FYFD

Category: Research

  • Adaptive Meshing

    Adaptive Meshing

    The use of numerical simulations in fluid dynamics has exploded over the past half century with new computational techniques being developed constantly. Most methods involve solving the equations of motion (or an approximation thereof) on a grid of points known as a mesh. To accurately capture the physics, meshes must often be quite closely packed in areas where detail is needed, but they can be more widely spaced in areas where the flow is not changing quickly. An increasingly common technique is adaptive meshing in which the mesh of grid points shifts between time steps; this places more grid points where the flow requires them and removes them from less important areas in order to reduce computational time.

    An example of adaptive meshing is shown above. On the left particles are falling into salt water. The colors show the concentration of particles. The right side shows the solid particles and the fluid mesh around them. Notice how the grid shifts as the particles fall. (Image credit: C. Jacobs et al., source)

  • Wriggling Threads

    Wriggling Threads

    A thread of mineral oil laid across a pool of water twists and turns like a river run wild. Because the oil has a lower surface tension than the water, Marangoni forces spread it outward (far left). Small variations in the thread make the areas of highest oil concentration start to bend just a bit. Inside the bends, the gradient of surface tension – the difference between the lowest and highest surface tensions – is very high, which pulls at these regions more than others. So bends beget more bends, causing the entire thread to wrinkle. Although the behavior is driven by a completely different process than the one that causes rivers to meander, the end result looks remarkably similar; this is because, in both cases, forces act to make each bend increasingly sinuous. (Image credit: B. Néel et al., source)

    Editor’s note: Starting tomorrow I’ll be on a trip that takes me out of range of the Internet until next week. Regular posts are queued up and should post as usual, but we’ll all have to trust Tumblr to handle everything because I won’t be able to check. Thanks!

  • Soaring Pelicans

    Soaring Pelicans

    Earlier this summer, I looked up on a bright, sunny day and saw a quartet of black and white figures soaring overhead. Initially, I thought it might be a formation of kites or unmanned aerial vehicles (UAVs) because I saw no flapping as the group wheeled about. With the help of the Cornell Lab of Ornithology’s awesome Merlin app, I was able to identify the soarers as American white pelicans – not a species I’d expected to find flying along the Front Range of the Rocky Mountains! (Turns out, they breed on lakes around here.)

    The reason I saw so little flapping is that the birds were riding thermals. As the sun heats the ground, air near the surface warms up and begins to rise due to its buoyancy. Pelicans interested in flying between breeding and foraging grounds will start testing the thermals early in the day, as soon as they begin to form. As the heating continues, the intensity of thermals strengthens and they extend higher into the atmosphere. This is where the birds can really excel at using atmospheric energy for their flight. Pelicans will circle within a thermal until they reach roughly the middle of its height. Then they will glide, gradually losing altitude until they reach another thermal where they can climb without expending their own energy. With a 2.7 meter wingspan and a relatively low drag coefficient, the pelicans can glide and soar remarkably well. Researchers have even suggested using them as a sort of biological UAV for studying atmospheric dynamics! (Image credits: D. Henise, M. Stratmoen; research credit: H. Shannon et al., pdfs – 1, 2)

  • Featured Video Play Icon

    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.)

  • 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)

  • 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)

  • Rocket Launch Systems

    Rocket Launch Systems

    If you’ve ever watched a rocket launch, you’ve probably noticed the billowing clouds around the launch pad during lift-off. What you’re seeing is not actually the rocket’s exhaust but the result of a launch pad and vehicle protection system known in NASA parlance as the Sound Suppression Water System. Exhaust gases from a rocket typically exit at a pressure higher than the ambient atmosphere, which generates shock waves and lots of turbulent mixing between the exhaust and the air. Put differently, launch ignition is incredibly loud, loud enough to cause structural damage to the launchpad and, via reflection, the vehicle and its contents.

    To mitigate this problem, launch operators use a massive water injection system that pours about 3.5 times as much water as rocket propellant per second. This significantly reduces the noise levels on the launchpad and vehicle and also helps protect the infrastructure from heat damage. The exact physical processes involved – details of the interaction of acoustic noise and turbulence with water droplets – are still murky because this problem is incredibly difficult to study experimentally or in simulation. But, at these high water flow rates, there’s enough water to significantly affect the temperature and size of the rocket’s jet exhaust. Effectively, energy that would have gone into gas motion and acoustic vibration is instead expended on moving and heating water droplets. In the case of the Space Shuttle, this reduced noise levels in the payload bay to 142 dB – about as loud as standing on the deck of an aircraft carrier. (Image credits: NASA, 1, 2; research credit: M. Kandula; original question from Megan H.)

  • Tightrope Walkair

    Tightrope Walkair

    A bubble rising through water can get caught on an aerophilic (air-attracting) fiber. The bubble will then adhere to the fiber and be guided to the surface by it. In the poster above, the image is a composite photo of such a bubble every 40 milliseconds. Once captured by the fiber, the bubble first accelerates and then reaches a terminal velocity, indicated by the equal spacing of the bubble photos toward the right end of the picture. The terminal velocity strikes a balance between buoyancy, which pulls the bubble upward, and skin friction between the bubble and the water, which acts like drag on the bubble. At the terminal velocity, these forces are equal; neither is able to speed up or slow down the bubble. (Image credit: H. de Maleprade et al.)