When a fluid is vibrated, instabilities can form along its surface. With a sufficient amplitude, voids form inside the fluid and their collapse leads to a jet that shoots out from the fluid. A very different process leads to air cavities forming in a vibrated granular medium, but the jets produced are remarkably similar, as seen in this video. (Video credit: M. Sandtke et al.)
Tag: jets
Jets from Hollows
Bubbles rising through a viscous fluid deform and interact. As they collapse into one another, the lower bubble induces a gravity-driven jet that projects upward into the higher bubble. The more elongated the bubble, the faster the jet. The same behavior is seen in the rebound of a cavity at the free surface of a liquid. The authors suggest a universal scaling law for this behavior. (Video credit: T. Seon et al.)
Viscoelastic Jets
Unlike Newtonian fluids, such as air and water, viscoelastic fluids exhibit non-uniform reactions to deformation. In this video, researchers explore the effects of this behavior when a liquid jet falls into another fluid. When fluids move past one another at different speeds in this manner, there is a shearing force which often leads to the wave-like Kelvin-Helmholtz instability between the fluids. Here we see for a variety of wavelengths how the breakdown of a Newtonian and viscoelastic jet differ. The Newtonian jets form clean lines and complicated tulip-like shapes, but the viscoelasticity of the non-Newtonian jets inhibits the growth of these instabilities, surrounding the central jet with wisps of escaping fluid. For more, see Keshavarz and McKinley. (Video credit: B. Keshavarz and G. McKinley)
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)
Bubbles and Jets
In the photo sequence above, a bubble is created at the interface between two immiscible liquids–water on top and denser hydrofluroether (HFE) below. Initially, the bubble expands explosively due to the vaporization of water generated by a short laser pulse. As the bubble collapses, a jet forms and accelerates into the HFE. After collapse, the bubble remnants injected in the HFE cause the formation of a jet that shoots back into the water above. Surface instabilities make the jet assume a mushroom or crown-like structure that detaches from the jet. Eventually gravity will return the system to its initial undisturbed fluid-fluid interface. (Photo credit: S. Avila et al. 1,2)
