FYFD

Category: Research

  • Denticles and Sharkskin

    Denticles and Sharkskin

    Look closely enough at a shark’s skin, and you will find it is covered in tiny, anvil-shaped denticles (lower left). To try and discover how and why these denticles help sharks, researchers are 3D printing denticles in different patterns onto flexible sheets to create biomimetic shark skin (lower right). 

    They test the artificial shark skin in a water tunnel by moving it with prescribed motions and measuring different characteristics, like the swimming speed attained and the power required. When compared to a smooth but flexible control surface, one pattern came out ahead. The staggered-overlapped denticle pattern (shown in C of the lower right figure) achieved swimming speeds 20% higher than the smooth control despite having far more surface area due to the denticles. The cost of that speed was only 13% greater than the smooth case on average, and was about equal to the smooth case for small amplitude motion. This suggests that the patterning of a shark’s skin may help it swim faster with little to no additional cost in effort.

    For more on shark hydrodynamics, check out my previous posts on the topic, and if you want even more shark science, check out these great videos. (Image credit: R. Espanto; J. Oeffner and G. Lauder; L. Wen et al.; research credit: L. Wen et al., 1, 2)

  • Drawing Up Dew

    Drawing Up Dew

    Desert plants have evolved to efficiently collect and capture whatever water they can. Each leaf of the moss Syntrichia caninervis ends in a hairlike fiber called an awn (seen in white in the top image). Tiny as they are, awns are vital to the moss’s water collection, correlating to more than 20% of their dew collection. Extremely tiny grooves on the surface of the awn provide nucleation sites where dew condensed from fog collects. Once a droplet forms on the awn, it grows larger as more fog condenses (middle image). When the droplet grows large enough, the conical shape of the awn will cause surface tension to draw the droplets along the awn and toward the leaf (bottom image).

    (Credits: Syntrichia caninervis moss image – M. Lüth; videos and research – Z. Pan et al., Supplementary Videos 3 and 4; h/t to T. Truscott)

  • Microscale Rockets

    Microscale Rockets

    Shown above are a trio of microscale rockets, each about 10 microns in length. These tiny rockets are roughly cylindrical in shape, with a narrower diameter at the front than the back. Like their space-faring brethren, these microrockets are chemically propelled. They draw in fuel from their surroundings, which reacts with the catalysts coating the interior of the microrocket to produce gases. Those gases bubble out the back end of the microrocket, creating thrust capable of propelling the rockets more than 1000 body lengths/second. Researchers have already demonstrated that these tiny rockets can haul cargo along with them. Scientists hope one day to use these self-propelled microrockets to help deliver drugs or isolate cancer cells. (Image credit: J. Li et al., source)

  • When Lasers Strike

    When Lasers Strike

    Lasers are a great way to deliver a lot of energy very quickly. In this animation, you see a jet of water get struck by a pulse from a powerful X-ray laser. The energy from that laser pulse gets absorbed by the water in a matter of picoseconds – that’s trillionths of a second. All that energy in so little time makes the water vaporize explosively. It’s this vapor explosion that breaks the jet in two. As the vapor expands outward, it forces water from the jet into a thin film that forms a cone. The conical film bends back on itself until it strikes the jet and coalesces. For more, check out this video of a similar experiment that looked at laser impacts on droplets. (Image credit: C. Stan et al., from Supplementary Movie 5; via Gizmodo)

  • Wingtip Vortices Visualized

    Wingtip Vortices Visualized

    In flight, airplane wings produce dramatic wingtip vortices. These vortices reduce the amount of lift a 3D wing produces relative to a 2D one. How much they influence the lift depends on both the strength and proximity of the vortex. The stronger and closer it is, the more detrimental its effect. One way airplane designers reduce the effects of wingtip vortices is by adding an extra section, called a winglet, to the end of the wing. Among other effects, the winglet moves the wingtip vortex further away from the main wing, which reduces its influence and allows the airplane to regain some of the lift that would otherwise be lost. (Image credits: A. Wielandt et al., source)

  • HIFiRE

    HIFiRE

    Earlier this month, an international team launched a successful hypersonic flight test in Australia. The Hypersonic International Research Experimentation (HIFiRE) Flight 5b was launched atop a two-stage rocket and reached its maximum speed of Mach 7.5, well above Mach 5, which defines the start of the hypersonic regime. The purpose of this particular flight test was not to test new propulsion technologies – there was no scramjet engine on this flight. Instead, researchers wanted to study aerodynamics at high Mach number, specifically the behavior of the air very close to the vehicle, its boundary layer.

    The payload being tested was an elliptical cone mounted on the front of the vehicle and shown in images above. The shape of the payload is such that flow will curve around the cone rather than following straight lines. The image on the lower right contains black streamlines that show how air twists around the cone. This complex flowfield complicates the physics of the boundary layer near the cone’s surface and increases the likelihood that the boundary layer will transition from laminar flow to turbulent flow, thereby increasing heating on the payload. Ideally, the data from the test flight will let engineers test their ability to understand and predict this boundary layer transition in the future. For more on boundary layer transition and its effects at hypersonic speeds, check out my latest FYFD video. (Image credit: Australia Department of Defense, R. Kimmel et al., F. Li et al.; topic requested by Guido)

  • Skating on Vapor

    Skating on Vapor

    Turn the stove up high enough and you may have noticed that drops of water stop boiling away and instead skate across the surface. This is the Leidenfrost effect, which occurs when a surface is so much hotter than a liquid’s boiling point that any liquid that contacts instantly vaporizes. That thin vapor layer insulates the rest of the drop and makes it skate around on very little friction. Previously, researchers found that putting these drops on patterned surfaces causes them to self-propel. Here you see Leidenfrost drops on a V-shaped “herringbone” surface. The grooves in the surface catch and direct the vapor out the Vs. If it seems counter-intuitive that the drops move in the same direction as their vapor, you’re not alone! It turns out that Leidenfrost drops aren’t propelled by vapor moving away from them – like, say, a rocket is. Instead the drops are being dragged along by friction between them and the escaping vapor. By controlling the direction of the vapor, researchers were able to create race tracks (top) and even traps (bottom) for the drops. (Image credit: D. Soto et al., from Supplemental Movies 2 and 3)

  • Whiskey Stains

    Whiskey Stains

    Photographer Ernie Button discovered that whiskey left behind intriguing patterns after it evaporated. Unlike coffee rings, the whiskey leaves behind a more uniform residue. Curious, he contacted researchers at Princeton, who were eventually able to explain why whiskey and coffee dry so differently. They observed three major effects in drying whiskey mixtures. Firstly, the alcohol in whiskey evaporates faster than other components, creating differences in concentration and, therefore, surface tension along the droplet. These variations in surface tension create Marangoni flow, which tends to mix the droplet. Coffee, being non-alcoholic, does not do this.

    Whiskey also contains surfactants, low surface tension chemicals, which help pull particulates away from the edge of the droplet so they aren’t trapped there like in coffee. And finally, they found that the polymers in whiskey helped glue particles to the glass so that they were less likely to be carried by the flow. Taken together, these three ingredients – alcohol, surfactants, and polymers – all help make the whiskey stain more uniform. For more, watch the video below, see Button’s website, or check out the research paper. (Image credit: E. Button; research credit: H. Kim et al.; video credit: C&EN; submitted by @tommyjwilson)

     

  • Flying with Large Ears

    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.)

  • The Blue Whirl

    The Blue Whirl

    Researchers studying the use of fire whirls to burn off oil spills have discovered a new type of fire whirl – the blue whirl. Their results are currently reported in a pre-print paper on arXiv and await peer-review. In their experiment, the scientists ignited a puddle of fuel floating atop water. Compared to a typical flame, they observed that a tightly-spinning fire whirl burns hotter and produces less soot by burning more of the fuel. To the researchers’ surprise, their lab-scale yellow fire whirl evolved into a compact, bright blue whirl. The blue whirl has a laminar flame and makes little to no noise. Its bright blue color indicates even more efficient combustion than the yellow fire whirl. The lack of yellow color means the whirl is burning without producing any soot, a by-product of incomplete combustion. The authors hope a better understanding of blue whirls will lead to better methods for responding to oil spills. (Image credit: H. Xiao et al.)