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)
Category: Research
Liquid Crystal Films
Smectic liquid crystals can form extremely thin films, similar to a soap bubble, that are sensitive to electrically-induced convection. Here an annular smectic film lies between two electrodes. When a voltage is applied across it, positive and negative charges build up on the surface of the film near their respective electrodes. The electrical field surrounding the fluid pushes on the surface charges, causing flow inside the film. Above a threshold voltage, an instability forms and the film develops into a series of counter-rotating vortices, which spin faster as the voltage increases. The color variations in the video above are due to differences in the film’s thickness, much like iridescence of a soap bubble. (Video credit: P. Kruse and S. Morris)
Shocked Interfaces
The Richtmyer-Meshkov instability occurs when two fluids of differing density are hit by a shock wave. The animation above shows a cylinder of denser gas (white) in still air (black) before being hit with a Mach 1.2 shock wave. The cylinder is quickly accelerated and flattened, with either end spinning up to form the counter-rotating vortices that dominate the instability. As the vortices spin, the fluids along the interface shear against one another, and new, secondary instabilities, like the wave-like Kelvin-Helmholtz instability, form along the edges. The two gases mix quickly. This instability is of especial interest for the application of inertial confinement fusion. During implosion, the shell material surrounding the fuel layer is shock-accelerated; since mixing of the shell and fuel is undesirable, researchers are interested in understanding how to control and prevent the instability. (Image credit: S. Shankar et al.)The APS Division of Fluid Dynamics conference begins this Sunday in Pittsburgh. I’ll be giving a talk about FYFD Sunday evening at 5:37pm in Rm 306/307. I hope to see some of you there!
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)
Avoiding Splashback
Here’s a likely Ig Nobel Prize candidate from the BYU SplashLab: a study of splashing caused by a stream of fluid entering a horizontal body of water or hitting a solid vertical surface. In other words, urinal dynamics. The researchers simulated this activity using a stream of water released from a given height and angle and observed the resulting splash with high-speed video. They found a stream falls only 15-20 centimeters before the Plateau-Rayleigh instability breaks it into a series of droplets, and that this is the worst-case scenario for splash-back. The video above shows how a stream of droplets hits the pool, creating a complex cavity driven deeper with each droplet impact. Not only does each impact create a splash, the cavity’s collapse does as well. Similarly, when it comes to solid surfaces, they found that a continuous stream splashes less. They’ve also put together a helpful primer on the best ways to avoid splash-back. (Video credit: R. Hurd and T. Truscott; submitted by Ian N., bewuethr, John C. and possibly others)
For readers attending the APS DFD meeting, you can catch their talk, “Urinal Dynamics,” Sunday afternoon in Session E9 before you come to E18 for my FYFD talk.
Particle-Tracking in Granular Flows
One of the challenges of experimental fluid dynamics is gathering sufficient data in environments that can be fast-changing, visually dense, and sometimes harsh. Ideally, researchers want to gather as much data–velocities, temperatures, pressures–at as many points as possible and do so without disturbing the flow with a probe. No technique can provide everything, and thus new diagnostics are always under development. This video shows a new particle tracking method developed for fluidized granular flows where the high concentration of particles makes other techniques unsuitable. Such flows are often seen in industrial applications in chemical processing, pharmaceuticals, and powder transport. Interestingly, the technique can also be used in particle-seeded fluid flows like those normally studied with particle image velocimetry (PIV). (Video credit: F. Shaffer and B. Gopalan; submitted by @ASoutolglesias)
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)
Wavy Swimmers
Animals often move in ways engineers find counter-intuitive. Take, for example, the glass knifefish, an undulatory swimmer that controls its motion through wavelike oscillations of its fin. One might expect the knifefish to move its fin so that a single continuous wave moves from one end to the other. Instead two opposing waves move down the knifefish’s fins, one travelling from head to tail and the other travelling from the tail forward. The intersection of these waves is the nodal point, and, by shifting the nodal point fore or aft, the knifefish can hover in place, move forward or swim backward. At first glance, this seems like a wasteful system since a significant portion of each wave cancels the other, but, through mathematical modeling and experiments with a biomimetic robot, the researchers found that the dual-wave locomotion increases both the stability and maneuverability of the fish. (Video credit: N. Cowan et al.; via phys.org)
Making Better Tags for Tracking Turtles
Tagging equipment is used on all manner of aerial and marine creatures to gather data about animal behavior in their natural environments. It can be difficult, though, for researchers to gauge what effects the tags have on an animal. A recent study by T. T. Jones et al. used drag measurements on marine turtle casts to estimate the effects of common tagging equipment. They found that, on large turtles, the equipment increases a turtle’s drag by as little as 5%, but for smaller species or juvenile turtles, the drag cost can be much larger – in some cases doubling a turtle’s drag when swimming. Such large increases in drag may significantly change a tagged turtle’s behavior and skew results or even endanger the animal. The researchers suggest a model that allows others to estimate a tag’s drag effects across species. (Image credits: T. Gray and M. Carey; research credit: T. T. Jones et al.; via PopSci; submitted by Chi M.)
Holey Splashes
A liquid’s surface tension can have a big effect on its splashes. In this video, a 5-mm droplet hits a surface covered in a thin layer of a liquid with lower viscosity and surface tension. The result is a dramatic effect on the spreading splash. As the initial curtain grows and expands, the lower surface tension of the impacted fluid thins the splash curtain. Fluid flows away from these areas due to the Marangoni effect, causing holes to grow. The sheet breaks up into a network of liquid filaments and ejected droplets before gravity can even bring it all to rest. For more, see this previous post and review paper. (Video credit: S. Thoroddsen et al.)
