FYFD

Month: August 2017

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    Paintball Collisions

    In their latest video, the Slow Mo Guys collide paintballs in mid-air, creating some pretty great paint splashes. The high-speed video does a nice job of revealing some of the typical stages a splash goes through. Initially, the paint spreads in a liquid sheet. As it expands and (necessarily) thins, holes form and grow, driving the paint into string-like ligaments. These ligaments are also stretching and eventually break up into an spray of droplets, much like the jet dripping from your faucet does. If you’d like to see some more awesome high-speed liquid collisions, check out what happens when a solid projectile hits a falling drop and then look at when a laser pulse hits a droplet. (Image and video credit: The Slow Mo Guys; submitted by Omar M.)

  • Review: “ABCs From Space”

    Review: “ABCs From Space”

    For me, one of the most fun aspects of studying science is seeking out examples of it in the world around us. Adam Voiland – who writes for NASA Earth Observatory, one of FYFD’s favorite sources for excellent fluids in action – takes this a step further with his children’s book “ABCs From Space: A Discovered Alphabet”. Voiland has sought out satellite imagery from around the world to illustrate all twenty-six letters, creating a lovely book for budding scientists of all sorts.

    Each letter has its own full-page image with no added text, like the G and H shown above. Younger children will have fun identifying and tracing out each letter. The back of the book provides more detail for older kids and adults, including brief descriptions of where and what each image shows, a map of all image locations, and some FAQs about satellite imagery and the geology, meteorology, and earth science on display. There are enough specifics to satisfy casual interest, but I suspect that science-inclined adults will find the book a fun springboard for more in-depth discussions with curious kids.

    Fluid dynamics itself makes a solid showing in the book. Several letters are formed by vortices (like G above) and various types of clouds, including the ship track clouds (like H) that form when water condenses on aerosols released by ship exhaust. There are also meandering rivers, creeping glaciers, and erosion features among the letters.

    I’m often asked about resources for teaching kids about fluid dynamics, and Voiland’s book is a great option for introducing that subject, as well as many other fields of science. (Image credits: A. Voiland/Simon & Schuster)

    Disclosure: I received a review copy of this book but was not otherwise compensated by the author or publisher. All opinions are my own. Additionally, this post contains affiliate links. Purchases made using these links do not cost you anything extra but may provide FYFD with a commission. Thanks!

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    Inside Earth’s Core

    Without our magnetic field, life as we know it could not exist on Earth. Instead, our atmosphere would be stripped away and the surface would be bombarded by charged particles in the solar wind. Relatively little is known about the dynamo process that governs our magnetic field, though it’s thought to be the result of liquid iron moving in the Earth’s outer core. The video above shows a slice of a recent 3D simulation of this liquid iron segment of our core. The colors show variations in the temperature, revealing vigorous convection in the core. This motion, combined with the spinning of the Earth, is the likely source of our magnetic field. Researchers hope that simulations like these can help us understand features we observe in our magnetic field – like local variations in field strength and the pole reversals in our geological record. (Video credit: N. Schaeffer et al.; CNRS via Gizmodo)

  • Optimal Swimming

    Optimal Swimming

    What do trout, sharks, and whales have in common? All are fast swimmers and share remarkable similarities in their swimming dynamics despite different sizes, shapes, and environments. A new study analyzing aquatic locomotion examines the characteristics of these swimmers. The researchers found that a typical parameter for studying swimming fish – the Strouhal number, which relates swimming speed, body length, and tail-beat frequency – only tells part of the story. When cruising at minimum power input, a fish cannot choose its Strouhal number – that characteristic is completely determined by the fish’s shape, which determines its drag.

    Instead, researchers found that a second additional number – the ratio of the tail-beat amplitude to the body length – was also needed to describe optimal swimming. Taken together, their model predicts that optimal swimming performance lies within a narrow range of the two numbers. And when the researchers examined cruising behaviors of a diverse variety of fish and whales, they found that they did indeed swim in the ranges predicted by the model. Now that we better understand characteristics of efficient swimming, engineers can use the model to guide designs of new biologically-inspired robot swimmers.   (Image credit: N. Sharp, source; research credit: M. Saadat et al.)

  • Hair in the Flow

    Hair in the Flow

    Humans are hairy on the inside. Not in the way that we are on the outside, but in the sense that many interior surfaces of our bodies are covered in small, flexible, hair-like protrusions like the papillae on our tongues or the cilia in our intestines. Many of these fibers are immersed in fluids, raising the question of how the flow and the hairs interact. An elastic fiber immersed in a flow will bend in the direction of the flow (bottom); this helps reduce the drag and widens the channel flow goes through compared to a stiff, upright fiber. 

    But what happens when the fibers are all mounted at an angle? In this case, researchers found an asymmetric response. If flow moves in the direction of the fibers’ bend, the hairs don’t impend the flow at all. If flow moves against that direction, however, the hairs start to stand upright, blocking the flow channel and increasing the drag. The researchers suggest this sort of mechanism could be use in micro-hydraulic devices in the same way as a diode in a circuit – allowing flow in only one direction. For another biological example of flow control, check out how a shark’s denticles can prevent flow separation. (Image credits: hairy surface – J. Alvarado et al., flow around a hair – J. Wexler et al.; research credit: J. Alvarado et al.)

  • Pressing Non-Newtonian Fluids

    Pressing Non-Newtonian Fluids

    For many fluids, the relationship between force and deformation is not simple. The catch-all name for these materials is non-Newtonian fluids. In a recent episode, the Hydraulic Press Channel did some experiments extruding a couple non-Newtonian fluids: oobleck and a temperature-sensitive putty. What they demonstrated is that a fluid’s response to the forces it experiences can change depending on the rate at which force is applied.

    Take their putty example from the latter half of the video. When the hydraulic press pushes the putty slowly, it extrudes in a smooth, semi-solid string. When they increase the pressure driving the hydraulic press, it pushes the putty more quickly, causing it to spray out of the die in a shredded mess. What they actually did here is surpass a threshold for what’s known in manufacturing as the sharkskin instability. This behavior occurs due to long-chain polymer molecules in the fluid. Inside the die, flow near the walls is slowed down by friction but moves freely in the middle of the pipe. When the walls are suddenly gone, flow at the outside accelerates to match the inside of the stream, which stretches the polymers until they can snap free of the die. The result is the rough, saw-tooth-like pattern seen here. (Video and image credit: Hydraulic Press Channel, source)

  • Fluid Black Holes

    Fluid Black Holes

    Fluid systems can sometimes serve as analogs for other physical phenomena. For example, bouncing droplets can recreate quantum effects and a hydraulic jump can act like a white hole. In this work, a bathtub vortex serves as an analog for a rotating black hole, a system that’s extremely difficult to study under normal circumstances. In theory, the property of superradiance makes it possible for gravitational waves to extract energy from a rotating black hole, but this has not yet been observed. A recent study has, however, observed superradiance for the first time in this fluid analog.

    To do this, the researchers set up a vortex draining in the center of a tank. (Water was added back at the edges to keep the depth constant.) This served as their rotating black hole. Then they generated waves from one side of the tank and observed how those waves scattered off the vortex. The pattern you see on the water surface in the top image is part of a technique used to measure the 3D surface of the water in detail, which allowed the researchers to measure incoming and scattered waves around the vortex. For superradiance to occur, scattered waves had to be more energetic after interacting with the vortex than they were before, which is exactly what the researchers found. Now that they’ve observed superradiance in the laboratory, scientists hope to probe the process in greater detail, which will hopefully help them observe it in nature as well. For more on the experimental set-up, see Sixty Symbols, Tech Insider UK, and the original paper. (Image credit: Sixty Symbols, source; research credit: T. Torres et al., pdf; via Tech Insider UK)

  • Surge Flows

    Surge Flows

    Sandy beaches can be a great place to play with neat flows. In a recent video, Frank Howarth describes playing with beach rivers on the Oregon coast and observing a surge flow there. Under the right conditions, a current flowing over sand will build up sand ripples large enough that they form miniature dams in the flow. This traps additional water, which eventually collapses the sand ripples, releasing a surge of water. The surge tends to smooth out the sand and cause the ripple-making process to start over. It’s a fairly unusual phenomenon, but it’s one known to happen seasonally in a few specific places, like at Medano Creek in Colorado’s Great Sand Dunes National Park. There the snowmelt-fed creek surges during the late spring and early summer, releasing a fresh wave every 20 seconds or so. (Image credit: F. Howarth, source; h/t to Sebastian E.)

  • Solar Eclipses and Coronal Mass Ejections

    Solar Eclipses and Coronal Mass Ejections

    Observations of many solar phenomena have only become accessible to humans relatively recently with the advent of satellites. Prior to that, it simply wasn’t feasible to observe dynamics in the sun’s atmosphere, like solar prominences or coronal mass ejections – the sun was simply too bright to see them – except during the occasional total solar eclipse!

    In the 1970s, scientists identified massive bursts of solar plasma as coronal mass ejections. These solar storms are responsible for so-called space weather and, when directed toward Earth, can pose a hazard to technologies on the ground and astronauts in orbit. Scientists initially thought this was the first time such storms had been observed, but they later recognized that photographs and sketches of an 1860 total eclipse revealed that humanity had seen a coronal mass ejection more than 100 years before! Check out the NASA video below for the full story. You can also learn about some of the science that will be going on in today’s eclipse. And, for those in the U.S. today, have a fun and safe time viewing the ecliipse!  (Image credit: S. Habbal, M. Druckmüller and P. Aniol, source; video credit: NASA Goddard)

  • Oreo Dunking Physics

    Oreo Dunking Physics

    As most people know, cookie dunking is serious business. Everyone has their own preference for cookie saturation and stiffness. Happily, scientists have examined this problem and have advice to offer those seeking cookie dunk perfection. Previously, we discussed Len Fisher’s Ig Nobel Prize-winning work on the physics of cookie dunking. In that work, Fisher found that Washburn’s equation for flow through cylindrical pores worked well to describe the uptake of tea or milk into a cookie.

    More recently, Splash Lab researchers have investigated just how much milk several common American cookies – including Oreos – take up in a given dunk. Because these cookies are quite dry, they take up liquid quickly, soaking in about 80 percent of the liquid weight within the first 2 seconds when dipped in 2% milk. Within five seconds, the cookies take on 99% of their liquid weight capacity, so there’s no point to a longer dunk – unless you like your cookie to disintegrate into the milk. The fat and sugar content of the dunking liquid does affect how quickly capillary action can whisk fluid into the cookie’s pores, but, overall, the research shows that milk users should be well-served by a three second dunk. If you like your cookie softer than that, simply pull it out of milk and let it sit for a bit while the milk soaks in. That way, your cookie doesn’t crumble! (Image credits: A. Melton; research credit: R. Hurd et al.; h/t to Randy H. and Mental Floss)