Two jets of sugar syrup collide and interact to form very different patterns. On the left, the two jets have a low flow rate and create a chain-like wake. The jets on the right have a higher flow rate and produce a liquid sheet that breaks down into filaments and droplets. The result is often likened to fish bones. (Photo credit: Rebecca Ing)
Tag: fluid dynamics
Vortex Ring Collision
Two vortex rings collide head-on in this video. If their vorticities and velocities are matched in magnitude and opposite in direction, their collision results in a stagnation plane–essentially a wall across which the fluid does not pass. In reality, there are slight variations that result in non-zero velocities where the vortices meet, so some mixing occurs, but the overall symmetry remains striking. The collision breaks up the vortex ring into filaments, some of which cross-link with the other vortex’s filaments, resulting in the little halo-like eddies around the perimeter. Videos of the same experiment at different Reynolds numbers can be found here. (Submitted by Charlie H; Video credit: T. Lim and T. Nickels)
Ejecting Drops
Large droplets ejected from a liquid pool do not coalesce immediately back into the whole. Instead, a thin layer of air gets trapped beneath them, much like the oil lubricating bearings. The weight of the droplet causes the air to drain away, and eventually the droplet comes in contact with the pool. Some of the droplet gets drained away before surface tension snaps the interface back into a low energy state. A new smaller droplet then bounces upward before repeating the process over again. Eventually the droplet becomes small enough that its entire mass gets sucked away by the pool. Researchers call this process the coalescence cascade.
Freezing in a Microchannel
Fluid mechanics at the microscale can behave quite differently than in our everyday experience. Microfluidic devices–sometimes known as labs on a chip–are becoming increasingly important in research and daily life. For example, the test strips used by diabetics to check their blood sugar levels are microfluidic devices. In this video, researchers use a microfluidic channel to observe the freezing of supercooled water droplets. As the droplet first passes into the cold zone of the channel, it flash freezes, filling from the inside out with ice crystals. As it continues through the cold zone, the drop freezes fully, beginning at the outside surface and working inward. As it does so, the ice droplet fractures due to stresses. (Video credit: Stan et al)
How Shock Waves Form
Most people are familiar with the Doppler effect–in which the frequency of a wave changes depending on the motion of the observer relative to the wave source–from the shifting pitch of sirens as they pass. But the effect is important for pressure waves in addition to acoustic waves. When an object moves through air, its motion disturbs the surrounding air via pressure waves, which travel at the speed of sound. If an object moves slower than the speed of sound (top right), then those pressure waves extend in front of the object, carrying information about the object and allowing the air to shift and move smoothly around it.
If the object is moving at the speed of sound (bottom left), then it arrives at the same time as the pressure waves. In essence, the object is striking a stationary wall of air–this is what was meant by “breaking the sound barrier”. At Mach 1, the physics of the problem have fundamentally shifted. Now the only way for air to deflect to allow the object’s passing is by the sudden compression of a shock wave.
Moving even faster than the speed of sound (bottom right) the pressure and sound waves created by the object’s motion stretch in a cone behind it. The cone, known as a Mach cone, is the shock wave that deflects air around the moving object. The result is that the object will actually pass an observer before the observer will hear it. This is because no information can travel forward of the Mach cone’s leading edge. That’s why the area outside of the Mach cone is sometimes called the Zone of Silence. When the Mach cone passes an observer, the shock wave will register as a boom, like when the space shuttle passes overhead while landing. (via fyeahchemistry)
Artificial Fins in Tandem
For this image, two artificial fish fins are placed side-by-side and flapped in phase. Flow in the image is upward. The wakes of the fins interact in a complicated vortex street. Researchers hope that studying such flows can help in designing the next generation of autonomous underwater vehicles. (Photo credit: B. Boschitsch, P. Dewey, and A. Smits)
Ultrasonic Levitation of Drops
This video shows an ultrasonically levitated 3 mm drop of propylene glycol changing shape. A couple of things are happening here. Firstly, the drop is suspended due to the acoustic radiation pressure from intense ultrasonic sound waves being produced by a transducer vibrating at 30kHz. Then the power input to the ultrasonic transducer is increased, which strengthens the acoustic field, and this is what causes the drop to flatten. Currently, acoustic levitation is used for containerless processing of very pure materials or chemicals. As with many methods for levitation, it is currently restricted to objects of relatively light weight. (Video credit: J. R. Saylor et al, Clemson University)
Oceanic Swirls
Mixing of surface waters with deeper ocean currents brings together the minerals and nutrients used by phytoplankton, resulting in gorgeous swirls of color in the ocean. These phytoplankton blooms are most common in the spring and summer, and while lovely, can be harmful to other marine life, either through the production of toxins or by depleting the waters of oxygen. Because the phytoplankton move according to the wind and waves, they can also form a sort of natural flow visualization. (Photo credit: ESA)
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Dove in Flight
This spectacular high-speed video shows a dove in flight. Note how its wings flex through its stroke and the way the wings rotate over the course of the downstroke and reversal. There is incredible beauty and complexity in this motion. The change in wing shape and angle of attack is what allows the bird to maximize the lift it generates. Note also how the outer feathers flare during the downstroke. This promotes turbulence in the air moving near the wing, which prevents separated flow that would cause the dove to stall. (See also: how owls stay silent. Video credit: W. Hoebink and X. van der Sar, Vliegkunstenaars project)
Inside a Blender
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High-speed video visualizes the complicated flow field inside a blender. Note that the video is placed in reverse for artistic effect. This flowfield is clearly too turbulent for reversible flow. That said, it is possible to mix two fluids and then unmix them, under the right circumstances.
