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Search results for: “turbulence”

  • Elastic Turbulence

    Elastic Turbulence

    Decades ago, engineers pumping polymer-filled drilling liquids into porous rock noticed sudden and dramatic increases in the viscosity of the liquid. Within the tiny pores of the rock, conventional (i.e., inertial) turbulent flow should be impossible — the Reynolds number is simply too low. Now a new experiment points to the source of the high viscosity: elastic turbulence.

    To observe the phenomenon, researchers watched flow in the spaces between glass beads packed into a narrow channel. Videos of flow through one of these pores — roughly 250 microns across — are shown below. When flow rates are low (left), the fluid moves smoothly through the pore, but at higher flow rates (right), chaotic fluctuations emerge, creating the dramatic increase in apparent viscosity. In their analysis, the researchers found that the polymers’ motions generated the flow fluctuations, but most of the viscosity increase was inherent to the fluid’s movement, not to the polymers’ resistance to stretching. (Image credit: top – M. van den Bos, pore flow – Datta Lab; research credit: C. Browne and S. Datta; via Quanta Magazine; submitted by Kam-Yung Soh)

    Video of smooth flow through a pore (left) and flow with elastic turbulence (right).
    At low flow rates (left), the fluid moves smoothly through the tiny pores, but at higher flow rates (right), the polymers in the flow generate elastic turbulence that greater increases the fluid’s apparent viscosity.

  • Elastic Turbulence

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    Shear and Convection in Turbulence

    In nature, we often find turbulence mixed with convection, meaning that part of the flow is driven by temperature variation. Think thunderstorms, wildfires, or even the hot, desiccating winds of a desert. To better understand the physics of these phenomena, researchers simulated turbulence between two moving boundaries: one hot and one cold. This provides a combination of shear (from the opposing motion of the two boundaries) and convection (from the temperature-driven density differences).

    Please note that, despite the visual similarity, these simulations are not showing fire. There’s no actual combustion or chemistry here. Instead, the meandering orange streaks you see are simply warmer areas of turbulent flow, just as the blue ones are cooler areas. The shape and number of streaks are important, though, because they help researchers understand similar structures that occur in our planet’s atmosphere — and which might, under the wrong circumstances, help drive wildfires and other convective flows. (Image, research, and video credit: A. Blass et al.)

  • Bacterial Turbulence

    Bacterial Turbulence

    Conventional fluid dynamical wisdom posits that any flows at the microscale should be laminar. Tiny swimmers like microorganisms live in a world dominated by viscosity, therefore, there can be no turbulence. But experiments with bacterial colonies have shown that’s not entirely true. With enough micro-swimmers moving around, even these viscous, small-scale flows become turbulent.

    That’s what is shown in Image 2, where tracer particles show the complex motion of fluid around a bacterial swarm. By tracking both the bacteria motion and the fluid motion, researchers were able to describe the flow using statistical methods similar to those used for conventional turbulence. The characteristics of this bacterial turbulence are not identical to larger-scale turbulence, but they are certainly more turbulent than laminar. (Image credits: bacterium – A. Weiner, bacterial turbulence – J. Dunkel et al.; research credit: J. Dunkel et al.; submitted by Jeff M.)

  • Bacterial Turbulence

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    Celebrating Turbulence

    Laminar flow is easy to love, but turbulence is a far richer phenomenon. That’s the premise behind Veritasium’s new video (and, yes, I agree with him). In the video Derek provides a nice introduction to turbulence, including a checklist of qualities a turbulent flow must have.

    Personally, I don’t classify flows as simply being either laminar or turbulent; I view those two states as ends of a spectrum, which means there are many flows that fall somewhere in-between. (For more on what happens between laminar and turbulent, check out my video on transition.)

    As neat and eye-catching as laminar flow can be, turbulence is critical to life as we know it. It’s a necessary ingredient in cloud and raindrop formation. It drives the mixing of blood in our hearts. It keeps the leaves on trees from overheating. Without it, your coffee would be cold long before your cream mixes in. Turbulence is even critical to star formation; without turbulence, our entire solar system might have lacked the matter and time necessary to form! (Video and image credit: Veritasium)

  • Veritasium Turbulence

  • New Signs of Turbulence in Blood Flow

    New Signs of Turbulence in Blood Flow

    Our bodies are filled with a network of blood vessels responsible for keeping our cells oxygenated and carrying away waste products. In many ways, our blood vessels are tiny pipes, but there’s a crucial difference in the flow they carry: it’s pulsatile. Because the flow is driven by our hearts, rather than a continuous pump, every heartbeat creates a distinct cycle of acceleration and deceleration in the flow. And new research has found that this cycle, when combined with curvature or flow restrictions like plaque build-up, can create turbulence in unexpected places.

    Specifically, the researchers found that decelerating pipe flows can develop a helical instability that breaks down into turbulence, even in vessels where purely laminar flow would be expected. In the animations above, you can see the flow slow, develop swirls and then break into turbulence. The flow becomes laminar again as it accelerates, but during that brief bout of turbulence there’s much higher forces on the walls of a blood vessel. Over time, that extra force could contribute to inflammation or even hardening of the arteries. (Image and research credit: D. Xu et al.; via phys.org)

  • Marangoni Drops and Turbulence

  • Vortex Collisions Leave Clues to Turbulence

    Vortex Collisions Leave Clues to Turbulence

    Vortex ring collisions have long been admired for their beauty, but they’re now shedding light on the fundamental interactions that lead to turbulence. By dying just the cores of colliding vortex rings (Image 2), researchers observed anti-symmetric perturbations that develop along each core as they interact. These are indicative of what’s known as the elliptical instability.

    But the breakdown doesn’t stop there. Instead, as the elliptical instability develops, it generates a set of secondary vortex filaments that wrap around the original cores (Image 3). Just like the original vortex cores, those counter-rotating secondary filaments interact with one another, develop their own elliptical instability, and generate a set of smaller, tertiary filaments (Image 4).

    What’s exciting is that this process gives us a physical mechanism for the turbulent energy cascade. Researchers have talked for decades about energy passing from large-scale eddies to smaller and smaller ones, but this work lets us actually observe that cascade in the form of smaller and smaller pairs of vortex filaments interacting. To see more, check out some of our previous posts on this work. (Image and research credit: R. McKeown et al.; via Cosmos; submitted by Ryan M. and Kam-Yung Soh)

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