Tag: fluid dynamics

Patterns of Flame
In nature, the way a system behaves often depends on multiple competing factors. This is particularly apparent for chemical reactions, some of of which can oscillate in wild patterns as different forces compete. Similar patterns can occur in combustion, as shown above.
What you see here are patterns formed on a flame propagating down a tube. They’re a result of what’s known as a thermal-diffusive instability. Flames like these typically propagate by conducting heat into the fuel-air mixture ahead of the flame front, thereby raising its temperature, while, simultaneously, fuel and air diffuse into the flame to sustain the chemical reactions. If the rates of heat transfer and chemical diffusion are balanced, the flame moves steadily. But if there’s an imbalance between those factors, instabilities occur.
In this case, the temperature rises much faster than the time needed for fresh fuel to move into the flame. As the temperature goes up, the reaction rate increases exponentially, and the flame surges forward. But the slow resupply of fuel makes the reaction rate drop, causing the flame’s progress to stall. This interplay results in the complex, pulsating instabilities we see here. (Image and submission credit: H. Pearlman; research credit: H. Pearlman and D. Ronney)
February 22, 2019
Even Mountains Flow
Over about 5 months of 2018, the summit of Mount Kilauea slowly collapsed as the volcano erupted. Seen in timelapse, it’s a remarkable reminder of the ancient Greek philosopher Heraclitus’s observation, “Everything flows.” All things change, so given enough time, just about everything can flow.
Fluid dynamicists actually capture this concept in a dimensionless ratio known as the Deborah number. Named for a Biblical prophet who states, “The mountains flow before the Lord,” the Deborah number is defined as the ratio between the time needed for a material to respond applied stress and the time over which the process is observed. In practice, a lower Deborah number indicates a more fluid-like material while a higher one represents more solid-like behavior.
Be sure to check out the full video. There’s some spectacular lava flow footage near the end – definitely a small Deborah number! (Video and image credit: USGS via Science; research credit: C. Neal et al.)
February 21, 2019
The Sharpshooter Insect
The sharpshooter is a small, sap-sucking insect capable of consuming more than 300 times its body weight in fluid each day. To sustain that level of intake, the insect also has to have a robust mechanism for expelling excess fluid, and that particular talent has earned the insect the nickname of the “pissing fly”. Together a group of sharpshooters can expel enough fluid to imitate rain (top).
Individually, the insects form a droplet on hydrophobic hairs near their anus. Once the droplet is large enough, those hairs bend like a spring, and the droplet gets catapulted off the insect with an acceleration greater than 20g. That makes it among the fastest reactions in the natural world – more than twenty times the acceleration of a cheetah. Understanding this mechanism is valuable for engineers building robotics as well as for finding ways to counter the agricultural menace the sharpshooters present when it comes to spreading diseases among infected crops. (Image and video credit: E. Challita et al.; via WashPo; submitted by Marc A.)
February 20, 2019
Ice Cream Vortex
[original media no longer available]
Here’s a fun demonstration of vorticity: sticking an ice cream cone in a bathtub vortex. Now, before someone points out that this is clearly a sink, not a bathtub, the term “bathtub vortex” actually has a standard scientific usage; it’s used to describe a vortex that forms when water drains out a small hole in a larger container.
Vortices like this have a surprisingly complex flow structure. Although there is some flow dragged into the vortex near the surface, flow visualization shows that most of the flow actually occurs along the bottom of the container. Fluid there gets dragged along the surface, then sucked upward near the center of the vortex, and finally gets pulled down the drain.
So what’s going on here? As long as the ice cream cone stays balanced inside the center of the vortex, it spins with the fluid due to viscous drag. When it’s unbalanced – like when it precesses too far or throws a chunk of cone off – I suspect the bottom of the cone is encountering that area of upwelling, which tips the cone completely. The surface flow then pulls it back into the center of the vortex, allowing it to right itself. (Video credit: Cheesemadoodles; research credit: A. Anderson et al.; submitted by randumblrposts and eclecticca)
February 19, 2019
Keeping Bubbles Around
Bubbles don’t stick around in pure water. Surfactants are needed to stabilize the thin liquid film for longer than the blink of an eye. But that’s not necessarily the case for other liquids. As the video below shows, a bubble in isopropyl alcohol is quite stable. This is because of the alcohol’s volatility – its ability to evaporate easily.
As the alcohol in the bubble film evaporates, it cools the film, creating a difference in surface tension that pulls fresh alcohol up into the bubble film. It’s so efficient at pulling alcohol up that the alcohol can’t evaporate fast enough to use it all. Once the excess alcohol is heavy enough, it slides back down the side of the bubble. Overall, though, the process is enough to keep a bubble in pure isopropyl alcohol from rupturing for minutes to hours at a time. (Image and video credit: M. Menesses et al.)
February 18, 2019
Massive Worthington Jet
The FloWave facility in Scotland is one of the coolest ocean simulators out there. Equipped with 168 individual wave makers and 28 submerged flow-drive units, it’s capable of recreating almost any ocean conditions imaginable. So naturally the Slow Mo Guys used it to create a giant spike wave.
Essentially, this is an oversized Worthington jet, the same as the ones you see after a droplet hits the surface. But with several thousand tonnes of crystalline clear water, the effect of that wave focusing is pretty spectacular. When you’re watching the high-speed footage, be sure to pay attention to the details, like the glassy surface of the collapsing jet, or the way holes open and expand as the splash curtain comes down around Dan’s head (above). Longtime readers will recognize many familiar features. (Image and video credit: The Slow Mo Guys)
February 15, 2019
Flow in the Heart
Few flows are more integral to our well-being than blood flow through the heart. Over the course of our lives, our hearts develop from a few cells pushing viscous blood through tiny arteries to the muscular center of a vast circulatory network, capable of powering us through incredible physical feats. What’s most astonishing about all this is that the heart goes through all these changes and adaptations without ever pausing.
Peering into the heart to see it in action is difficult, but researchers today are combining imaging techniques like CT and MRI with computational fluid dynamics to build patient-specific heart models. Not only does this help us understand hearts in general; it’s paving the way toward predicting how a specific treatment may affect a patient. Imagine, for example, being able to simulate and compare different models of an artificial heart valve to see which will work best for a particular patient. We’re not to the point of doing so yet, but it’s a very real possibility in the future.
To see some examples of predicted and measured heart flows, check out this video by J. Lantz. In the meantime, happy Valentine’s Day! (Image credits: Linköping University Cardiovascular Magnetic Resonance Group, video source; via Another Fine Mesh)
February 14, 2019
Forming an Oxbow
Without human intervention, meandering rivers become more sinuous over time. This is driven by the flow around a river bend, which tends to push sediment from the outer bank of the curve to the inner, making the bend more pronounced. Eventually, loops in the river can pinch off and form a separate oxbow lake, as seen in the animation above and video below.
By studying many photo sequences like this one, researchers have concluded that how quickly a river bend meanders depends on its curvature. In general, the higher the curvature, the faster the river bend will migrate. When rivers deviate from this rule of thumb, it’s typically because part of a river bank is tougher to erode than other sections. (Image and video credit: Z. Sylvester/Geolounge; research credit: Z. Sylvester; via Landsat; submitted by Aatish B.)
February 13, 2019
Swallowing Physics
Swallowing – whether of food, beverage, or medication – is an important process for humans, but it’s one many struggle with, especially as they age. To help study the physics behind swallowing, one research group has built an artificial mouth and throat model, shown in the bottom row of images. The model uses rollers to imitate the wave-like motion of swallowing.
In our mouths, chewed food typically combines with saliva to form a soft ball we can move from our tongue and down our throat with a series of reflex actions. How easily we swallow something depends on its flow properties, our saliva, shape, and more.
In their early studies of model swallowing, researchers have focused on what it takes to swallow pills (suspended in liquid). What they found is probably consistent with your own experience: smaller pills are easier to swallow than large ones, and elongated pills are easier to swallow than round ones of the same volume. That seems to be a function of elongated pills’ smaller cross-section when aligned with flow going down the throat. As the research continues, scientists hope to explore what can be done to make food easier to swallow for those who struggle with it. (Image credits: meal – D. Shevtsova; model – M. Marconati; via APS Physics; submitted by Kam-Yung Soh)
February 12, 2019
Swirling Polygons
We don’t usually think of fluids forming corners, but they can. Here you see liquid nitrogen in a simple pot. Since the pot is much hotter than the boiling point of the nitrogen, the liquid nitrogen is floating on a layer of its own vapor. This is called the Leidenfrost effect. That nearly frictionless contact with the pot means that stirring the nitrogen conveniently spins it up into these rotating polygons, visible in high-speed footage. The faster you stir the nitrogen, the more points you get.
Check out the full video below for instructions on how the researchers constructed their set-up. If you try it, though, remember to have plenty of ventilation. When the nitrogen vaporizes, its volume increases dramatically, and if you’re not careful, it will displace too much oxygen and make it hard to breathe. (Image and video credit: A. Duchesne et al., source)
February 11, 2019