FYFD

Category: Research

  • Crowns On Impact

    Crowns On Impact

    Dropping a partially-filled test tube of water against a table makes the meniscus at the air-water interface invert into a jet of liquid. In some cases, the impact is strong enough to generate splashing crowns of water around the base of the jet. These crowns come in two forms – one with many splashes layered upon one another and the other with only a few splashes and a faster jet. 

    The many-layered splash crowns come from the pressure wave that reflects back and forth from the bottom of the tube to the surface and back. This pressure wave moves at the speed of sound and vibrates the water surface, creating the many splashes. The same reflected pressure wave occurs in the second type of splash crown, but it gets disrupted by cavitation bubbles that form in the water (visible in the lower left image). Instead the splash crowns form from the shock waves generated when the cavitation bubbles collapse. (Image credits: A. Kiyama et al.)

  • Featured Video Play Icon

    Paint Spilling Physics

    There is a remarkable amount of physics contained in art. In this video, scientists from The Splash Lab explore some of the physics involved in pouring paint atop a rectangular post. The spreading paint transforms its shape repeatedly, and, at the corners of the post, it preserves a tiny history of all the colors poured. Paint sliding down the sides shifts from a thin sheet to a thicker jet that deposits color in waves. For tall posts, the distance the paint falls is long enough for instabilities to set in, producing a paint puddle that’s riddled with curves and waves between each color of paint. It’s a lovely reminder of the complexity inherent even within a simple action. (Video credit: R. Hurd et al.)

  • When Jets Collide

    When Jets Collide

    Two liquids that collide don’t always coalesce. The image above shows two jets of silicone oil colliding. On the left, the jets collide and bounce off one another. On the right, at a slightly higher flow rate, the two jets coalesce. This bouncing, or noncoalescence, observed at lower speeds is due to an incredibly thin layer of air separating the two jets. This air layer is constantly being replenished by air that gets dragged along by the flowing oil. But if the oil flows too quickly, that air layer becomes unstable–in the same way that a droplet that falls too quickly will splash on impact. When the separating air layer becomes unstable and breaks down, the jets collide and merge. (Image credit: N. Wadhwa et al., pdf)

  • Featured Video Play Icon

    Simulating the Earth

    Computational fluid dynamics and supercomputing are increasingly powerful tools for tracking and understanding the complex dynamics of our planet. The videos above and below are NASA visualizations of carbon dioxide in Earth’s atmosphere over the course of a full year. They are constructed by taking real-world measurements of atmospheric conditions and carbon emissions and feeding them into a computational model that simulates the physics of our planet’s oceans and atmosphere. The result is a visualization of where and how carbon dioxide moves around our planet.

    There are distinctive patterns that emerge in a visualization like this. Because the Northern Hemisphere contains more landmass and more countries emitting carbon, it contains the highest concentrations of carbon dioxide, but winds move those emissions far from their source. As seasons change and plants begin photosynthesizing in the Northern Hemisphere, concentrations of carbon dioxide decrease as plants take it up. When the seasons change again, that carbon is re-released.

    These visualizations underscore the fact that these carbon emissions impact everyone on our planet–nature does not recognize political borders–and so we share a joint responsibility in whatever actions we take. (Video credit: NASA Goddard; h/t to Chris for the second vid)

  • Falling Atop Sheets

    Falling Atop Sheets

    A sphere falling into water is a classic problem in fluid dynamics, but scientists are becoming increasingly interested in what happens when they introduce new dimensions to the problem. Here researchers float an extremely thin elastic sheet atop water and study how it wrinkles when a steel sphere impacts it. Despite its elasticity, the sheet does not stretch when the ball hits. Instead it compresses and forms wrinkles. Some of those wrinkles deepen into folds, but the wrinkle pattern that forms right at impact determines the way the film will bunch up. If the ball is heavy enough, it will drag the sheet entirely underwater; if not, the sheet will catch the ball and continue floating. Scientists are interested in these interactions between liquids and thin solids because sheets could be used to encapsulate liquids for applications like targeted drug delivery. (Image credit: M. Inizan et al., source)

  • Featured Video Play Icon

    Visualizing Flow with Snowfall

    One of the challenges in engineering and operating wind turbines is that full-scale turbines rarely behave as predicted in smaller-scale laboratory experiments and simulations. One way to reconcile these differences (and discover what our experiments and simulations are missing) is to take the experiments out into the field. One research group has done this by using snowfall to visualize the flow around wind turbines. In this video, they share some of their observations, which include interactions of tip vortices with one another and with the vortex from the tower. My favorite part starts around 1:50 where you can observe tip vortices leap-frogging one another behind the wind turbine! (Video credit: Y. Liu et al.)

  • Featured Video Play Icon

    Freezing Drops

    A water droplet deposited on a cold surface freezes from the bottom up. As anyone who has made ice cubes knows, water expands when it freezes. But watch the outline of the drop carefully. The drop isn’t expanding radially outward while it freezes. Instead the remaining liquid part of the drop forms what’s known as a spherical cap, a shape like the sliced-off top of a sphere. Surface tension creates that spherical shape, but the water still has to expand when it freezes. The result? The last bit of the drop freezes into a point! This means that surface tension maintains the drop’s spherical shape, for the most part, and all the expansion the water does takes place vertically. (Video credit: D. Lohse et al.)

  • Inside Cavitation

    Inside Cavitation

    Cavitation bubbles live a short and violent life. It begins when a low-pressure void forms in a fluid–for example, when a liquid is accelerated so that the pressure drops below the vapor pressure, which can happen at the tips of a boat’s propeller or when striking a bottle. The bubbles that form expand and then collapse rapidly as the higher pressure of the liquid surrounding them squeezes them down. That collapse of the bubble is so violent that it heats the fluid inside the bubble to temperatures hotter than the surface of the sun, generating both a flash of light and a shock wave. It’s these shock waves that cause much of the damage associated with cavitation in engineering, but they can be used for good as well. Shock wave lithotripsy uses cavitation-induced shock waves to break down kidney stones. (Image credit: O. Supponen et al., source)

  • Water Bottle Flipping Physics

    Water Bottle Flipping Physics

    Water bottle flipping has become quite the craze, and in a recent video The Backyard Scientist presented his own take on the subject, testing whether you could flip a bottle with mercury rather than water. As it turns out, fluid dynamicists have studied this topic, too, by dropping partially-filled elastic spheres containing water, isopropyl alcohol, and glycerin. The key physics here comes from the sloshing of liquid inside the container.  When the elastic ball bounces, energy that would otherwise go into the sphere’s rebound instead gets distributed into sloshing the fluid inside. The result is that the sphere bounces less on its subsequent impacts.

    Interestingly, the researchers found that the properties of the fluid inside the ball made very little difference to its rebound height. Instead, the most important feature was the volume of fluid in the container. Balls filled to approximately 30% of their volume had the most damping – that’s totally consistent with the best water bottle flips, which use bottles about 1/3rd full.

    The main difference between flipping a bottle and dropping a ball is what goes on in the first bounce. When a bottle hits a surface, the liquid inside has already been disturbed by the bottle’s rotation. For a ball being dropped, that first impact is what disturbs the fluid. So while a water-filled ball’s first rebound will reach nearly the same height as an empty ball, the spinning water bottle is, in effect, already on its second bounce. The motion of the fluid inside the bottle acts as a damper, allowing the bottle to stick the landing. (Image credit: Mercury Bottle Flip – The Backyard Scientist, source; Water Ball Bounce – The Splash Lab, source; research credit: T. Killian et al.)

  • Laser Goggles for Parrotlets

    Laser Goggles for Parrotlets

    Many experimental techniques in fluid dynamics use lasers. One such technique, particle image velocimetry (PIV), introduces tiny particles into the flow and uses a laser to illuminate the particles. By taking pictures in rapid succession and comparing them, researchers can measure the velocity in different parts of the flow. This technique is incredibly powerful but it’s rarely used to study topics like animal flight, except using mechanical substitutes for live animals.

    Part of the reason researchers don’t typically use live animals in this type of experiment is that these very powerful lasers can blind people or animals that aren’t properly protected. So to protect their test subject, Stanford researchers designed and built a special pair of laser safety goggles for their parrotlet. This let the bird fly safely despite the lasers and enabled the researchers to measure flow around realistic bird flight conditions. (Image credit: Stanford News, source, and E. Gutierrez; research credit: E. Gutierrez et al.; submitted by Simon H. via Wired)