Tag: physics

Melting
File this one under “Oddly Satisfying” – this timelapse video shows the process of melting a jawbreaker candy using a blowtorch. Over a minute and a half, each colorful layer of candy melts away to reveal the strata beneath. There’s a definite connection here to some of the previous research we’ve discussed on erosion, dissolution, and melting. The blowtorch’s flame generates a hot boundary layer around the candy surface; it’s thickest and hottest at the central stagnation point, but judging by the melting layer we see running all the way to the candy’s shoulder, its size and effect are substantial even there. It’s hard to tell from the video whether the surface of candy is getting roughened (a la scalloping) or whether that’s just an uneven layer of melted candy flow. Regardless, it’s a fun watch. (Video and image credit: Let’s Melt This; via Colossal)
March 4, 2019
Noisy Jets
One major problem that has plagued supersonic aircraft is their noise. The Concorde – thus far the only supersonic commercial airliner – was plagued with noise complaints that ultimately restricted its usability. Noise reduction is a major area of inquiry in aerospace, and the video below shows one experiment trying to understand the connections between supersonic flow and noise.
Above you see a supersonic, Mach 1.5 microjet emanating from a nozzle at the top of the image. The jet is hitting a flat plate at the bottom of the image. Just beyond nozzle’s exit, you can see the X-shape of shock waves inside the jet. The position of that X is oscillating up and down.
In the background, you can see horizontal light and dark lines traveling up and down. Those horizontal lines in the background are acoustic waves. When they hit the bottom plate, they reflect and travel upward until they hit another surface (outside the picture) and reflect back down. As they travel, they interact with the jet, causing those X-shaped shock waves to move up and down. This coupling between flow and acoustic waves makes the jet much louder – up to 140 dB – than it would be otherwise.
Researchers hope that unraveling the physics of simpler systems like this one will help them quiet more complicated aircraft. (Image and video credit: F. Zigunov et al.)
March 1, 2019
Landslide Lubrication
In 2008, an 8.2 magnitude earthquake in China caused the enormous Daguangbao landslide, which loosed over one cubic kilometer of rocks and debris. That material rushed down the mountainside, running more than 4 kilometers before coming to a stop. A new study uses field measurements and laboratory experiments to explain how the landslide could run so far from its source.
The researchers found that friction between the sliding material and the stable rock heated that layer to over 850 degrees Celsius, hot enough to start decomposing the dolomite in the fall. That vaporized carbon dioxide out of the rock, which helped lower the friction. Simultaneously, the high temperatures and high pressures within in the landslide caused recrystallization in the falling rocks; this created a viscous layer that helped lubricate the slide. The team estimated that the two mechanisms working in tandem enabled the landslide to reach an estimated 60 m/s. (Image and research credit: W. Hu et al.; via Nature; submitted by Kam-Yung Soh)
February 28, 2019
“The World Below”
Since the first cosmonauts and astronauts entered orbit around our planet, they’ve held a unique perspective. Thanks to the timelapse photography of recent astronauts aboard the ISS and the editing skills of photographer Bruce W. Berry, Jr, the rest of us can enjoy a taste of that viewpoint. Turn up the volume, fire up the big screen, and enjoy.
I particularly like how several of the sequences show off the depth of the atmosphere. Earth’s atmosphere is incredibly thin compared to the size of our planet – less than one percent of Earth’s radius – but thanks to the shadows that clouds cast on one another, you can really appreciate their height in sequences like the one at 2:26. (Video credit: B. Berry, Jr. using NASA footage)
February 27, 2019
Bats in Ground Effect
As pilots can tell you, flying near the ground (or an open expanse of water) gives one an aerodynamic boost. Essentially, the surface acts like a mirror, reflecting and dissipating the wingtip vortices that create downwash. That reduces the power necessary to fly, as long as you’re flying within about a wingspan of the surface.
Theoretically, flapping fliers like bats and birds should also benefit from this ground effect, but measurements have been hard to come by. A new study using bats trained to fly in a wind tunnel provides some of the first detailed measurements of ground effect for flapping animals. The researchers found a 29% reduction in the power necessary for flight when in ground effect compared to being out of it! That’s twice the savings predicted by modeling, meaning we still have a ways to go to accurately capture the physics of flapping flight under these circumstances.
Such a substantial savings also strengthens arguments for flight developing from the ground up. Using ground effect, surface-dwelling animals could have evolved flight gradually, taking advantage of the energy savings offered by sticking close to the surface. (Image and research credit: L. Johansson et al.; submitted by Marc A.)
February 26, 2019
Inside a Wind Tunnel
When I was in graduate school, I worked in a facility known as the High-Speed Wind Tunnel Lab. We were located next door to the Low-Speed Wind Tunnel, and every few months we’d receive a phone call asking whether we could film someone in the high-speed wind tunnel. This was impossible for several reasons – the size of human beings and the necessity of drawing the hypersonic tunnels down to vacuum-like pressures before initiating flow being only two of them – but what it really did was highlight the difference in definitions.
What these (usually) weather forecasters wanted was to simulate hurricane force winds on a human being. And to an aerodynamicist, that hundred mile-an-hour flow is still low-speed. Because we’re comparing it to the speed of sound, not the normal range of wind speeds a human experiences. That said, watching humans struggle inside a wind tunnel is always entertaining.
As you can see from the Slow Mo Guys here, counteracting the lift and drag forces from these wind speeds is tough. On the bottom left, Dan has managed to balance his weight and the drag forces to hold himself in a virtual chair. Meanwhile, Gav’s attempt to jump forward against the wind just pushes him backward as his lab coat parachutes behind him. (Image and video credit: The Slow Mo Guys)
February 25, 2019
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