Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "jet"

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)

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)

This high-speed video of a bullet fired into a water balloon shows how dramatically drag forces can affect an object. In general, drag is proportional to fluid density times an object’s velocity squared. This means that changes in velocity cause even larger changes in drag force. In this case, though, it’s not the bullet’s velocity that is its undoing. When the bullet penetrates the balloon, it transitions from moving through air to moving through water, which is 1000 times more dense. In an instant, the bullet’s drag increases by three orders of magnitude. The response is immediate: the bullet slows down so quickly that it lacks the energy to pierce the far side of the balloon. This is not the only neat fluid dynamics in the video, though. When the bullet enters the balloon, it drags air in its wake, creating an air-filled cavity in the balloon. The cavity seals near the entry point and quickly breaks up into smaller bubbles. Meanwhile, a unstable jet of water streams out of the balloon through the bullet hole, driven by hydrodynamic pressure and the constriction of the balloon. (Video credit: Keyence)

Whenever a hollow cavity forms at the surface of a liquid, the cavity’s collapse generates a jet—a rising, high-speed column of liquid. The composite images above show snapshots of the process, from the moment of the cavity’s greatest depth to the peak of the jet. The top row of images shows water, and the bottom row contains a fluid 800 times more viscous than water. The added viscosity both smooths the geometry of the process and slows the jet down, yet strong similarities clearly remain. Focusing on similarities in fluid flows across a range of variables, like viscosity, is key to building mathematical models of fluid behavior. Once developed, these models can help predict behaviors for a wide range of flows without requiring extensive calculation or experimentation. (Image credit: E. Ghabache et al.)

Cephalopods like the octopus or squid are some of the fastest marine creatures, able to accelerate to many body lengths per second by jetting water behind them. Part of what makes its high speed achievable, though, is the way the animal changes its shape. In general, drag forces are proportional to the square of velocity, meaning that doubling the velocity increases the drag by a factor of four. The energy necessary to overcome such large drag increases generally prevents marine animals from going very fast (compared to those of us used to moving through air!) But drag is also proportional to frontal area. Like the bio-inspired rocket in the video above, jetting cephalopods begin their acceleration from a bulbous shape and then shrink their exposed area as they accelerate. Not only does this shape change help mitigate increases in drag due to velocity, it prevents flow from separating around the animal, shielding it from more drag. The result is incredible acceleration using only a simple jet for thrust. For example, the octopus-like rocket in the video above reaches velocities of more than ten body lengths per second in less than a second. (Video credit: G. Weymouth et al.)

Granular materials like sand are sometimes very fluid-like in their behaviors. The high-speed video above shows a ball bearing being dropped into packed sand. Many features of the splash are fluid-like; the initial impact creates a spreading crownlike splash, followed by a strong upward jet that eventually collapses back into the medium. At the same time, many of the impact characteristics are decidedly non-fluidic. Sand has no surface tension, so both the crown and the jet readily break up into small particles. The granular jet is very narrow and energetic, reaching heights greater than the impacter’s drop height. Interestingly, the column begins collapsing on its lower end before the jet even reaches its highest peak. This may be due to the lower energy of the sand particles that were ejected later in the crater formation process. (Video credit: J. Verschuur, B. van Capelleveen, R. Lammerink and T. Nguyen)

For the right angles and flow rates, it’s possible to bounce a fluid jet off a pool of the same fluid. As the jet flows, it pulls a thin layer of air with it, entraining the air. This air film is what keeps the jet separate from the pool when it initially hits. In the photo above, the jet is flowing right to left; notice how it maintains its integrity within the dimple during the bounce. The pool’s surface tension acts almost like a trampoline, redirecting the jet’s momentum into the bounce. It’s even possible to get a double bounce. In this video, the mechanism is the same, although the apparatus is different. In the photo above, the jet is introduced with a horizontal velocity to induce air entrainment and bouncing. In the video, the pool is spinning, which provides the necessary horizontal velocity between the jet and the liquid pool. (Photo credit: J. Bomber and T. Lockhart)

Jets of high-energy plasma and sub-atomic particles explode outward from the Hercules A elliptical galaxy at the center of this photo. The jets are driven to speeds close to that of light due to the gravitation of the supermassive black hole at the center of the elliptical galaxy. Relativistic effects mask the innermost portions of the jets from our view, but, as the jets slow, they become unstable, billowing out into rings and wisps whose turbulent shapes suggest multiple outbursts originating from Hercules A. (Photo credit:NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble HeritageTeam (STScI/AURA); via Discovery)

Underwater explosions often behave non-intuitively. Here researchers explore the effects of surface explosions by setting off charges at the air/water interface. Initially, an unconfined explosion’s blast wave expands a cavity radially into the water. This cavity collapses back toward the surface from the bottom up, ultimately resulting in a free jet that rebounds above the water level. Confined explosions behave very differently, expanding down the glass tube containing them in a one-dimensional fashion. The cavity never extends beyond the end of the glass tube, likely due to hydrostatic pressure. (Video credit: Adrien Benusiglio, David Quéré, Christophe Clanet)

For the right flow speeds and incidence angles, a jet of Newtonian fluid can bounce off the surface of a bath of the same fluid. This is shown in the photo above with a laser incorporated in the jet to show its integrity throughout the bounce. The walls of the jet direct the laser much the way an optical fiber does. The jet stays separated from the bath by a thin layer of air, which is constantly replenished by the air being entrained by the flowing jet. The rebound is a result of the surface tension of the bath providing force for the bounce. (Photo credit: T. Lockhart et al.)