Tag: multiphase flow

Squeeze or Splatter?
Many a white shirt has met the disaster of a nearly-empty condiment bottle. One moment, you’re carefully squeezing out ketchup, and the next — sppplltlttt — you’re covered in red splatters. This messy phenomenon of gas displacing a liquid is widespread, showing up in condiments, some volcanic eruptions, and even the reinflation of a collapsed lung. Researchers have now constructed a mathematical model to fully capture and explain the process.
When you squeeze a container with both air and a liquid — like ketchup — in it, the air is easily compressed but the liquid is not. The extra pressure of the air creates a driving force that pushes the liquid out, despite its viscous resistance. Most of the time, these two forces are balanced, and the ketchup flows smoothly out of the container. But when the volume of ketchup is small compared to the air, squeezing can overpressurize the air, driving the ketchup out in an uncontrolled burst.
Luckily, the mathematics also suggest a solution to this problem: squeeze more slowly and double the size of the nozzle. You can also, they note, simply remove the top to avoid splatter. (Image credit: Rodnae Productions; research credit: C. Cuttle and C. MacMinn; via Ars Technica; submitted by Kam-Yung Soh)
December 27, 2022
Pumping Waste
Sewage systems rely on gravity to remove waste from our homes and carry it toward treatment plants. But that constant downward slope can’t always be maintained. Sometimes we have to bring the sewage back up to the surface to process it. For that, modern systems rely on pumps and other equipment to move the challenging slurry of liquid and solid materials. In this video, Grady from Practical Engineering breaks down the physics and engineering of sewage pumping. (Image and video credit: Practical Engineering)
June 13, 2022
Oil-Coated Bubbles
Bubbles in industrial applications are often more complicated than a simple pocket of air surrounded by water. Here researchers investigate the formation of an air bubble coated in oil before it rises through water. The photo above shows a series of snapshots as the bubble forms. Initially, a droplet of oil sits pinned on the surface. As air gets injected, the oil stretches around the growing bubble. Eventually, buoyancy pulls the bubble off the injector, creating a rising air bubble coated in oil. The team found that oil-coated bubbles could grow much larger than those in water alone. (Image and research credit: B. Ji et al.)
May 6, 2021
Measuring Contaminants in Drops and Bubbles
Rising bubbles and droplets are common in many chemical and industrial applications. But just a tiny concentration of contaminants on their surface can completely alter their behavior, disrupting coalescence and slowing down chemical reactions.
Historically, it’s been hard to measure the level of contamination in these some drops and bubbles, but a new study outlines a way to measure these small concentrations by perturbing the drops and watching how they deform. By analyzing how the drop shimmies and shakes, they’re able to measure its surface tension and, ultimately, the concentration of contaminants. (Image credit: S. Sørensen; research credit: B. Lalanne et al.; via APS Physics)
June 11, 2020
Fluid Dynamics and Disease Transmission
Right now people around the world are experiencing daily disruptions as a result of the recently declared coronavirus pandemic. There is a lot we don’t know yet about coronavirus, though researchers are working around the clock to report new information. Today’s video, though a couple years old, focuses on an area of medical knowledge that’s historically lacking but extremely relevant to our current situation: the mechanics behind disease transmission through sneezing or coughing.
Lydia Bourouiba is a leader in this area of research. Her studies have focused not on the size range of droplets produced but on the dynamics of the turbulent clouds that carry these droplets and what allows them to persist and spread. If you’ve wondered just why healthcare providers are recommending masks for sick people, keeping large distances between individuals, and frequent hand-washing, the image above hopefully helps explain why. Droplets carried in these turbulent clouds can travel several meters, and the buoyancy of the cloud’s gas components can help lift droplets toward ceiling ventilation. Right now, social distancing is one of our best tools against this disease transmission.
My goal in posting this is not to panic anyone. Rather, I hope you leave better informed as to why these precautions are needed. With coronavirus, our detailed knowledge of its characteristics — how long it remains viable in the air or on surfaces, how much is needed for an infection to take hold, etc. — is limited. But from research like Bourouiba’s, we know that coughing and sneezing are remarkably efficient ways to deliver respiratory pathogens, and that’s why caution is warranted. Stay safe, readers. (Video credit: TEDMED; image credit: Bourouiba Research Group, source; research credit: L. Bourouiba et al., see also S. Poulain and L. Bourouiba, pdf)
March 16, 2020
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)
September 8, 2014
Sneezes Vs. Coughs
Sneezing and coughing are major contributors to the spread of many pathogens. Both are multiphase flows, consisting of both liquid droplets and gaseous vapors that interact. The image on the left shows a sneeze cloud as a turbulent plume. The kink in the cloud shows that plume is buoyant, which helps it remain aloft. The right image shows trajectories for some of the larger droplets ejected in a sneeze. Like the sneeze cloud, these droplets persist for significant distances. The buoyancy of the cloud also helps keep aloft some of the smaller pathogen-bearing droplets. Researchers are building models for these multiphase flows and their interactions to better predict and counter the spread of such airborne pathogens. For similar examples of fluid dynamics in public health, see what coughing looks like, how hospital toilets may spread pathogens, and how adjusting viscoelastic properties may counter these effects. For more about this work, see the Bourouiba research group’s website. (Image credit: L. Bourouiba et al.)
March 27, 2014
Geyser Physics
Three basic components are necessary for a geyser: water, an intense geothermal heat source, and an appropriate plumbing system. In order to achieve an explosive eruption, the plumbing of a geyser includes both a reservoir in which water can gather as well as some constrictions that encourage the build-up of pressure. A cycle begins with geothermally heated water and groundwater filling the reservoir. As the water level increases, the pressure at the bottom of the reservoir increases. This allows the water to become superheated–hotter than its boiling point at standard pressure. Eventually, the water will boil even at high pressure. When this happens, steam bubbles rise to the surface and burst through the vent, spilling some of the water and thereby reducing the pressure on the water underneath. With the sudden drop in pressure, the superheated water will flash into steam, erupting into a violent boil and ejecting a huge jet of steam and water. For more on the process, check out this animation by Brian Davis, or to see what a geyser looks like on the inside, check out Eric King’s video. (Video credit: Valmurec; idea via Eric K.)
June 13, 2013
Labyrinth
A labyrinthine pattern forms in this timelapse video of a multiphase flow in a Hele-Shaw cell. Initially glass beads are suspended in a glycerol-water solution between parallel glass plates with a central hole. Then the fluid is slowly drained over the course of 3 days at a rate so slow that viscous forces in the fluid are negligible. As the fluid drains, fingers of air invade the disk, pushing the beads together. The system is governed by competition between two main forces: surface tension and friction. Narrow fingers gather fewer grains and therefore encounter less friction, but the higher curvature at their tips produces larger capillary forces. The opposite is true of broader fingers. Also interesting to note is the similarity of the final pattern to those seen in confined ferrofluids. (Video credit and submission: B. Sandnes et al. For more, see B. Sandes et al.)
June 20, 2012
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. #
December 29, 2011