FYFD

Tag: fluid dynamics

  • Vibrating in the Flow

    Vibrating in the Flow

    Objects can obviously affect flows, but that’s not a one-way street. Flows can also affect objects, even ones as simple a circular cylinder. If you live somewhere with traffic lights mounted to a horizontal bar, you’ve probably seen this. On a windy day, the beam holding the traffic lights will oscillate up and down. This is an example of vortex-induced vibration, a coupling between the flow structures formed by an object and the motion of the object itself. With cylinders, engineers have mostly studied a situation like the traffic light – one where the motion of the cylinder is perpendicular to the direction of the flow. 

    But it’s also possible to get vortex-induced vibration in the same direction as the flow. That’s what you see visualized in the images above. Notice how the oscillation of the cylinders is inline with the flow direction. As with the crossflow version of vortex-induced vibration, this inline example has several wake forms that vary based on flow conditions. (Image and research credit: T. Gurian et al.)

  • Communication Between Microswimmers

    Communication Between Microswimmers

    The elongated cells of Spirostomum ambiguum swim using hair-like cilia, but when threatened, the cells contract violently, sending out long-range hydrodynamic waves, like those visualized above. Along with these waves, the cells release toxins aimed at whatever predator threatens them. In a colony, these waves act like a communication beacon. The swirl of a previous cell’s reaction tugs on its neighbors. As they contract, the message–and the toxins–spread. If the colony density is high enough, the hydrodynamic trigger waves will propagate through the entire colony, releasing enough toxins to disable even large predators. (Image and video credit: A. Mathijssen et al.)

  • Soap Film Evolution

    Soap Film Evolution

    The beautiful colors of a soap film reflect its variations in thickness. As a film drains and evaporates, it turns to shades of gray and black as it gets thinner. More than fifty years ago, one scientist proposed a free-energy-based explanation for how such ultrathin films might evolve. But it’s taken another half a century for experimental techniques to reach a point where the thickness of these ultrathin films could be measured well enough to test that theory. The new mechanism, known as spinodal stratification, has been observed in both vertical films (top) and foam (bottom) but has so far not been observed in any horizontal configuration, suggesting that buoyant effects are likely important, too. (Image and research credit: S. Yilixiati et al.; submitted by James S.)

  • Paddling

    Paddling

    When I lived in New England, I often spent summers paddling around a lake in either a kayak or canoe. Every stroke was an opportunity to stare down into the dark water and watch how the flow curled around my oar. Here you see a bit of what that looks like from underwater.

    The animation above shows a flat plate – twice as tall as it is wide – submerged about 20 mm below the surface and accelerated steadily from rest. As it starts moving, there’s a clear vortex ring formed and shed behind it. You can also see how the plate distorts the free surface into large depressions. Both of these cause extra drag on the plate. Eventually, though, the plate reaches a steady state.

    All together, what you see here is a good representation of what’s going on when a rower first begins to accelerate their boat from rest. Hydrodynamically speaking, the best way to do that isn’t to dig in with a deep stroke. It’s to use a series of short, relatively shallow strokes to get the boat up to speed. This takes advantage of the efficiency of drag generation during acceleration to get the boat to its cruising speed quickly. (Image and research credit: E. Grift et al.)

  • Foam Collapse

    Foam Collapse

    Introduce the right additive and the bubble arrays in foam will collapse catastrophically. What you see above is high-speed video of a quasi-two-dimensional soap bubble foam collapsing. There are two main mechanisms in the collapse. The first is a propagating mode. When one section of the film breaks, a stream of liquid from the broken film can impact an adjacent section, causing it to break as well. This accounts for much of the breakage you see above.

    The second mode is through penetration by droplets. Watch carefully, and you’ll see that some of the breaking films generate tiny droplets which can fly through the wall of the next cell and impact against the far side. With the right conditions, that impact can trigger a new break along a non-adjacent film. Together, these two mechanisms can destroy foam in the blink of an eye. (Image and research credit: N. Yanagisawa and R. Kurita)

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    The Shaky Life of a Droplet

    An evaporating drop of ouzo goes through several stages due to the interactions of oil, alcohol and water. If you turn the situation around by placing a drop of (blue-dyed) water in a mixture of alcohol and anise oil (top image), you get some similarly odd behavior. The drop of water shimmies and grows as alcohol dissolves into it, carrying the occasional oil droplet with it. Eventually, the droplet grows large enough and buoyant enough that part of it detaches and floats to the surface (middle image). If you increase the alcohol ratio in the surrounding fluid, you speed up this process, causing droplets to stream up to the surface (bottom image). (Image and video credit: O. Enriquez et al., source)

  • Phase-Switching to Avoid Icing

    Phase-Switching to Avoid Icing

    Preventing ice and frost from forming on surfaces – especially airplane wings – is a major engineering concern. The chemical de-icing cocktails currently used in aviation are a short-lived solution, and while superhydrophobic surfaces can be helpful, they tend to be easily damaged and therefore impractical. Another possible solution, shown here, are so-called phase-switching liquids – substances like cyclohexane that have freezing points higher than that of water. This means that they form a solid coating near the freezing temperature of water.

    Water droplets on these coatings move in a random stick-slip walk (above) but they tend not to freeze. This is because freezing requires the droplets to release heat, which melts part of the phase-switching liquid. Now, instead of solidifying to the surface, the droplet moves on a film of the phase-switching liquid. Re-freezing that liquid is tough because it’s thermodynamically unfavorable, and the smoothness of the liquid layer makes it harder for ice to find a nucleation point. In lab tests, the phase-switching liquid surfaces resisted ice and frost more than an order of magnitude longer than conventional materials. (Image and research credit: R. Chatterjee et al.; video credit: Univ. of Illinois at Chicago; submitted by Night King)

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    Agnes Pockels: Surface Science Pioneer

    Today’s FYFD video tells a story I’ve wanted to share for a couple of years now. It’s about the life and work of Agnes Pockels, a woman born in the mid-nineteenth century who, despite a lack of formal scientific training, made major contributions to the understanding of surface tension and to the experimental apparatuses and methodologies used in surface chemistry in general. She accomplished all of this not in a scientific lab, but from her kitchen.

    Pockels’ story is one of curiosity, determination, and meticulous scientific inquiry. Chances are that you’ve never heard of her, but you really should. Check out the full video below to learn more! (Image and video credit: N. Sharp)

  • The Color of Droplets

    The Color of Droplets

    In nature, color comes from many sources: like the pigmentation of skin and hair, the structural iridescence of a butterfly’s wings, or the refraction of a rainbow from water droplets. Recently, scientists discovered another source of brilliant color in simple, hemispherical water droplets.

    When small droplets form on a transparent surface, they form concave shapes capable of total internal reflection. This means that two light rays entering from the same angle can follow different paths inside the droplet. After reflecting several times, the light rays exit the droplet with a phase difference and how large that phase difference is determines the color. Check out the video below for some brightly colored examples of the effect. The researchers hope the technique will eventually be suitable for creating dye-free, color-changing technologies. (Image credit: F. Frankel; video credit: MIT News; research credit: A. Goodling et al.)

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    Weirs

    Hydraulic engineers use weirs, like the one shown below, to control upstream flow conditions. Weirs can come in many forms, but they essentially look like a small dam with water flowing over the top. They’re used to control both the flow rate and the upstream water level. As Grady from Practical Engineering explains, there are a few characteristics hydraulic engineers can vary to help adapt to changing water conditions. Check out the full video above to learn more about these important engineering features you’ve likely seen but never learned about. (Video and image credit: Practical Engineering)