Supercomputing has been an enormous boon to fluid dynamics over the past few decades. Many problems, like the interaction between a supersonic shock wave and a bubble, are too complicated for analytical solutions and difficult to measure experimentally. Numerical simulation of the problem, combined with visualization of key variables, adds invaluable understanding. Here a shock wave strikes a helium bubble at Mach 3, and the subsequent interactions in terms of density and vorticity are shown. This situation is relevant to a number of applications, such as supersonic combustion and shockwave lithotripsy–a medical technique in which kidney stones are broken up inside the body using shock waves. After impact, an air jet forms and penetrates the center of the structure while the outer regions mix and form a persistent vortex ring. (Video credit: B. Hejazialhosseini et al.; via Physics Buzz)
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Donut-Shaped Bubbles
Here researchers simulate rain-like droplet impacts with large drops of water falling into a tank from several meters. The momentum of such an impact is significantly higher than many other droplet impact examples we’ve featured. In this case, the coronet, or crown-like splash, caused by the collision collapses quickly, closing the fluid canopy around a trapped bubble of air. The remains of the coronet fall inward, preventing the development of the usual Worthington jet associated with droplet impacts. Instead, the air bubble takes on an unstable donut-like shape. (Video credit: M. Buckley et al.)
Polygonal Jumps
Hydraulic jumps occur when a fast-moving fluid enters a region of slow-moving fluid and transfers its kinetic energy into potential energy by increasing its elevation. For a steady falling jet, this usually causes the formation of a circular hydraulic jump–that distinctive ring you see in the bottom of your kitchen sink. But circles aren’t the only shape a hydraulic jump can take, particularly in more viscous fluids than water. In these fluids, surface tension instabilities can break the symmetry of the hydraulic jump, leading to an array of polygonal and clover-like shapes. (Photo credits: J. W. M. Bush et al.)
The Archer Fish’s Arrows
The archer fish hunts by shooting a jet of water at insects in the leaves above and knocking them into the water. How the fish achieve this feat has been a matter of contention. A study of high-speed video of the archer’s shot shows that fluid dynamics are key. The fish releases a pulsed liquid jet, imparting greater velocity to the tail of the jet than the head. As a result, the tail tends to catch up to the head and increase the jet’s mass on impact while decreasing the duration of impact. Simultaneously, the jet tends to break down into droplets via the Rayleigh-Plateau instability caused by surface tension. Surface tension’s power to hold the water in droplets combined with the inertial effects of the pulsed jet create a ball of fluid that strikes the archer’s prey with more than five times the power than vertebrate muscles alone can impart. For more on archer fish, check out this video and the original research paper by A. Vailati et a. (Photo credits: Scott Linstead and BBC; submitted by Stuart R)
Rebounding
A ping pong ball bounces off a puddle, drawing a liquid column upward behind it. This photo shows the instant after the fluid has disconnected from the ball, allowing it to rebound without further loss of momentum to the fluid. The fluid column begins to fall under gravity, the tiny undulations in its radius growing via the Rayleigh-Plateau instability and eventually causing the column to separate from the puddle. You can see the whole process in action in this high-speed video. (Photo credit: BYU Splash Lab)
Air Entrainment
When a liquid jet falls into a pool, air is often entrained along with the liquid, creating a cavity and, often, bubbles. Shown above is video of a low-speed laminar jet entering a quiescent pool. The jet appears to entrain a thin film of gas, which then breaks up in a three-dimensional fashion, despite the symmetry of the incoming jet. As the speed of the incoming jet is increased and turbulence is introduced, the resulting air entrainment becomes violent and chaotic. For additional information and videos, see Kiger and Duncan 2012 and their supplemental videos. (Video credit: K. Kiger and J. Duncan)
Rocket Exhaust
A fiery jet of exhaust remains amid plumes of smoke as a Soyuz rocket lifts off from Baikonur Cosmodrome bound for the International Space Station. The lengthscales of such turbulence range from tens of meters to only millimeters, highlighting the incredible difficulty of accurately capturing and describing the fluid motion of a practical engineering problem. (Photo credit: NASA/Carla Cioffi; via Visual Science)
The Supersonic Plonk
Everyone knows the familiar plonk of a stone falling into a pond but few realize the complexity of the physics. When a solid object falls into a pool, a sheet of liquid, the crown splash, is sent upward. Simultaneously, the object pulls a cavity of air down with it. As the water moves inward, this cavity is pinched, creating an hourglass-like shape reminiscent of the shape of a rocket’s nozzle. As the diameter of that pinched cavity shrinks, the velocity of the upward escaping air increases, resulting in the formation of an air jet moving faster than the speed of sound. This air jet is followed by a slower liquid jet that may rebound to a height higher than then original height of the dropped object. So next time you throw a stone into a pond, enjoy the knowledge that you’ve broken the sound barrier. (Photo credit: D. van der Meer; see also Physics World)
Fireball in Slow Motion
The high-speed video above shows an atomized spray of flammable liquid being ignited using a lighter. It was filmed at 10,000 fps and is replayed at 30 fps. Although uncontained, this demonstration is similar to the combustion observed inside of many types of engines. Automobiles, jet engines, and rockets all break their liquid fuel into a spray of droplets to increase the efficiency of combustion. The turbulence of the flames dances and swirls, with small-scale motions close to the sprayed droplets and larger-scale motions around the vaporized fuel. This variation in size of the scales of motion is a hallmark feature of turbulence and can be used to characterize a flow.
Astro Puffs
Microgravity continues to be a fascinating playground for observing surface tension effects on the macroscale without pesky gravity getting in the way. Here astronaut Don Pettit has created a sphere of water, which he then strikes with a jet of air from a syringe. Initially, the momentum from the jet of air creates a sharp cavity in the water, which rebounds into a jet of water that ejects one or more satellite drops. Surface waves and inertial waves (inside the water sphere) reflect back and forth until the fluid comes to rest as a sphere once more. Note how similar the behavior is to the pinch-off of a water column. Both effects are dominated by surface tension, but on Earth we can only see this behavior with extremely small droplets and high-speed cameras! (Video credit: Don Pettit, Science Off the Sphere)