describe turbulence. It's still one of the great remaining challenges in the field. But a Russian physicist named Andrei Kolmogorov made considerable progress in the 1940s when he predicted there would be a mathematical connection (now known as Kolmogorov scaling) between how a flow's speed fluctuates over time and the rate at which it loses energy as friction.

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Posted in art, astronomy, astrophysics, fluid dynamics, Gaming & Culture, Physics, science, starry night, turbulence, Vincent van Gogh | Comments (0)

Nature’s skyscrapers: X-ray imaging reveals the secrets of termite mounds

April 4th, 2019
Nature’s skyscrapers: X-ray imaging reveals the secrets of termite mounds

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Visit the African savannas in Zimbabwe or Namibia, and you might notice large, towering termite mounds dotted about the landscape—nature's skyscrapers, if you will. And nature is quite the engineer: those mounds are self-cooling, self-ventilating, and self-draining. New 3D X-ray images have revealed that one of the secrets to this impressive efficiency is a vast network of micropores in the walls of the mounds, according to a recent paper in Science Advances.

Termite mounds, with their ingenious mechanisms for climate control, have been providing inspiration for architectural design for at least the last 20 years, most notably when Zimbabwean architect Mick Pearce based his design for the Eastgate Center in his nation's capital of Harare on the termite mounds he observed in the region. He wanted to move away from the big glass block designs previously favored for office buildings and wanted his design to be heated and cooled almost entirely by natural means. The Eastgate Center is the country's largest commercial and shopping complex, and yet it uses less than 10 percent of the energy consumed by a conventional building of its size, because there is no central air conditioning and only a minimal heating system.

The termite mounds are basically fungus farms, since fungus is the termites' primary food source. Conditions have to be just right in order for fungus to flourish. So the termites must maintain a constant temperature of 87°F in an environment where the outdoor temperatures range from 35°F at night to 104°F during the day. Swiss entomologist Martin Lüscher suggested that the termite mounds worked a bit like air conditioners, facilitating a continuous exchange of air. Hot air in the nest below rises up while cooler air diffuses down, aided by the height of the mounts. The termites construct tunnels that act a bit like heating and cooling vents, which can be opened and closed as needed over the course of the day.

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Posted in air flow, architecture, biomimicry, fluid dynamics, science, termites, thermal regulation | Comments (0)

Alligator gar both sucks and chomps to catch its prey, new study finds

March 30th, 2019

Scientists had assumed the alligator gar catches its prey simply by slamming its powerful, tooth-y jaws shut. But according to a new study posted to the pre-print site bioRxiv, the fish also creates a fast, powerful suction force to suck prey into its jaws by moving the bones in its skull and shoulder. The paper is currently undergoing peer review for the Journal of Morphology.

Alligator gar are the largest species in the gar family of freshwater fish, and they can grow as large as 10 feet and 300 pounds. They are often dubbed "living fossils" because their earliest ancestors in the fossil record date back over one hundred million years to the Early Cretaceous period. Once considered a "trash fish," they are primarily found in the southern US (Arkansas, Louisiana, Texas) along the Gulf of Mexico.

They get their colloquial name from the fact that they share a broad snout and long teeth with the American alligator—most other gar have long slender snouts. Because of this, it was assumed that the alligator gar used a similar lateral snap of the jaw for feeding, but according to lead author Justin Lemberg of the University of Chicago, the jaws of the alligator gar have a lot more joints, and hence greater mobility, than their reptile namesakes.

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The secret of how the mako shark swims so fast lies in its flexible scales

March 8th, 2019
A shortfin mako shark off the coast of Cancun, Mexico. Tiny flexible scales on its skin control flow separation as it swims, reducing pressure drag.

Enlarge / A shortfin mako shark off the coast of Cancun, Mexico. Tiny flexible scales on its skin control flow separation as it swims, reducing pressure drag. (credit: YouTube/Guy Harvey)

Mako sharks can swim as fast as 70 to 80 MPH, earning them the moniker "cheetahs of the ocean." Now scientists at the University of Alabama have determined one major factor in how mako sharks are able to move so fast: the unique structure of their skin, especially around the flank and fin regions of their bodies. The team described their work at the American Physical Society's 2019 March meeting this week in Boston.

University of Alabama engineer Amy Lang conducted a series water tunnel experiments in her lab to test samples of real mako shark skin from the animal's flanks, using a technique called particle image velocimetry to measure the velocity of the water flowing over and around the skin. Anyone who has touched a shark knows the skin feels smooth if you stroke from nose to nail. Reverse the direction, however, and it feels like sandpaper. That's because of tiny translucent scales, roughly 0.2 millimeters in size, called "denticles" (because they strongly resemble teeth) all over the shark's body, especially concentrated in the animal's flanks and fins. It's like a suit of armor for sharks.

Mako sharks have evolved a distinct passive "bristling" aspect on some of their scales to swim faster. Lang's lab coordinated their project with biologists at the University of South Florida, who imaged shark skin and mapped out the scales, noting particularly how many of the scales were capable of this passive bristling and the angles at which such bristling occurs. They found that near regions like the nose, the scales aren't especially flexible, more like molars embedded in the skin. But near the flanks and fins, the scales are much more flexible.

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Posted in APS March Meeting, biophysics, fluid dynamics, mako shark, Physics, science | Comments (0)

Georgia Tech scientists figured out how maggots can eat so much, so fast

February 17th, 2019
Studying the collective feeding behavior of black soldier fly larvae.

Enlarge / Studying the collective feeding behavior of black soldier fly larvae. (credit: Hu lab/Georgia Tech)

How do the larvae of black soldier flies eat so much, so fast, despite their tiny size? Scientists at Georgia Tech have been studying this "collective feeding" behavior and found that one strategy for maximizing the larvae's feeding rate involves forming maggot "fountains." The scientists described the results in a recent paper in the Journal of the Royal Society Interface, along with an entertaining video showing a swarm of larvae consuming an entire pizza in just two hours.

"This is the first time, as far as I know, that we've really tried to quantify how much they were able to eat, and how they are able to do it," said graduate student and co-author Olga Shishkov, who demonstrated the research on Saturday at the American Association for the Advancement of Science meeting in Washington, DC. It's not the first time she's had fun demonstrating the maggots' hearty appetite in creative ways: last year, she videotaped the critters devouring a heart-shaped donut for Valentine's Day.

Shishkov's advisor is David Hu, who runs a biolocomotion laboratory at the Georgia Institute of Technology studying how various creatures move. He is perhaps best known for his work with fire ants, but his lab also studies cat tongues, water striders, snakes, various climbing insects, mosquitos, and, of course, black soldier fly larvae.

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Posted in Biology, collective behavior, fluid dynamics, Physics, science | Comments (0)

It’s the drag that helps the humble hagfish slime predators so quickly

January 17th, 2019

Courtesy of University of Wisconsin, Madison.

The homely hagfish might look like just your average bottom feeder, but they have a secret weapon: they can unleash a full liter of sticky slime in less than one second. That slime can clog the gills of a predatory shark, for instance, suffocating it. Scientists are unsure just how the hagfish (affectionately known as a "snot snake") accomplishes this feat, but a new paper in the Journal of the Royal Society Interface suggests that turbulent water flow (specifically, the drag such turbulence produces) is an essential factor.

Scientists have been studying hagfish slime for years because it's such an unusual material. It's not like mucus, which dries out and hardens over time; hagfish slime stays slimy, giving it the consistency of half-solidified gelatin. That's due to long, thread-like fibers in the slime, in addition to the proteins and sugars that make up mucin, the other major component. Those fibers coil up into "skeins" that resemble balls of yarn. When the hagfish lets loose with a shot of slime, the skeins uncoil and combine with the salt water, blowing up more than 10,000 times its original size.

Yet the precise mechanism for slime deployment is still poorly understood, according to co-author Gaurav Chaudhary of the University of Illinois, Urbana-Champaign. Recent research showed that sea water is essential to the formation of the slime, and that hagfish skeins can unravel spontaneously if ions in the sea water mixes the adhesives that hold the fibrous threads together in skeins. Chaudhary says that what's missing in this earlier work is taking the fast time scales into account. A 2014 study, for instance, showed that any spontaneous unraveling of the skeins would take several minutes—yet the hagfish deploys its slime in about 0.4 seconds.

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