Archer fish hunt by shooting jets of water at their prey to knock them into the water where the fish can eat them. Previous research showed that the archer fish’s projectile jet is pulsed such that the water released at a later time has a greater velocity. This makes the jet bunch up so that a ball of liquid hits the prey with greater force than the jet would otherwise. A recently released paper shows that the archer fish actively adjust their liquid jets in order to strike targets at different distances while maintaining this bunching effect. To control the jets, the fish adjust both how long they jet and what speed they impart to the fluid by changing how they open and close their mouths. (VIdeo credit: Nature; research credit: P. Gerullis and S. Schuster; via phys.org; submitted by @jchawner)
Search results for: “jet”
The Physics of Sneezing
Sneezing can be a major factor in the spread of some illnesses. Not only does sneezing spew out a cloud of tiny pathogen-bearing droplets, but it also releases a warm, moist jet of air. Flows like this that combine both liquid and gas phases are called multiphase flows, and they can be a challenge to study because of the interactions between the phases. For example, the buoyancy of the air jet helps keep smaller droplets aloft, allowing them to travel further or even get picked up and spread by environmental systems. Researchers hope that studying the fluid dynamics and mathematics of these turbulent multiphase clouds will help predict and control the spread of pathogens. Check out the Bourouiba research group for more. (Video credit: Science Friday)
Breaking Drops with Vibration
Atomization is the process of breaking a liquid into a spray of fine droplets. There are many methods to accomplish this, including jet impingement, pressure-driven nozzles, and ultrasonic excitement. In the images above, a drop has been atomized through vibration of the surface on which it rests. Check out the full video. As the amplitude of the surface’s vibration increases, the droplet shifts from rippling capillary waves to ejecting tiny droplets. With the right vibrational forcing, the entire droplet bursts into a fine spray, as seen in the photo above. The process is extremely quick, taking less than 0.4 seconds to atomize a 0.1 ml drop of water. (Photo and video credit: B. Vukasinovic et al.; source video)
Water-Based Tractor Beam
Researchers in Australia have demonstrated a “tractor beam” capable of manipulating floating objects from a distance using surface waves on water. And, unlike some research, you can try to replicate this result right in the comfort of your own bathtub! When a wave generator oscillates up and down, it creates surface waves that move objects and particles on the water’s surface. When the wave amplitudes are small, the outgoing wave fronts tend to be planar, as in part (a) of the figure above. These planar waves push surface flow away from the wave generator in a central outward jet, and new fluid is entrained from the sides to replace it. This creates the kind of flowfield shown in the streaklines of part (b).Increasing the amplitude of the surface waves drastically changes the surface flow’s behavior. Larger wave amplitudes are more susceptible to instabilities due to the nonlinear nature of the surface waves. This means that the planar wave fronts seen in part (a) break down into a three-dimensional wavefield, like the one shown in part (c). Near the wave-maker, the surface waves now behave chaotically. This pulsating motion ejects surface flow parallel to the wave-maker, which in turn draws fluid and any floating object toward the wave-maker. The corresponding surface flowfield is shown in part (d). The researchers are refining the process, but they hope the physics will one day be useful in applications oil spill clean-up. (Video credit: Australia National University; image and research credit: H. Punzmann et al. 1, 2; via phys.org; submitted by Tracy M)
The Real Shape of Raindrops
We often think of raindrops as spherical or tear-shaped, but, in reality, a falling droplet’s shape can be much more complicated. Large drops are likely to break up into smaller droplets before reaching the ground. This process is shown in the collage above. The initially spherical drops on the left are exposed to a continuous horizontal jet of air, similar to the situation they would experience if falling at terminal velocity. The drops first flatten into a pancake, then billow into a shape called a bag. The bags consists of a thin liquid sheet with a thicker rim of fluid around the edge. Like a soap bubble, a bag’s surface sheet ruptures quickly, producing a spray of fine droplets as surface tension pulls the damaged sheet apart. The thicker rim survives slightly longer until the Plateau-Rayleigh instability breaks it into droplets as well. (Image credit: V. Kulkarni and P. Sojka)
4th Birthday: The Kaye Effect
Today’s post continues my retrospective on mind-boggling fluid dynamics in honor of FYFD’s birthday. This video on the Kaye effect was one of the earliest submissions I ever received–if you’re reading this, thanks, Belisle!–and it completely amazed me. Judging from the frequency with which it appears in my inbox, it’s delighted a lot of you guys as well. The Kaye effect is observed in shear-thinning, non-Newtonian fluids, like shampoo or dish soap, where viscosity decreases as the fluid is deformed. Like many viscous liquids, a falling stream of these fluids creates a heap. But, when a dimple forms on the heap, a drop in the local viscosity can cause the incoming fluid jet to slip off the heap and rebound upward. As demonstrated in the video, it’s even possible to create a stable Kaye effect cascade down an incline. (Video credit: D. Lohse et al.)
Inside a Rocket
Rockets often utilize liquid propellants for their combustion. To maximize the efficiency during burning, the liquid fuel and oxidizer must mix quickly and break up into an easily vaporized spray. One method to achieve this is to inject the fuel and oxidizer as liquid jets that collide with one another. For high enough flow rates, this creates a highly unstable liquid sheet that quickly atomizes into a spray of droplets. The animation above shows an example of two impinging jets, but rockets using this method would typically have more than just two injection points. Other rockets use co-axial or centrifugal injectors to mix and atomize the fuel and oxidizer prior to combustion. (Image credit: C. Inoue; full-scale GIF)
Fireworks Taking Off
Aerial fireworks are essentially semi-controlled exploding rockets. Here Discovery Channel shares high-speed video of fireworks taking off. The turbulent billowing exhaust on the ground is reminiscent of other rocket launches. The tube-launched firework clip is a great example of an underexpanded nozzle. The pressure of the gases in the tube is higher than the ambient air, so when the gases escape, the exhaust fans out to equalize the pressure. And, finally, the explosion that propels the colorful chemicals outward forms jets that can affect the final form of the display. To my American readers: Happy 4th of July! And be safe! (Video credit: Discovery Slow-Down)
Reader Question: Rib Vortices
Reader tarastarr1 asks:
For the (awesome) wave gif and explanation, I think the asker was wondering about that little branch-like projection you can see in the top-middle part of the gif right after the camera submerges. Your explanation of the wave is great, but now I’m also wondering: if the wave is so powerful, how can that little jet form?
I think you’re probably right about the original question. I actually didn’t even notice that tiny vortex until after the post went up today! I think the little vortex is probably similar to the rib vortices I referenced at the end of the last post. If there happened to be some small localized rotation in the water initially, the wave’s passage would stretch it out. Stretching a vortex causes it to spin faster, exactly the way that an ice skater pulling her arms in causes her to spin faster – conservation of angular momentum! In that situation, the wave’s passage actually strengthens the vortex rather than destroying it.
Colliding in Microgravity
On Earth, it’s easy for the effects of surface tension and capillary action to get masked by gravity’s effects. This makes microgravity experiments, like those performed with drop towers or onboard the ISS, excellent proving grounds for exploring fluid dynamics unhindered by gravity. The video above looks at how colliding jets of liquid water behave in microgravity. At low flow rates, opposed jets form droplets that bounce off one another. Increasing the flow rate first causes the droplets to coalesce and then makes the jets themselves coalesce. Similar effects are seen in obliquely positioned jets. Perhaps the most interesting clip, though, is at the end. It shows two jets separated by a very small angle. Under Earth gravity, the jets bounce off one another before breaking up. (The jets are likely separated by a thin film of air that gets entrained along the water surface.) In microgravity, though, the jets display much greater waviness and break down much quicker. This seems to indicate a significant gravitational effect to the Plateau-Rayleigh instability that governs the jet’s breakup into droplets. (Video credit: F. Sunol and R. Gonzalez-Cinca)
