Tag: science

Fluidic Public Art by Charles Sowers
Artist Charles Sowers creates exhibits and public art focused on illuminating natural phenomenon that might otherwise go unnoticed, and much of his work features fluid dynamics directly or indirectly. “Windswept” and “Wave Wall” are both outdoor exhibits that show undulations and vortices corresponding to local wind flow. Other pieces explore ferrofluids through magnetic mazes or feature foggy turbulence. My own favorite, “Drip Chamber”, oozes with viscous fluids whose dripping forms patterns reminiscent of convection cells. Be sure to check out his website for videos of the exhibits in action. (Photo credits: Charles Sowers; submitted by rreis)
November 9, 2012
Reader Question: How Useful is Flow Viz?
Reader Andrew asks:
I’ve noticed you’ve posted a bunch of flow visualization/wind tunnel content. I’m just curious where how useful information is obtained from these. Is it just observation? Or are there instruments that are usually used in conjunction with these techniques to provide data?
Great question, Andrew! The answer can vary based on the technique and application. In some cases, flow visualization is used for purely qualitative observation, but in others it can provide more quantifiable data. For example, the water tunnel flow visualization of Google’s heliostat array gave very qualitative data about flow around a given configuration but allowed quick evaluation of many configurations. Flow visualization can also help identify key features for additional study like vortices in a wake. This identification of structure can be so useful that even in computational fluid dynamics, where researchers have all possible information about pressure, temperature, and velocity in a flow field, flow visualization is regularly used to identify underlying structures.
Some flow visualization methods can also give very specific information. Oil-flow visualization gives a snapshot of shear stress at the surface of an object, letting an engineer identify at a glance areas of laminar and turbulent flow as well as regions with vortices and streaks. Naphthalene flow visualization and infrared thermography are both great for identifying the location of laminar-turbulent transition and can do so across the span of an object, which is much easier than trying to traverse a probe across the entire object. And some forms of flow visualization allow for extraction of velocity field information, as in particle image velocimetry. In this technique, tiny particles seed the flow and carefully timed image pairs are taken and correlated to determine the flow field velocity based on the changes in particle positions between images.
Like every measurement, flow visualization methods have their strengths and limitations. But for many applications, flow visualization provides much more than just pretty pictures and thus remains an important tool in any fluid dynamicist’s arsenal!
November 8, 2012
Detonation in a Bubble
Accidental releases of combustible gases in unconfined spaces can be difficult to recreate in a laboratory environment. Here researchers simulate the conditions using detonation inside a soap film bubble. Combustible gases are pumped inside the soap film and then a spark creates ignition. The resulting flame propagation is visualized using high-speed schlieren photography, making the density gradients in the flame visible. When the mixture of hydrogen fuel to air is balanced, the flame is spherically symmetric with a high flame speed. In contrast, weaker mixtures of fuel/air produce slow flame speeds and mushroom-like flames that leave behind unreacted fuel. This is due to buoyant effects; the time scale associated with buoyancy is smaller than that of the flame speed and chemical reactions when the fuel/air mixture is lean. (Video credit: L. Leblanc et al.)
November 7, 2012
Accidental Painting
Artist D. A. Siqueiros sometimes used a technique he referred to as “accidental painting” in his work, in which he would pour a layer of one color of paint and then pour a second color over it. The two colors would mix in striking patterns. Here researchers recreate the technique and analyze the fluid dynamics of it. Each paint has a slightly different density thanks to the pigments used to color them. When a denser paint is poured over a less dense one (as in the white on black in the video), this activates the Rayleigh-Taylor instability. The white paint will tend to sink down below the black paint due to gravity. At the same time, the spreading of the two paints also affects the shapes and patterns through mixing and diffusion. (Video credit: S. Zetina and R. Zenit)
November 6, 2012
Donut-Shaped Bubbles
Here researchers simulate rain-like droplet impacts with large drops of water falling into a tank from several meters. The momentum of such an impact is significantly higher than many other droplet impact examples we’ve featured. In this case, the coronet, or crown-like splash, caused by the collision collapses quickly, closing the fluid canopy around a trapped bubble of air. The remains of the coronet fall inward, preventing the development of the usual Worthington jet associated with droplet impacts. Instead, the air bubble takes on an unstable donut-like shape. (Video credit: M. Buckley et al.)
November 5, 2012
Green Fingers
Differences in surface tension between two layers of fluid can cause fascinating finger-like instabilities. Here glycerol is spread in a thin film on a silicon wafer. Then a wire coated in oleic acid, which has a lower surface tension than glycerol, was touched to the wafer. As the oleic acid spreads across the film surface, Marangoni and capillary stresses cause variations in the film thickness, which results in the dendritic patterns seen here. (Photo credit: B. Fischer et al.)
November 2, 2012
“Millefiori”
In “Millefiori” artist Fabian Oefner mixes watercolors with ferrofluids to create bright fluid microcosms. Each photograph represents an area about the size of a thumbnail. Ferrofluids contain iron-based nanoparticles suspended in a carrier fluid and thus respond to magnetic fields. They can form sharp points, labyrinthine mazes, or even brain-like patterns depending on the magnetic field and the substances surrounding them. For more on this art project, see this interview with the artist. (Photo credit: Fabian Oefner)
November 1, 2012
Dribbling Droplets
Ethanol droplets on a hot copper plate bounce under the influence of electrostatic forces from a charged rod. The temperature of the plate is high enough that the droplet is supported by a thin vapor film, which is what keeps it from wetting the plate. Ethanol does not have the strong polarity that water does, but the hydroxyl group on one end does make it susceptible to the electrostatic charge built up on the teflon rod. As a result, the droplets oscillate under electrostatic and gravitational forces, resulting in a dribbling effect. (Video credit: S. Wildeman et al.)
October 31, 2012
Polygonal Jumps
Hydraulic jumps occur when a fast-moving fluid enters a region of slow-moving fluid and transfers its kinetic energy into potential energy by increasing its elevation. For a steady falling jet, this usually causes the formation of a circular hydraulic jump–that distinctive ring you see in the bottom of your kitchen sink. But circles aren’t the only shape a hydraulic jump can take, particularly in more viscous fluids than water. In these fluids, surface tension instabilities can break the symmetry of the hydraulic jump, leading to an array of polygonal and clover-like shapes. (Photo credits: J. W. M. Bush et al.)
October 30, 2012
Spray Starch
High speed video of of spray starch from a can. Once the initial transients die down, a cone-shaped annular sheet forms. Disturbances propagate in the sheet, tearing it into filaments that break down into droplets. Beautiful complexity hidden in a simple everyday device. (Video credit: John Savage)
October 29, 2012