Knots have long fascinated humans, appearing in art for thousands of years and generating entire fields of study. Until recently, however, the idea of a knotted fluid was purely theoretical. To knot fluids, researchers used 3D printing to create twisted hydrofoil shapes. When towed through water, fluid travels around the shape and spins up at the trailing edge, creating a knotted vortex ring. The knotted vortices were captured with 3D imaging, allowing scientists to observe how they evolve. So far the knots they’ve created have all been unstable, deforming until two vortex lines approach one another. Upon contact, the vortices disconnect and reconnect with one another, unraveling the knot. Intriguingly, these vortex reconnections seem remarkably similar to the vortex reconnections observed between quantized vortices in superfluids. (Video credit: D. Kleckner et al.)
Tag: vortices
Bubble Vortices
Vortices appear in scales both large and small, from your shower and the flap of an insect’s wing to cyclones and massive storms on other planets. Especially with these large-scale vortices, it can be difficult to understand the factors that affect their trajectories and intensities over time. Here researchers have studied the vortices produced on a heated half bubble for clues as to their long-term behavior. Heating the base of the bubble creates large thermal plumes which rise and generate large vortices, like the one seen above, on the bubble’s surface. Researchers observed the behavior of the vortices with and without rotation of the bubble. They found that rotating bubbles favored vortices near the polar latitudes of the bubble, just as planets like the Earth and Saturn have long-lived polar vortices. They also found that the intensification of both bubble vortices and hurricanes was reasonably captured by a single time constant, which may lead to better predictions of storm behaviors. Their latest paper is freely available here. (Image credit: H. Kellay et al.; research credit: T. Meuel et al.; via io9)
Vibrations from Vortices
Vortex shedding frequently happens in the wakes of non-streamlined bodies as a result of flow around the obstacle. Newton’s third law states that forces come in equal and opposite pairs, meaning that the vortex shedding behind an obstacle is accompanied by a force on the obstacle. For a fixed cylinder, this is not always apparent, but for a pendulum, like the ones demonstrated in this video, this vortex-induced vibration causes significant motion. This same effect can make traffic lights and industrial chimneys sway. You’ve likely experienced it yourself as well, if while swimming you’ve ever spread your fingers underwater and spun in place. Try it sometime with your arm out and you’ll feel the vortices make your arm vibrate up and down as you spin. (Video credit: Harvard Natural Sciences Lecture Demonstrations)
The Physics of a Flying-V
New research using free-flying northern bald ibises shows that during group flights the birds’ positioning and flapping maximize aerodynamic efficiency. In flight, a bird’s wings generate wingtip vortices, just as a fixed-wing aircraft does. These vortices stretch in the bird’s wake, creating upwash in some regions and downwash in others as the bird flaps. According to theory, to maximize efficiency a trailing bird should exploit upwash and avoid downwash by flying at a 45-degree angle to its leading neighbor and matching its flapping frequency. The researchers found that, on average, this was the formation and timing the flock assumed. In situations where the birds were flying one behind the next in a straight line, the birds tended to offset their flapping by half a cycle relative to the bird ahead of them–another efficient configuration according to theory. Researchers don’t yet know how the birds track and match their neighbors; perhaps, like cyclists in a peloton, they learn by experience how to position themselves for efficiency. For more information, see the researchers’ video and paper. (Photo credit: M. Unsold; research credit: S. Portugal; via Ars Technica; submitted by M. Piedallu van Wyk)
Holiday Fluids: Santa’s Aerodynamics
Today we have some holiday-themed fluid dynamics: visualization of flow around Santa’s sleigh! This is a flowing soap film visualization at a low speed (author Nick Moore has some other speeds as well). Santa’s sleigh is what aerodynamicists call a bluff body–a shape that is not streamlined or aerodynamic–and sheds a complicated wake of vortices. Like any object moving through a fluid, Santa’s sleigh generates drag forces made up of several components. There is viscous drag, which comes from friction between the sleigh’s surface and the fluid, and form drag (or pressure drag), which comes from the shape of the sleigh. That wake full of complicated vortices significantly increases the sleigh’s pressure drag, requiring Rudolph and the other reindeer to provide more thrust to counter the sleigh’s drag. Speaking thereof, the visualization does not take into account the aerodynamics of the reindeer, who, in addition to providing the sleigh’s thrust, would also affect the flowfield upstream of the sleigh. This post is part of this week’s holiday-themed post series. (Video credit: N. Moore)
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!
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.)
Fluids Round-up – 2 November 2013
Fluids round-up time! Here are your latest links:
- Over at PhysicsFocus, Colin White discusses the Bernoulli fallacy and other zombie myths of physics. (Via @JenLucPiquant)
- Aviation Week has an exclusive look at Skunk Works’ SR-72 next-gen hypersonic aircraft.
- MinutePhysics asks if it’s better to walk or run through rain. This post has another take on the question.
- io9 describes why bubbles lose their color as they pop.
- Physics Buzz looks at knotted fluid vortices. They also have a nice write-up on the foaming of a struck beer, which we talked about last week.
- Enjoy the beauty of mathematics next to the physics they describe. (via io9)
- More fun fluids from Physics Buzz, this time looking at new tiny jellyfish-like flying robots.
- Remember the Chelyabinsk meteor from February? Discovery reports on an analysis of the air burst and its probability.
- Is there fluid mechanics in neck cracking? (?!?)
- New research shows that mesoscale self-assembly can be achieved using capillary charges.
- Finally, our lead image shows a simulation of turbulent flow in a tightly packed lattice of spheres. It’s an entry from Argonne National Laboratory’s annual “Art of Science” contest. Take a look at the entries and vote for your favorites!
While not strictly fluid dynamical, I want to take a moment to talk about education. I receive a lot of stunned reactions and self-deprecation when people learn I study aerospace engineering. Many people say, “Oh, I could never do that!” or “You must be some kind of genius.” I’m not. It’s true that studying engineering and fluid dynamics involves a lot of math and some it is complex (no pun intended). There’s a lot of unfounded fear about science and math in our society, when really they are just skills that any of us can improve with practice and effort. So, for those out there who have ever thought, “I can’t do that, there’s too much math,” please watch this young woman address mathphobia. She sums up just about everything I’ve always wanted to tell you.(Photo credit: Argonne National Laboratory)
Dynamic Stall
In nature, birds and other flying animals often use unsteady flow effects to enhance the lift their wings generate. When a wing sits at a high angle of attack, it stalls; the flow separates from the upper surface, and its lift force is suddenly lost. If, on the other hand, that wing is in motion and pitching upward, lift is maintained to a much higher angle of attack. The reason for this is shown in the flow visualization above. This montage shows a rectangular plate pitching upwards. Flow is left to right. Each row represents a specific angle of attack and each column shows a different spanwise location on the plate. As the plate pitches upward, a vortex forms and grows on the leading edge of the plate. Eventually, the leading-edge vortex separates, but not until a much higher angle of attack than the plate could sustain statically. This effect allows birds to maintain lift during perching maneuvers and is also key to helicopter rotor dynamics. (Image credit: K. Granlund et al.)
