FYFD

Month: July 2018

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    Vortex Ring Collisions

    One of the most enduringly popular submissions I receive is T. Lim’s experimental footage of two vortex rings colliding head-on. It’s an devilishly tough experimental set-up to master because perfectly aligning the rings is incredibly difficult. The pay-off, however, is huge because the breakdown of the colliding rings and their transformation into secondary rings is breathtaking. Destin at Smarter Every Day and his team have worked hard to recreate the experiment (top video), but they’re not the only ones – nor are they the first in decades – to do so.

    Ryan McKeown and a team at Harvard have a set-up of their own for vortex ring collisions, and you can see a little of it in action in the middle video. Ryan’s set-up is, frankly, incredible. It scans a light sheet through the vortex rings at high-speed, allowing him to capture the collision and break-up in minute detail in both space and time. What you see in the latter half of his video is a digital reconstruction of that data – not a simulation but real data! His work is capturing vortex collisions in unprecedented detail, allowing researchers to probe the smallest scales of the phenomenon.

    When two vortex rings approach one another, they can undergo what’s known as a vortex reconnection event. Bubbles rings are a great place to see this. The vortex cores get distorted when they’re close to one another due to the influence of the other vortex ring’s velocity field. This often stretches and flattens the vortex core. It’s impossible for the rings to simply break apart, though, (per Helmholtz’s second theorem). So when the original vortex rings thin to the point of breaking, they immediately reconnect to a piece of the other ring, creating a series of small vortex rings around the remains of the originals. The exact details of how this works are what investigators like Ryan and his colleagues are trying to understand. You can hear a little more about their work in my interview with Ryan in the bottom video, starting at ~2.54. (Video credits: Smarter Every Day, R. McKeown et al., and N. Sharp and T. Crawford; submission credit: a huge number of readers)

  • 2D Turbulence

    2D Turbulence

    Turbulence, the chaotic regime of fluid dynamics, is a complicated beast. It’s hard to analyze or predict, but we do understand some general ideas about it, like the fact that energy starts out in large eddies, cascades down smaller and smaller ones, and finally gets dissipated at the smallest scales, where viscosity snuffs them out. But that’s only true in three dimensions.

    Two-dimensional turbulence – what you get when you confine your fluid to a flat plane – is even weirder. When turbulence is flat, you can actually get an inverse energy cascade, where the energy of small eddies can add up to feed bigger ones. For awhile, this was treated as a mathematical curiosity; after all, we live in a three-dimensional world. But there are situations in life that are nearly two-dimensional, like the surface of a soap bubble or the atmosphere of a planet (which is typically exceptionally thin compared to the planet’s radius). And, little by little, scientists are collecting evidence that this inverse cascade – a flow of energy from small scales to larger ones – does actually happen in the real world. Understanding how this works may explain why hurricanes can intensify even when conditions are “wrong” and how Jupiter’s Great Red Spot has persisted for centuries. To learn more, check out Quanta Magazine’s full article on the work. (Image credit: NASA et al., M. Appel; via Quanta; submitted by Kam-Yung Soh)