Fluid dynamics appear at all kinds of scales. The animation above shows two comets, Encke and ISON, on their recent approach toward the sun. The darker wisps emanating from the right side of the image are part of the solar wind, a plasma stream continuously emitted by the sun’s upper atmosphere. Although the solar wind is very rarefied by terrestrial standards, its density is sufficient to whip the comets’ tails of gas and dust from side-to-side. Scientists use images like these to learn more about the structure of the solar wind based on its interaction with the comets. For more great images of ISON’s journey around the sun, check out NASA Goddard. (Image credit: K. Battams/NASA/STEREO/CIOC; submitted by John C)
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North Dakota Ice Disk
Cold weather can create some wild fluid dynamics, so pay attention to your local rivers and waterfalls during the next cold snap. The video above comes from North Dakota where a combination of cold dense air and a stable river eddy created a spinning ice disk, roughly 16 meters in diameter. The disk forms as a collection of ice chunks–not one solid, spinning piece–because the ice formed gradually. As ice pieces form, they get caught in the river eddy and begin to spin as part of the disk, rather like dust and ice do in the rings of Saturn. Such formations are rare but not unheard of; here’s a video showing a similar disk as it grows. (Video credit: G. Loegering; via Yahoo and io9; submitted by Simon H and John C)
Liquid Umbrella
When a water drop strikes a pool, it can form a cavity in the free surface that will rebound into a jet. If a well-timed second drop hits that jet at the height of its rebound, the impact creates an umbrella-like sheet like the one seen here. The thin liquid sheet expands outward from the point of impact, its rim thickening and ejecting tiny filaments and droplets as surface tension causes a Plateau-Rayleigh-type instability. Tiny capillary waves–ripples–gather near the rim, an echo of the impact between the jet and the second drop. All of this occurs in less than the blink of an eye, but with high-speed video and perfectly-timed photography, we can capture the beauty of these everyday phenomena. (Photo credit: H. Westum)
Pathlines vs. Streaklines
When considering fluid motion, there are many ways to describe trajectories through the flow. One is the pathline, the trajectory followed by an individual fluid particle. Imagine releasing a rubber duck down a stream. Following the duck’s position over time would give you a pathline. Now imagine that instead of releasing a single rubber duck you release lots of them – say one every half-second from the exact same starting spot. You would end up with a line of rubber ducks stretching downstream, each of them sharing the same origin but with a different starting time. This is called a streakline. Would the streakline of rubber ducks follow the same trajectory as the lone duck? Not if the flow is time-varying! In fact, for unsteady flows, pathlines and streaklines can give completely different pictures of a flow, as illustrated in the video above. Knowing and understanding the difference between these types of trajectories is extremely important when it comes interpreting flow visualizations in unsteady flows because some visualization methods produce pathlines and others produce streaklines. (Video credit: V. Miller and M. Mungal)
Bouncing Off The Surface
For the right angles and flow rates, it’s possible to bounce a fluid jet off a pool of the same fluid. As the jet flows, it pulls a thin layer of air with it, entraining the air. This air film is what keeps the jet separate from the pool when it initially hits. In the photo above, the jet is flowing right to left; notice how it maintains its integrity within the dimple during the bounce. The pool’s surface tension acts almost like a trampoline, redirecting the jet’s momentum into the bounce. It’s even possible to get a double bounce. In this video, the mechanism is the same, although the apparatus is different. In the photo above, the jet is introduced with a horizontal velocity to induce air entrainment and bouncing. In the video, the pool is spinning, which provides the necessary horizontal velocity between the jet and the liquid pool. (Photo credit: J. Bomber and T. Lockhart)
The Challenges of Trapping Carbon Dioxide
One way to reduce carbon dioxide in the atmosphere is to pump the CO2 into saline aquifers deep below the surface. Such aquifers are thin but stretch over large areas and are sometimes gently sloping. Since carbon dioxide is relatively buoyant, it may migrate up-slope after injection and potentially leak elsewhere. Dissolving the carbon dioxide into the groundwater helps prevent this undesirable migration. The video above shows a laboratory analog of the fluid instability at the heart of this trap. Imagine the video tilted by a few degrees so it slopes upward toward the right. The initially buoyant carbon dioxide, represented by the dark fluid, rises on the left and moves rightward, up-slope. As the CO2 dissolves into the ambient groundwater, the water becomes denser and fingers of the CO2-rich water drift downward, effectively halting the carbon dioxide’s escape. This is known as convective dissolution. (Video credit: C. MacMinn and R. Juanes)
Fluids Round-up – 16 November 2013
Time for another fluids round-up. Here are your links:
- PhysicsBuzz takes a look at the use of plasma actuators to control airflow.
- Over at Deep Sea News, you can learn about parasitic capillary waves.
- NanoWerk reports on self-steering particles in microfluidic devices.
- The 9th drop of the Queensland pitch drop experiment–believed to be the longest continuously running experiment in the world–is expected to fall at any time. Want to be part of the historic moment? Check out their Ninth Watch website.
- Aatish Bhatia examines the concepts behind the Fourier transform, an important mathematical technique used throughout fluid dynamics and physics. (via io9)
- Fluid dynamics and adaptive control might help alleviate traffic jams. (via @AIP_Publishing)
- On the whimsical side, take a look at these beautiful flying model boats built by Luigi Prina. (submitted by jshoer)
- Finally, our lead image was created with the app Frax, which allows users to make their own fractal-based art. Fluid dynamics has a lot of fractal behaviors. iOS users who want to play with fractals should check it out.
(Image credit: Ath3na)
Flow Behind a Cylinder
Flow over blunt bodies produces a series of alternating vortices that are shed behind an object. The image above shows the turbulent wake of a cylinder, with flow from right to left. Red and blue dyes are used to visualize the flow. This flow structure is known as a von Karman vortex street, named for aerodynamicist Theodore von Karman. The meander of the wake is caused by the shed vortices, each of which has a rotational sense opposite its predecessor. The rapid mixing of the two dyes is a result of the flow’s turbulence. In low Reynolds number laminar cases of this flow the structure of individual vortices is more visible. Similar flow structures are seen behind islands and in the wakes of flapping objects. (Photo credit: K. Manhart et al.)
Beads-on-a-string
Viscoelastic fluids are a type of non-Newtonian fluid in which the stress-strain relationship is time-dependent. They are often capable of generating normal stresses within the fluid that resist deformation, and this can lead to interesting behaviors like the bead-on-a-string instability shown above. In this phenomenon, a uniform filament of fluid develops into a series of large drops connected by thin filaments. Most fluids would simply break into droplets, but the normal stresses generated by the viscoelastic fluid prevent break-up. For this particular photo, the stresses are generated by clumps of surfactant molecules within the wormlike micellar fluid. Similar effects are observed in polymer-laced fluids. (Photo credit: M. Sostarecz and A. Belmonte)
Engineering Sediment Transport
Sediment transport via fluid motion is a major factor in engineering, geology, and ecology. This video shows two common forms of sediment transport: particle suspension and saltation. Suspension, in which the fluid carries small solid particles, is visible high in the blue water layer. Saltation occurs closer to the surface when loose particles are picked up by the flow before being redeposited downstream. Watch some of the individual particles near the surface to see the process. Kuchta has several more demo videos of flow in this desktop flume, sold by Little River Research & Design. (Video credit: M. Kuchta; submitted by gravelbar)