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    The Reynolds Experiment

    One of the most famous and enduring of all fluid dynamics experiments is Osborne Reynolds’ pipe flow experiment, first published in 1883 and recreated in the video above. At the time, it was understood that flows could be laminar or turbulent, though Reynolds’ terminology of direct or sinuous is somewhat more poetic:

    Again, the internal motion of water assumes one or other of two broadly distinguishable forms-either the elements of the fluid follow one another along lines of motion which lead in the most direct manner to their destination, or they eddy about in sinuous paths the most indirect possible. #

    There had, however, been no direct evidence of these eddies in a pipe. Reynolds built an apparatus that allowed him to control the velocity of flow through a clear pipe and simultaneously introduce a line of dye into the flow. He carefully varied the velocity and temperature (and thus viscosity) in his apparatus and not only documented both laminar and turbulent flow but found that the transition from one to another could be described by a dimensionless number he derived from the Navier-Stokes equation. This number was dependent on the fluid’s velocity and kinematic viscosity as well as the diameter of the pipe. This was the birth of the Reynolds number, one of the most important parameters in all of fluid dynamics. (Video credit: S. dos Santos; research credit: O. Reynolds)

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    The Structure of Turbulence

    Though they may appear random at first glance, turbulent flows do possess structure. The video above shows a numerical simulation of a mixing layer, a flow in which two adjacent regions of fluid move with different velocities. The upper third of the frame shows a top view, and the bottom frame shows a side view, in which the upper fluid layer moves faster than the lower one. The difference in velocities creates shear which quickly drives the mixing layer into turbulence. But watch the chaos carefully, and your eye will pick out vortices rolling clockwise in the largest scales of the mixing layer. These features are known as coherent structures, and they are key to current efforts to understand and model turbulent flows. (Video credit: A. McMullan)

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    Hydraulic Jump in the Lab

    When fast-moving liquids encounter regions of slow-moving liquids, they decelerate rapidly, trading their kinetic energy for potential energy and creating a hydraulic jump. Flow in the video above is from left to right. The depth difference between the incoming and outgoing water can be directly related to the velocity of the incoming fluid. Hydraulic jumps in rivers and spillways are often extremely turbulent, like the one in this video, but laminar examples exist as well. In fact, with the right height and flow rate, you can create stable hydraulic jumps right in your kitchen sink. The hydraulic jumps formed from a falling jet are typically circular, but with the right conditions, all sorts of wild shapes can be observed. (Video credit: H. Chanson)

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    Vibrations from Vortices

    Vortex shedding frequently happens in the wakes of non-streamlined bodies as a result of flow around the obstacle. Newton’s third law states that forces come in equal and opposite pairs, meaning that the vortex shedding behind an obstacle is accompanied by a force on the obstacle. For a fixed cylinder, this is not always apparent, but for a pendulum, like the ones demonstrated in this video, this vortex-induced vibration causes significant motion. This same effect can make traffic lights and industrial chimneys sway. You’ve likely experienced it yourself as well, if while swimming you’ve ever spread your fingers underwater and spun in place. Try it sometime with your arm out and you’ll feel the vortices make your arm vibrate up and down as you spin.  (Video credit: Harvard Natural Sciences Lecture Demonstrations)

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    Inksplosion

    Chemical Bouillon are a trio of artists who use the chemistry of surface reactions to create abstract videos full of exploding and imploding droplets and colors. As chemicals react, local concentrations at the interface vary, which changes the local surface tension. These gradients drive flow from areas of low surface tension to those of higher surface tension. This is called the Marangoni effect – the same behavior that drives tears in a glass of wine. Chemical Bouillon have a whole YouTube channel dedicated to these kinds of videos, with everything from inks to ferrofluids. Be sure to take a look at some of their other videos and, if you like them, subscribe. (Video credit: Chemical Bouillon)

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    Measuring Wind Turbines with Snowfall

    One of the challenges in large-scale wind energy is that operating wind turbines do not behave exactly as predicted by simulation or wind tunnel experiments. To determine where our models and small-scale experiments are lacking, it’s useful to make measurements using a full-scale working turbine, but making quantitative measurements in such a large-scale, uncontrolled environment is very difficult. Here researchers have used natural snowfall as seeding particles for flow visualization. The regular gaps in the flow are vortices shed from the tip of the passing turbine blades. With a searchlight illuminating a 36 m x 36 m slice of the flow behind a wind turbine, the engineers performed particle image velocimetry, obtaining velocity measurements in that region that could then be correlated to the wind turbine’s power output. Such in situ measurements will help researchers improve wind turbine performance. (Video credit: J. Hong et al.)

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    Air Pressure Affects Splashes

    When a drop falls on a dry surface, our intuition tells us it will splash, breaking up into many smaller droplets. Yet this is not always the case. The splashing of a droplet depends on many factors, including surface roughness, viscosity, drop size, and–strangely enough–air pressure. It turns out there is a threshold air pressure below which splashing is suppressed. Instead, a drop will spread and flatten without breaking up, as shown in the video above. For contrast, here is the same fluid splashing at atmospheric pressure. This splash suppression at low pressures is observed for both low and high viscosity fluids. Although the mechanism by which gases affect splashing is still under investigation, measurements show that no significant air layer exists under the spreading droplet except near the very edges. This suggests that the splash mechanism depends on how the spreading liquid encroaches on the surrounding gas. (Video credit: S. Nagel et al.; research credit: M. Driscoll et al.)

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    Simulating Early Planetary Impacts

    Early in our geological history, Earth was a hellish landscape of molten oceans into which metallic impactors would sometimes collide. Geophysicists have been curious how the impactors behaved after collision: did they maintain their cohesion, or did they break up into a cloud of droplets? Here the UCLA Spinlab simulates this early planetary formation by dropping liquid gallium through a tank of viscous fluid. As the video shows, the impactor’s behavior varies strongly with size. Smaller impactors stick together as a single diapir, but, as the initial size increases, the diapir becomes unstable, eventually breaking down into a cascade of droplets – a metallic rain through an ocean of magma. (Video credit: J. Wacheul et al./UCLA Spinlab; submitted by J. Aurnou)

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    Shooting a Bullet Through a Water Balloon

    This high-speed video of a bullet fired into a water balloon shows how dramatically drag forces can affect an object. In general, drag is proportional to fluid density times an object’s velocity squared. This means that changes in velocity cause even larger changes in drag force. In this case, though, it’s not the bullet’s velocity that is its undoing. When the bullet penetrates the balloon, it transitions from moving through air to moving through water, which is 1000 times more dense. In an instant, the bullet’s drag increases by three orders of magnitude. The response is immediate: the bullet slows down so quickly that it lacks the energy to pierce the far side of the balloon. This is not the only neat fluid dynamics in the video, though. When the bullet enters the balloon, it drags air in its wake, creating an air-filled cavity in the balloon. The cavity seals near the entry point and quickly breaks up into smaller bubbles. Meanwhile, a unstable jet of water streams out of the balloon through the bullet hole, driven by hydrodynamic pressure and the constriction of the balloon. (Video credit: Keyence)

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    “Porgrave”

    Artist Sandro Bocci uses macro imagery of fluids in his new piece “Porgrave” to create scenes reminiscent of celestial landscapes and the first moments of life. Surface tension, the Marangoni effect, and diffusion create pulsating motion in some frames whereas immiscible liquids form untouchable islands in others. “Porgrave” reminds me of work by Pery Burge and Julia Cuddy as well as sequences from films like 2001 and The Fountain, both of which created some of their effects with macro photography of fluids. Still images from “Porgrave” are available on Bocci’s site. (Video credit and submission: S. Bocci)

    ETA: This article originally misprinted the artist’s name as “Sandro Bocchi” and has been updated with the correct spelling.