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)
Category: Research
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)
Staining Patterns
This timelapse video shows a particulate suspension as it dries and the pattern formation that results. The mixture of silicon dioxide particles and water is spread over a glass slide. As the water evaporates, capillary action generates a flow toward the edges, but the fluid meniscus pins larger particles to the glass, trapping them. As more and more water evaporates, smaller particles are trapped, causing the formation of uneven stripes in the particulate deposits. You’ve probably seen these patterns before on the side of a muddy car after a rainy day! (See also: how coffee rings form; Video credit: Bjornar Sandnes)
Sloshing to Dampen
In this high-speed video, two flexible spheres are dropped from the same height. The one on the left is filled with air, the other is partially filled with a liquid. Although both spheres rebound to nearly the same height after the first bounce, their behavior differs drastically after that. The sloshing of the liquid inside the sphere acts as a damper, absorbing energy that would otherwise cause the ball to continue bouncing. The effects of contained liquids sloshing are important for understanding the dynamics of tankers, fuel on spacecrafts, and even how to walk without spilling your coffee.
Fragmenting Raindrops
This numerical simulation demonstrates the fragmentation of droplets of water falling through a quiescent medium–essentially how a raindrop behaves. As the initial droplet falls, drag forces deform the droplet, contorting it until surface tension causes it to break into smaller droplets, which can themselves be broken up by the same mechanisms.
Star-Shaped Nozzles
Efficient mixing of fluids is vital for many applications, including fuel injection for all types of combustion and masking the exhaust of stealth fighters. Star-shaped lobed nozzles can produce jets that mix more effectively than conventional jets. This photo shows cross-sections of the jet at several downstream distances from the nozzle exit. (Photo credit: H. Hu et al)
Breakup of an Annular Sheet
A thin annular sheet of water is sandwiched between two concentric air streams. This airflow on either side of the water causes shearing and Kelvin-Helmholtz-type instabilities develop, causing the sinuous waves along the water surface. Periodic behavior of the sort observed here is frequently observed in fluid mechanical instabilities. #
Viscous Fluid Falling on a Moving Belt
In this video a very viscous (but still Newtonian) fluid is falling in a stream onto a moving belt. Initially, the belt is moving quickly enough that the viscous stream creates a straight thread. As the belt is slowed, the stream begins to meander sinusoidally and ultimately begins to coil. Aside from some transient behavior when the speed of the belt is changed very quickly, the behavior of the thread is very consistent within a particular speed regime. This is indicative of a nonlinear dynamical system; each shift in behavior due to the changing speed of the belt is called a bifurcation and can be identified mathematically from the governing equation(s) of the system. (Video credit: S. Morris et al)
Particle Jets
During explosions, solid particles and liquids packed around the explosive charges can form jets, making a blast wave appear more porcupine-like than spherical. The instability mechanisms that cause this behavior are not well-understood, but researchers suspect the jets are formed due to perturbations in the particle bed on the timescale of the initial shock propagation. The presence of these jets can affect the blast wave’s subsequent growth as well as the mixing in its wake. The number of jets produced depends on many factors, including particle type, the geometry of the charge, the ratio of explosive to particles, and even whether the particles are wet or dry. Note the very different natures of the explosions in the video when shown side by side. (Video credit: D. Frost et al)
Stick-Slip Bubbles
Varying the rate of injection of air into a wet granular mixture contained in a Hele Shaw cell results in very different flow patterns. At low injection rates, stick-slip bubbles form. As the injection rate increases, patterns are affected by “temporal intermittency” where continuous motion is occasionally interrupted by jamming. Increasing the injection rate still further results in Saffman-Taylor-like fingering. #
