Making electronics water-resistant can be a challenge, but as this Slow Mo Guys video demonstrates, engineers have some clever ways to deal with unwanted liquids. The Apple Watch, for example, uses its speakers to eject water that gets into the watch during immersion. As seen above, the vibration of the speakers ejects most of the water as tiny droplets. Occasionally, surface tension makes this tough and drops instead coalesce on the watch’s surface. To counter this tendency, the speakers sometimes pause, allowing water to collect before they begin vibrating again. (Video and image credit: The Slow Mo Guys)
Differences in temperature and salinity create distinct layers within the ocean. When combined with flow over submerged topography — underwater canyons, mountains, and reefs — it makes waves. But those waves aren’t always apparent when sitting at the surface. Instead, they travel along those ocean layers as internal waves that can be as tall as hundreds of meters in height.
When the sun glints just right off the ocean, these massive internal waves can be caught by satellite imagery, as shown in the above image of the Andaman Sea near Thailand and Myanmar. Even seemingly calm waters can roil in the deep. (Image credit: USGS; via NASA Earth Observatory)
We’ve all experienced the frustration of traffic jams that seem to come from nowhere — standstills that occur with no accident, construction, or obstacle in sight. Traffic shares a lot of similarities with fluid flows, including its waves and instabilities.
These disturbances propagate and grow when traffic surpasses a critical density. Once that happens, any small speed adjustment made by a lead driver gets amplified by the larger and larger braking of each driver downstream. Effectively, this creates a wave of slower speed and higher density that travels downstream through the traffic.
Each driver brakes more than the last largely because they can’t tell what the conditions upstream of them are. But that lack of knowledge may be less of an issue for driverless cars, which have the potential to communicate with cars and traffic sensors ahead of them. With enough automated vehicles on the highway, phantom traffic jams may become a thing of the past. (Video and image credit: TED-Ed)
As part of its shakedown, the new Inouye Solar Telescope has captured the surface of the sun in stunning new detail. Seen here are some of the sun’s turbulent convection cells, each about the size of the state of Texas. Hot plasma rises in the center of each cell, cools, and then sinks near the dark edges. Also visible within these dark borders are bright spots thought to mark magnetic fields capable of channeling energy out into the corona. Researchers hope the new telescope will help them uncover the physics behind these processes. (Image and video credit: Inouye Solar Telescope)
Editor’s note: Like several other telescopes located in Hawai’i, the Inouye Solar Telescope was built against the wishes of many native Hawaiians. Although FYFD supports scientific progress, it is my personal belief that scientific advances should not come at the expense of indigenous populations. I strongly urge my scientific colleagues to listen to and work alongside those with concerns about future facilities.
Laminar flow is easy to love, but turbulence is a far richer phenomenon. That’s the premise behind Veritasium’s new video (and, yes, I agree with him). In the video Derek provides a nice introduction to turbulence, including a checklist of qualities a turbulent flow must have.
Personally, I don’t classify flows as simply being either laminar or turbulent; I view those two states as ends of a spectrum, which means there are many flows that fall somewhere in-between. (For more on what happens between laminar and turbulent, check out my video on transition.)
As neat and eye-catching as laminar flow can be, turbulence is critical to life as we know it. It’s a necessary ingredient in cloud and raindrop formation. It drives the mixing of blood in our hearts. It keeps the leaves on trees from overheating. Without it, your coffee would be cold long before your cream mixes in. Turbulence is even critical to star formation; without turbulence, our entire solar system might have lacked the matter and time necessary to form! (Video and image credit: Veritasium)
With spring heading into summer, many parts of the United States enjoy a nighttime chorus of frogs and toads. These amphibians are singing to attract mates and delineate territory. Some, like this American toad, sing from the water, and the vibration of their vocal sac creates ripples that last as long as they’re vocalizing. The toad sings by closing its nostrils and mouth, then forcing air from its lungs over its vocal cords. Those vibrations are amplified by resonance in its vocal sac, generating the high chirp we hear. (Image credit: cassiescisco)
One of my favorite aspects of fluid dynamics is how well it pairs with so many other fields, from mathematics and space exploration to biology, medicine, and even paleontology. That last field is key to today’s question, namely: how did a prehistoric reptile the size of an F-16 manage to fly?
As Joe’s video describes, many factors went into Quetzalcoatlus’ flight. The pterosaur had strong but hollow bones to save on weight while anchoring flight muscles. Its wing shape mimicked an airfoil’s. And, finally, it overcame the challenge of taking off by using both its front and hind limbs to leap off the ground, much like modern bats do.
There’s no doubt that it would be stunning (and probably terrifying!) to see these creatures in action. But you may wonder how scientists piece together these animals from incomplete fossils. Don’t worry! There’s a video for that question, too. (Video and image credit: It’s Okay to Be Smart; see also the video’s references)
Recent changes to the Golden Gate Bridge’s guardrails have created a new soundscape in the Bay Area. Under high winds, the bridge gives off an eerie, otherworldly wail that can be heard even miles away. The new guardrails are substantially thinner than the previous ones, which reduces the wind load the bridge has to endure. But that thinner profile is also what causes the noise, through a well-known phenomena known as vortex shedding.
Animation of vortex shedding behind a cylinder. (Image credit: Wikimedia)
As air moves past a non-streamlined body, like a cylinder, it forms counter-rotating vortices that peel off the body at a set frequency. Fluid dynamicists use a non-dimensional number, the Strouhal number, to characterize this vortex shedding. For a simple shape like a cylinder, the Strouhal number is relatively constant, so I decided to do a quick and dirty calculation to examine the wind velocities responsible for the sound. (See also my analysis of Star Trek Voyager’s opening sequence.)
I began by collecting several videos with samples of the bridge’s singing (1, 2, 3). Then I used Adobe Audition to analyze the frequency content of the bridge noise. Below is a sample snapshot from a video taken on the bridge’s bike path, right next to the guardrail. The analysis shows three broad, but distinct peaks: a primary peak at 430 Hz, a small harmonic of that frequency at 860 Hz, and a separate, secondary peak centered at 1070 Hz. The broadness of the peaks, along with the competition between the primary and secondary peaks, is probably responsible for the disconcerting, discordant nature of the sound.
Frequency analysis of the Golden Gate Bridge’s “singing”, taken from a section of this video. (Image credit: N. Sharp)
Of the other videos I analyzed, a second video from near the bridge also showed the 430 Hz peak, while a video from further away had a dominant frequency of 517 Hz. There’s a lot of uncertainty introduced in not knowing exactly when each video was filmed, but given the agreement between videos 2 and 3, I suspect that video 1’s higher frequency may be caused by interference and modulation as the sound travels.
With the major frequency in hand, I estimated the size of the new guardrail wires as 10mm in diameter. After some tweaking to adjust the Reynolds number and Strouhal numbers, that gave me an estimated wind speed of 21 meters per second, or about 47 miles per hour. That’s right in line with the 43 miles per hour discussed by the news anchors.
What if the guardrails are a little thinner? If the wires are about 7.5 mm in diameter, then it only takes winds at about 15 meters per second (34 miles per hour) to create that 430 Hz note.
Keep in mind that this analysis doesn’t predict the minimum wind speed needed to create the audible noise; all I’m able to do is a back-of-the-envelope calculation of what the likely wind speed was when a video was recorded. Nevertheless, I hope you’ll find it interesting! (Video credit: KPIX CBS News; image credits: vortex shedding – Wikimedia, frequency analysis – N. Sharp; submitted by Christina T.)
Lens-shaped lenticular clouds are not terribly rare in mountainous areas, but observers at Mount Washington caught a very unusual cloud near sunrise in late February. This lenticular cloud had an added curl on top thanks to the Kelvin-Helmholtz instability!
Lenticular clouds form when air is forced to flow up over a mountain in such a way that its temperature and pressure drop and water vapor in the air condenses. The resulting water droplets form a cloud that appears stationary over the mountain, even though the air continues to flow.
To get that added wave-like curl, there needs to be another, faster-moving layer of air just above the cloud. As that air flows past, it shears the cloud layer, causing the interface to curl. Neither of these cloud types is long-lived — Kelvin-Helmholtz formations often last only a few minutes — so catching such a great dual example is lucky, indeed! (Image credit: Mount Washington Observatory; via Smithsonian Magazine; submitted by Kam-Yung Soh)
The Glen Canyon Dam lies on the Colorado River, upstream of the Grand Canyon. Because the dam blocks sediment from upstream, the region’s only sediment sources are two tributary rivers downstream of the dam. Periodically, the Bureau of Reclamation releases high flows from the dam in order to mimic the seasonal floods that existed on the river before the dam was built. These surge flows pick up hundreds of thousands of tonnes of sediment from the tributary rivers and push it downstream, creating and renewing sand bars and beaches along the Colorado. (Video and image credits: Bureau of Reclamation, 1, 2)
