FYFD

Category: Phenomena

  • Featured Video Play Icon

    “It’s All About Flow”

    Fluid dynamicists, like other scientists, have lives and interests well beyond our research. Ivo Nedyalkov, for example, is a professional rapper in addition to a PhD-level fluid dynamicist. In “It’s All About Flow,” Dr. Ivo brings those areas of expertise together with a rap all about fluid dynamics. The version embedded here is a bit shorter than the full version, which digs not only into experimental fluid dynamics but into computational work as well.

    Check it out, and if you’d like to see the full lyrics and explanation behind them, he’s posted those as well. You can also ping me here or on Twitter if you’d like to know more about the phenomena he discusses. (Video and image credit: I. Nedyalkov/ASME; full video here; lyrics and explanation)

  • A Microfluidic Zoo

    A Microfluidic Zoo

    Microfluidic channels are excellent at creating a steady supply of droplets. But depending on the characteristics of the two viscous fluids being used, as well as factors like flow rate and channel geometry, the results can be anything from well-defined and separated drops to steady jets to wild instabilities. The image above shows a series of different outcomes, including waves that break on the edges of drops and ligaments that stretch around the central fluid. (Image and research credit: X. Hu and T. Cubaud)

  • Ice Patterns

    Ice Patterns

    Periods of freezing and thawing can leave complicated patterns in ice, as seen in this aerial photo of Binnewater Lake in New York. Ice rarely forms evenly on large bodies like this, so there are always underlying weaknesses. A hard freeze may have caused the ice to contract, forming the initial radial pattern. Then warmer periods of melting allowed water to rise into the cracks and expand them. As the process repeats, the visible pattern emerges.

    Also note the star-like crack patterns near the shore. These may have formed in spots where something like a stick protruding from the water’s surface allowed warmer water up onto the ice to melt the snow sitting atop it. (Image credit: D. Spitzer; via EPOD; submitted by Kam-Yung Soh)

  • Featured Video Play Icon

    Boiling Water Using Ice Water

    Steve Mould demonstrates a neat thermodynamic trick in this video by using ice water to boil hot water. The key to understanding this is recognizing that the boiling point of water depends both on its temperature and its pressure.

    Here’s the set-up (which, to be clear, neither he nor I recommend you try yourself): microwave some water in an open bottle until the water is hot enough to boil. Remove the bottle from the microwave and screw on the lid. At this point, you’ve confined any water vapor coming off the hot water, thereby raising the pressure inside the bottle. Even though it’s still quite hot, the water will stop visibly boiling.

    Now pour ice water over the top of the bottle. Because water vapor has a lower heat capacity than liquid water, this will preferentially cool the vapor. As its temperature drops, its pressure will also drop. Liquid water boils at lower temperatures when the pressure is lower. (This is part of why cooking and baking instructions are quite different in Denver than they are in Miami.) When the internal pressure in the bottle drops, the remaining hot water will start to visibly boil. (Image and video credit: S. Mould)

    Animation of boiling water using ice water.
  • Featured Video Play Icon

    The World in a Droplet

    Capturing refracted images in a droplet is a popular pastime among high-speed photographers, and in this solo Slow Mo Guy outing, we get to see that process in video. Physically, the subject is a simple drop of water, which on impact with a pool, rebounds into a Worthington jet and ejects one or more droplets from its tip. Despite hundreds of years of study, it’s still a joy to watch, especially at 12,000 frames per second.

    It’s also not the easiest image to capture, and one thing I rather enjoy about this video is how it gives you a sense of the trial and error involved in capturing just the right view. Even without having to worry about the timing issues, there is a lot of fiddling with lenses, focus, lights, and positioning — something familiar not just to photographers and videographers but to many researchers as well! (Image and video credit: The Slow Mo Guys)

  • Where are Titan’s Deltas?

    Where are Titan’s Deltas?

    Saturn’s moon Titan is the only other planetary body in our solar system known to have bodies of liquid on its surface. But where Earth has lakes and seas of water, Titan’s are hydrocarbon-based, primarily ethane and methane. As on Earth, these liquids rain from skies and run down rivers and streams into larger bodies. What they do not do, as far as scientists can tell, is form deltas.

    On Earth (and ancient Mars), rivers tend to slow and branch out as they run into larger, still bodies. Many of these river deltas — like the Nile, Ganges, and Mississippi — are visible from space. But so far we’ve seen no equivalent formations on Titan, even though the radar resolution of Cassini should have allowed for it.

    There are currently two hypotheses to explain this absence. One posits that density differences between hydrocarbon rivers and lakes mean that deltas do not form. On Titan, the larger bodies are warmer and do not absorb as much atmospheric nitrogen, making them lighter overall. That means a cold, dense river might just sink immediately beneath the lake without slowing to deposit sediment.

    Another hypothesis is that deltas do form but that the shifting shorelines of Titan’s seas wash them out and make them unrecognizable. There’s evidence that Titan’s northern and southern hemispheres can swap their liquid hydrocarbons back and forth on a 100,000 year timescale. If that’s true, those shifts could obscure any evidence of deltas.

    Experiments are underway to test the first hypothesis, but the final answers may have to wait until NASA’s Dragonfly mission reaches Titan in 2034. (Image credit: Titan – NASA/JPL-Caltech/ASI/Cornell, Alaska – NOAA; via AGU Eos; submitted by Kam-Yung Soh)

  • Featured Video Play Icon

    Collective Catfish Convection

    Gather many birds, fish, or humans together and you often get collective motion that’s remarkably fluid-like in appearance. This video shows a group of juvenile striped eel catfish, an (eventually) venomous species that uses strength in numbers for protection while young. Their movement is rather mesmerizing, and if you watch individual catfish, you’ll see a sort of convective motion inside the blob. There’s a general downward trend near the front of the school and a rising one on the backside. Perhaps they’re taking turns feeding near the bottom of the pack? (Image and video credit: Abyss Dive Center; via Colossal)

  • Breaking the Euler Equations

    Breaking the Euler Equations

    Mathematicians like to break things. Or, more exactly, they like to know when the equations we use to describe physics break down. One popular target in fluid mechanics are the Euler equations, which describe the motion of frictionless, incompressible flows. Mathematicians have been on the hunt for centuries for situations where these equations predict singularities, points where the velocity or vorticity of a fluid change infinitely quickly. Since that can’t happen in reality (at least as far as we understand it), these singularities indicate weaknesses in our mathematical description and may help uncover fundamental flaws in our understanding.

    Despite centuries of effort, the Euler equations withstood mathematical assault… until recently. Since 2013, a series of mathematicians have been successfully chipping away at the Euler equations’ seeming perfection with a series of scenarios that seem to lead to singularities. One is similar to stirring a cup of tea, except that you stir the upper part of the cup in one direction and the bottom half in the opposite. As the flow develops, a singularity occurs where the secondary flows of these two stirring motions collide. For more, check out these two articles over at Quanta. (Image credit: L. Fotios; see also Quanta Magazine 1, 2)

  • Featured Video Play Icon

    When the Mediterranean Dried Up

    Geological evidence shows that millions of years ago, the Mediterranean Sea nearly dried out. In fluid mechanics, we’d describe this problem using one of our fundamental equations: conservation of mass, also known as continuity.

    Imagine a volume containing the entire Mediterranean. To describe the amount of sea water in that volume, you need to keep track of two major quantities: how much water is flowing into the volume and how much is leaving it. For the prehistoric (as well as today’s) Mediterranean, the sources feeding the sea are 1) an inflow from the Atlantic through the Strait of Gibraltar; 2) inflows from rivers; and 3) rainfall. Water is lost primarily to evaporation.

    As explained in the video, the Mediterranean’s dry spell was heralded by tectonic changes that sealed the Strait of Gibraltar, depriving it of its largest source of inflow. At the same time, warmer temperatures and less rainfall reduced influx from rivers and the atmosphere while increasing evaporation rates. The result? Water levels in the Mediterranean dropped by hundreds of meters, creating massive salt deposits, wiping out native marine life, and allowing mass migration by land-dwelling animals. Eventually, though, the Strait re-opened, creating what might have been a massive flood. (Video and image credits: PBS Eons)

  • Captured by Waves

    Captured by Waves

    Acoustic levitation and optical tweezers both use waves — of sound and light, respectively — to trap and control particles. Water waves also have the power to move and capture objects, as shown in this award-winning poster from the 2019 Gallery of Fluid Motion. The central image shows a submerged disk, its position controlled by the arc-shaped wavemaker at work on the water’s surface. The complicated pattern of reflection and refraction of the waves we see on the surface draws the disk to a focal point and holds it there.

    On the bottom right, a composite image shows the same effect in action on a submerged triangular disk driven by a straight wavemaker. As the waves pass over the object, they’re refracted, and that change in wave motion creates a flow that pulls the object along until it settles at the wave’s focus. (Image and research credit: A. Sherif and L. Ristroph)