FYFD

Tag: instability

  • Splash Sheets

    Splash Sheets

    When a falling liquid jet hits a horizontal impacter, it is deflected into a sheet. The shape of the sheet is dependent upon the velocity of the jet and the viscosity of the fluid. At sufficiently high speeds the sheet will be circular; at lower speeds it may sag into a bell-shape. The circular sheets can also develop an instability that causes them to become polygonal, as shown in the photos above. The fluid then flows out along the sheet, into and along the rim, and then spouts outward in jets at the polygon’s corners. For some conditions, the jets at the corners even form a sort of fluid chain (top photo). (Photo credit: R. Buckingham and J. W. M. Bush; via 14-billion-years-later) 

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    Viscoelastic Fluids in Space

    In honor of astronaut Don Pettit’s launch to the International Space Station (and in the hope that he’ll do more neat microgravity fluids demonstrations while in space!), here’s a look a the behavior of viscoelastic fluids in microgravity. The elasticity of these fluids means that, when strained, the fluid deforms instantaneously and then returns to its initial shape when the strain is removed. Pettit demonstrates both Plateau-Rayleigh instability behavior, where a column of fluid breaks apart due to surface tension variations, and die swell, where a fluid jet expands beyond the diameter of nozzle from which it was extruded. Such swelling is commonly caused by the stretching and relaxation of polymers in the fluid as they react to forces caused by the nozzle opening.

  • Wave Clouds Over Alabama

    Wave Clouds Over Alabama

    Last week, Birmingham, Alabama got treated to a special cloudy day, thanks to some Kelvin-Helmholtz waves, shown above. When a layer of faster moving fluid shears a slower moving fluid, this instability can form and cause some spectacular mixing. In this case, the lower, slower fluid was cool and moist enough to contain clouds, enabling us to see the effect with the naked eye. The same mechanism is responsible for the shape of breaking ocean waves and can even be seen in the atmospheres of gas giants like Saturn and Jupiter. (submitted by David B)

  • Surface Tension Instability

    Surface Tension Instability

    Droplets of oleic acid spread across a thin film of glycerol on a silicon wafer. The shapes here are driven by hydrodynamic instabilities, particularly Marangoni effects due to the differences in surface tension between the two fluids. (Photo credit: A. Darhuber, B. Fischer and S. Troian)

  • Ink Sculptures

    Ink Sculptures

    Dripping ink into water can create fantastic structures as the two fluids mix. In this artwork there are numerous complex mixing phenomena: the eddies and multiple scales of turbulence; the long, thin streams of laminar flow; and the wispy mushrooms and umbrellas of the Rayleigh-Taylor instability. (Photo credit: Mark Mawson; via @thinkgeek)

  • Water Balloon Physics

    [original media no longer available]

    This video explores some of the physics behind the much-loved bursting water balloon. The first sections show some “canonical” cases–dropping water balloons onto a flat rigid surface.  In some cases the balloon will bounce and in others it breaks. The bursting water balloons develop strong capillary waves (like ripples) across the upper surface and have some shear-induced deformation of the water surface as the rubber peals away. Then the authors placed a water balloon underwater and vibrated it before bursting it with a pin. They note that the breakdown of the interface between the balloon water and surrounding water shows evidence of Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The Rayleigh-Taylor instability is the mushroom-like formation observed when stratified fluids of differing densities mix, while the Richtmyer-Meshkov instability is associated with the impulsive acceleration of fluids of differing density.

  • Transition to Turbulence

    Transition to Turbulence

    Smoke introduced into the boundary layer of a cone rotating in a stream highlights the transition from laminar to turbulent flow. On the left side of the picture, the boundary layer is uniform and steady, i.e. laminar, until environmental disturbances cause the formation of spiral vortices. These vortices remain stable until further growing disturbances cause them to develop a lacy structure, which soon breaks down into fully turbulent flow. Understanding the underlying physics of these disturbances and their growth is part of the field of stability and transition in fluid mechanics. (Photo credit: R. Kobayashi, Y. Kohama, and M. Kurosawa; taken from Van Dyke’s An Album of Fluid Motion)

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    High-Speed Ferrofluid

    High-speed video captures the behavior of a ferrofluid trapped between two magnets. Ferrofluids contain tiny ferromagnetic particles suspended in a carrier fluid like oil or water. The distinctive peaks and valleys of a ferrofluid subject to a strong magnetic field is due to the normal-field instability and is a result of the fluid minimizing its magnetic energy.

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    Viscous Fingers

    A Hele Shaw cell is little more than two glass plates separated by a thin layer of viscous fluid. The cell serves as a good test bed for viscous, low Reynolds number flows such as those found in microfluidics. Here a less viscous fluid is injected into the center of the cell, causing the finger-like protrusions of the less viscous fluid into the more viscous one via the Saffman-Taylor instability.

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    Convection Visualization

    Here on Earth a fascinating form of convection occurs every time we put a pot of water on the stove. As the fluid near the burner warms up, its density decreases compared to the cooler fluid above it. This triggers an instability, causing the cold fluid to drift downward due to gravity while the warm fluid rises. Once the positions are reversed, the formerly cold fluid is being heated by the burner while the formerly hot fluid loses its heat to the air. The process continues, causing the formation of convection cells. The shapes these cells take depend on the fluid and its boundary conditions. For the pot of water on the stove and in the video above, the surface tension of the air/water interface can also play a role in modifying the shapes formed. The effects caused by the temperature gradient are called Rayleigh-Benard convection. The surface tension effects are sometimes called Benard-Marangoni convection.