an animated map of global wind conditions
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)
The photo sequence in the upper image shows, left to right, a fluid-filled tube falling under gravity, impacting a rigid surface, and rebounding upward. During free-fall, the fluid wets the sides of the tube, creating a hemispherical meniscus. After impact, the surface curvature reverses dramatically to form an intense jet. If, on the other hand, the tube is treated so that it is hydrophobic, the contact angle between the liquid and the tube will be 90 degrees during free-fall, impact, and rebound, as shown in the lower image sequence. The liquid simply falls and rebounds alongside the tube, without any deformation of the air-liquid interface. (Photo credit: A. Antkowiak et al.)
Lasers and impacting droplets can’t fail to impress. PIV is a flow viz technique in which small ‘seeder’ particles are released into a flow and laser light and cameras are used to map the positions of these particles. From these measurements flow quantities like velocity can be calculated.
In this example the interplay between a liquid surface and the air around it is seen as a drop collides with the liquid surface. It looks like the seeding machine is just off the left of screen and producing some pretty good vortices.
Al Seckel, a cognitive neuroscientist and expert on illusions, created this “Levitating Water” installation, in which multiple streams of water appear as a series of levitating droplets thanks to a strobing light. The well-timed strobe lighting tricks the brain into seeing many different falling droplets as the same, nearly stationary droplet. The effect is similar to the one created by vibrating a stream of falling water. (Video credit: wunhanglo)
Most jellyfish swim through the water by periodically contracting their elastic bells. This shoots a jet out behind them propelling them forwards (albeit slowly). Here the affect on the surrounding water is shown through dye placed in front. Vorticity is shed into the fluid by the jellyfish’s motion which can be seen brilliantly in this video as the vortex ring coming off of the jelly fish body.
This video shows a multi-layered droplet, in which several droplets are formed one inside the other as an initial drop falls through a layer of oil sitting atop another liquid. When the drop falls, its potential energy gets transformed into interface energy, creating a fascinating interplay of surface tension, deformation, and miscibility between the fluids. Such self-contained multi-layered droplets, similar to multiple emulsions, could be helpful in pharmaceutical development. (Video credit: E. Lorenceau and S. Dorbolo 2004)
A double emulsion inside a double emulsion: These concentric quadrupole emulsions, a drop inside of a drop inside of a drop inside of a drop, are very challenging to produce. Furthermore, to controllably generate a large number of them with all the same dimensions is impossible with conventional techniques. Here, using a new single step emulsification technique with a glass microfluidic device, layered fluids are controllably mass produced. The walls of the device are surface treated with silanes to control the wetting properties of the fluids so that five immiscible fluids converge to the same location in the device. Using appropriate flow velocities, fluids are sheared with respect to each other for drop formation. (Video Credit: Shin-Hyun Kim )
There’s not much cooler than lasers and water which is what we see here. Although the lasers here aren’t really being used as a flow visualization technique, lasers are often used in wind tunnels and elsewhere to visualise flow patterns, as in PIV. There’s also some pretty cool total internal reflection.
Credit: University of Colorado, Boulder
Have you ever wondered what happens inside a jet of fluid as it breaks into droplets? Such events are not commonly or readily measured. This video uses a double emulsion—in which immiscible fluids are encapsulated into a multi-layer droplet—to demonstrate interior fluid flow during the Plateau-Rayleigh instability. The innermost drops and the fluid encapsulating them have a low surface tension between them, thanks to the addition of a surfactant to the inner drops. As a result, the inner drops are easily deformed by motion in the fluid surrounding them. Flow on the left side of the jet is clearly parabolic, similar to pipe flow. Closer to the pinch-off, the inner droplets shift to vertical lines, indicating that the interior flow’s velocity is constant across the jet. After pinch-off, the inner droplets return to a spherical shape because they are no longer being deformed by fluid movement around them. The coiling of the inner drops inside the bigger one is due to the electrical charges in the surfactant used. (Video credit: L. L. A. Adams and D. A. Weitz)
This numerical simulation shows a von Karman vortex street in the wake of a bluff body. As flow moves over the object, vortices are periodically shed off the object’s upper and lower surfaces at a steady frequency related to the velocity of the flow. The simulation takes place in a channel; note how the thickness of the boundary layers on the walls increases with downstream distance, forcing a slight constriction on the vortex street in the freestream.