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.
Category: Research
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. #
Structures in Turbulence
Despite its appearance, there is order in the chaos of turbulence. These snapshots from a turbulent channel flow simulation outline these coherent structures in black. The top photo shows a top view looking down on the channel and the bottom image shows a side view of the channel. It is thought that studying these coherent structures may help shed light on turbulence and its formation, which remains one of the great open questions of classical physics. (Photo credit: M. Green)
Splash Sheets
When a falling liquid jet hits a horizontal impacter, it is deflected into a sheet. The shape of the sheet is dependent upon the velocity of the jet and the viscosity of the fluid. At sufficiently high speeds the sheet will be circular; at lower speeds it may sag into a bell-shape. The circular sheets can also develop an instability that causes them to become polygonal, as shown in the photos above. The fluid then flows out along the sheet, into and along the rim, and then spouts outward in jets at the polygon’s corners. For some conditions, the jets at the corners even form a sort of fluid chain (top photo). (Photo credit: R. Buckingham and J. W. M. Bush; via 14-billion-years-later)
Particle Image Velocimetry
One common experimental technique for measuring velocity in a flow is particle image velocimetry (PIV), shown above. Special particles are introduced–seeded–into the flow. Typically, these particles are small, neutrally buoyant, and have a refractive index significantly different from the background flow. One or more lasers are used to illuminate a section of the flow–a plane for 2D measurements or a cube for 3D. Rather than operating continuously, the laser is pulsed, producing very short exposure times of the order of hundreds of nanoseconds. A camera (or more than one camera for 3D measurements) captures a pair of images separated by this short exposure. The time between frames is so small that the particles will not have moved much between frames. Researchers can then correlate the two frames and derive velocity data from the motion of the particles.
