FYFD

Tag: newtonian fluids

  • Spinning Tops

    Spinning Tops

    What does the flow look like around a spinning top? Here, researchers used dye to visualize what happens in a Newtonian fluid (like air or water) as well as a viscoelastic fluid. The Newtonian fluid (upper images) divides into two circulating zones, one below the top and one above. They both take the shape of a toroidal, or donut-shaped, vortex, visible here in cross-section.

    The long molecules of the viscoelastic fluid lend it elasticity to resist stretching. The result is a very different flow field. Beneath the top, there’s still a toroidal vortex, though it appears tighter. But around the upper part of the top, there’s a butterfly-like region of recirculation! (Image credit: B. Keshavarz and M. Geri)

  • Spin Cycle

    Spin Cycle

    Rotational motion is a great way to break up liquids, as anyone who’s watched a dog shake itself dry can attest. That same centrifugal force is what allows this rotary atomizer to break liquids into droplets. Relative to the photos above, the atomizer spins in a counter-clockwise direction. This motion stretches the fluid flowing off it into skinny, equally-spaced ligaments, which eventually break down into droplets.

    Just how and when that break-up occurs depends on the fluid, as well as the characteristics of the spin. For Newtonian fluids like silicone oil — shown in the first two pictures — the break-up is driven by surface tension and happens relatively quickly. But with a viscoelastic fluid — shown in the last image — the elasticity of polymers in the fluid allow it to resist break-up for much longer. Instead, the ligaments form the beads-on-a-string instability. See more flows in action in the video below. (Video, image, and research credit: B. Keshavarz et al., video)

  • Climbing Up the Walls

    Climbing Up the Walls

    You may have noticed when baking that fluids don’t always behave as expected when you agitate them. If you put a spinning rod into a fluid, we’d expect the rod to fling fluid away, creating a little vortex that stirs everything around. And for a typical (Newtonian) fluid, this is what we see. The fluid’s viscosity tries to resist deforming the fluid, but the momentum imparted by the rod wins out. With a viscoelastic fluid, on the other hand, the story is much different. As before, the spinning of the rod deforms the fluid. But the viscoelastic fluid contains long chains of polymers. As those polymers get stretched by the deformation, they generate their own forces, including forces parallel to the rod. Instead of being flung outward, the viscoelastic fluid starts climbing up the rod, with the stretchy elasticity of the polymers helping pull more fluid up and up.  (Image credit: Ewoldt Research Group, source)

  • Newtonian and Non-Newtonian Vortices

    Newtonian and Non-Newtonian Vortices

    Not all vortex rings are created equal. Despite identical generation mechanisms and Reynolds numbers, the two vortex rings shown above behave very differently. The donut-shaped one, on the top left in green and in the middle row in blue, was formed in a Newtonian fluid, where viscous stress is linearly proportional to deformation. As one would expect, the vortex travels downward and diffuses some as time passes. The mushroom-like vortex ring, on the other hand, is in a viscoelastic fluid, which reacts nonlinearly to deformation. This vortex ring first furls and expands as it travels downward, then stops, contracts, and travels backward! (Image credit: J. Albagnac et al.; via Gallery of Fluid Motion)

  • Bouncing Jet

    Bouncing Jet

    For the right flow speeds and incidence angles, a jet of Newtonian fluid can bounce off the surface of a bath of the same fluid. This is shown in the photo above with a laser incorporated in the jet to show its integrity throughout the bounce. The walls of the jet direct the laser much the way an optical fiber does. The jet stays separated from the bath by a thin layer of air, which is constantly replenished by the air being entrained by the flowing jet. The rebound is a result of the surface tension of the bath providing force for the bounce. (Photo credit: T. Lockhart et al.)

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    Viscoelastic Jets

    Unlike Newtonian fluids, such as air and water, viscoelastic fluids exhibit non-uniform reactions to deformation. In this video, researchers explore the effects of this behavior when a liquid jet falls into another fluid. When fluids move past one another at different speeds in this manner, there is a shearing force which often leads to the wave-like Kelvin-Helmholtz instability between the fluids. Here we see for a variety of wavelengths how the breakdown of a Newtonian and viscoelastic jet differ. The Newtonian jets form clean lines and complicated tulip-like shapes, but the viscoelasticity of the non-Newtonian jets inhibits the growth of these instabilities, surrounding the central jet with wisps of escaping fluid. For more, see Keshavarz and McKinley. (Video credit: B. Keshavarz and G. McKinley)

  • Viscous Fingers

    Viscous Fingers

    High viscosity silicon oil is sandwiched between two circular plates.  As the upper plate is lifted at a constant speed, air flows in from the sides. The initially circular interface develops finger-like instabilities, due to the Saffman-Taylor mechanism, as the air penetrates. Eventually the fluid will completely detach from one plate. (Photo credit: D. Derks, M. Shelley, A. Lindner)