Tag: fluid dynamics

Gathering Safely
One effect of the COVID-19 pandemic is a renewed interest in the physics of disease transmission and what measures can protect us from airborne respiratory illnesses. This recent study looks at how meetings — whether in classrooms, conferences, or care facilities — can transmit infections. Their mathematical model is able to handle many variables — room size, number of people, length of meeting, breaks between sessions, masking, ventilation, and so on. Without prescribing any one policy, the authors aim to inform decision makers so that they can choose what methods (testing, masking, ventilation, etc.) work best for their event.
That said, they find that ventilation and periodic breaks between meetings are highly effective in reducing a room’s viral load. Leaving enough time between sessions for ventilation to clear the room was as effective (or more effective) than masking and moderate isolation of those infected. Tools like these are vital in enabling gatherings that keep participants safe. (Image credit: Product School; research credit: A. Dixit et al.; submitted by Kam-Yung Soh)
June 27, 2023
Predicting Contamination in Urban Environs
The canyons of a city’s streets form a complex flow environment. To better understand the risks of a spreading contaminant, researchers simulated a release in lower Manhattan’s urban jungle. The released particles spread due to the dominant wind pattern of the area. Initially, the particles follow the street pattern and stay at a low elevation. But updrafts on the downwind side of skyscrapers lift the particles higher, spreading them to lower concentrations at more elevations.
Public officials study simulations like these to understand what response is needed to protect people in the event of an accidental or intentional release of harmful materials. (Image and video credit: W. Oaks and A. Khosronejad)
June 26, 2023
“Níłtsą́”
Living in the central and western United States, it’s easy to dismiss summer weather as just another storm, but the truth is that this region sees some of the most majestic and spectacular thunderstorms in the world. And no one captures that grandeur better than storm-chasing photographer Mike Olbinski. His latest film is named for the Navajo word for rain and features over 12 minutes of the best storms from 2021 and 2022. Towering turbulent clouds grow by convection, lightning splits the night sky, and microbursts pour down from above. As always, it’s a stunning depiction of the power of atmospheric fluid dynamics. (Image and video credit: M. Olbinski)
June 23, 2023
Washing By Vortex Ring
Spraying a surface clean with a jet of fluid can be an energy-intensive operation. But a recent experiment shows that pulsed flow — which creates vortex rings — could be a viable cleaning alternative. Here we see vortex rings impacting a porous, beaded surface that’s covered in oil. Vortex rings with lots of rotation actually pass through the beads, knocking oil off both the front and back surfaces (Image 1). Even with a lower rotation rate, a vortex ring can still help clean the upper surface (Image 2). (Image and research credit: S. Jain et al.; via APS Physics)
June 22, 2023
Disease and Placental Flows
The human placenta functions as a life-support system for a growing fetus. Despite its frisbee-like appearance, the organ is packed with nearly 10 square meters of blood vessels. On the fetal side, these blood vessels form villous trees where diffusion across the placental boundary exchanges molecules with the maternal blood that fills the space between villous trees. This setup allows oxygen, glucose, carbon dioxide and other key chemicals to cross between the parent and fetus while (ideally) keeping diseases out.
Views of the placenta. Beige areas show the intervillous space where maternal blood flows while pink areas show villous trees where exchanges between the fetus and mother take place. The first three images show a) preeclamptic, b) normal, and c) diabetic placentas. The final image d) shows a 3D view of placental tissue taken with x-ray tomography.
But when diseases directly affect the structure of the placenta, flow to the fetus gets disrupted. The image above shows cross-sections of placental tissues, with villous trees marked in pink, under (a) preeclamptic, (b) normal, and (c) diabetic conditions. Preeclampsia is associated with reduced density of villous trees, which restricts the amount of nutrients a fetus receives and can lead to reduced growth or stillbirth. In contrast, with gestational diabetes villous trees can proliferate, causing a high resistance to flow that also affects exchanges.
For more on the complex physics of the placenta, check out this article from Physics Today. (Image credit: sketch – L. da Vinci, placentas – A. Clark et al.; see also A. Clark et al.)
June 21, 2023
Sandgrouse Soak in Water
Desert-dwelling sandgrouse resemble pigeons or doves, but they have a very different superpower: males can soak in and hold 25 milliliters of water in their feathers, which they carry tens of kilometers back to their chicks. The key to this ability is the microstructure of the bird’s breast feathers. Unlike other species, where feathers have hooks and grooves that “zip” them together, the sandgrouse’s specialized feathers have tiny barbules with varying bending stresses. When dipped in water, their curled shape unwinds, allowing water to soak in through capillary action. Barbules at the tips curl inward, holding the water in place so that the sandgrouse can fly home with it.
Studying nature’s solutions for water-carrying will help engineers design better materials for human use, whether that’s a water bottle that avoids sloshing or a medical swab that’s better at absorbing and releasing fluids. (Image and video credit: Johns Hopkins; research credit: J. Mueller and L. Gibson; via Forbes; submitted by Kam-Yung Soh)
June 20, 2023
Stirring Up Sediment
In early February, Tropical Cyclone Gabrielle passed over the Bellona Plateau in the Coral Sea, stirring up sediment from the shallow reefs there. Once the storm cleared, large swirls of carbonate sediment mixed into the deeper waters around the plateau. As the sediment sinks to depths of kilometers, it will dissolve into the deep ocean waters, eventually getting captured as part of sedimentary rocks. This is a critical step in the ocean’s carbon capture cycle.
Unfortunately, climate change is disrupting the ocean’s ability to capture carbon. An excess of carbon dioxide acidifies ocean waters, making it harder for creatures like corals and crabs to incorporate carbon into their bodies. That reduces sources for carbonate sediments like those seen here. Changes in ocean chemistry also affect where and how much carbonate can get dissolved. In short, ocean carbon capture has been an important process for Earth’s carbon cycle in the past, but the process is a slow one, and human activity has overloaded the ocean’s system in ways we don’t fully understand. (Image credit: A. Nussbaum; via NASA Earth Observatory)
June 19, 2023
“Eternal Spring”
With every spring comes the thaw. Warming temperatures melt winter’s ice, carving it away to reveal the surfaces beneath. Christopher Dormoy’s macroscale timelapse “Eternal Spring” captures this dynamic, showing the process drop-by-drop and rivulet-by-rivulet. It’s also a commentary on melting in general as human-driven climate change chips away at ice that formed over millennia. (Video and image credit: C. Dormoy)
June 16, 2023
Overcoming Turbulence
Despite their microscopic size, many plankton undertake a daily migration that covers tens of meters in depth. As they journey, they must contend with currents, turbulence, and other flows that could knock them off-course. And, increasingly, research shows that a plankton’s shape makes a big difference in these flows.
Spherical plankton tend to cluster in areas of flow moving opposite to their direction of travel. But more elongated plankton can resist — or even reverse — this tendency, helping them stay on track. In turbulence, elongated swimmers are also better at keeping their thrust oriented in the desired direction of travel. So both nature and engineers should favor elongated microswimmers when contending with turbulence and potential crossflows. (Image credit: Picturepest/Flickr; research credit: R. Bearon and W. Durham)
June 15, 2023
Twisted Fibers
A drop sliding down a fiber can do so asymmetrically or symmetrically. The asymmetric configuration is unstable and will spontaneously shift to a symmetric one. Adding a second, parallel fiber stabilizes an asymmetric drop, letting it slide without shifting. And twisting the two fibers together gives even more control, allowing researchers to tweak drop shape, speed, and orientation independent of properties like the drop’s volume or viscosity. (Image and video credit: V. Kern and A. Carlson)
June 14, 2023