Ocean currents play a major role in the weather and climate of our planet. This video shows a simulation of the surface ocean currents in the Mediterranean and Atlantic over an 11-month period. Each second corresponds to 2.75 days. You’ll see many swirling eddies in the Mediterranean and more flow along the coastlines in the Atlantic. One observation worth noting: near the end of the video, you’ll notice that flow through the Strait of Dover between England and France changes its direction, flowing back and forth depending on tidal forces. In contrast, flow through the Strait of Gibraltar is always into the Mediterranean (within the timescale of the simulation, at least). This net in-flow to the Mediterranean is due in part to the warm waters there evaporating at a higher rate than the cooler Atlantic. (Video credit: NASA; via Flow Viz; h/t to Ralph L)
Tag: fluid dynamics
Bubbles and Films Merging
As we’ve seen before, a water droplet can merge gradually with a pool through a coalescence cascade. It turns out that the coalescence of a soap bubble with a soap film can follow a similar process! Initially, the bubble and film are separated by a thin layer of air. Once that air drains away and the bubble contacts the fluid, it starts to coalesce. But the bubble pinches off before its entire volume merges, leaving behind a daughter bubble with about half the radius of the previous bubble. This process repeats until the bubble is small enough that it merges completely. To see more great high-speed footage of this bubble merger, check out the full video below. (Image/video credit: D. Harris et al.)
A New Cloud
These unusual and spectacular clouds are known as undulatus asperatus. Though they have been proposed as a new type of cloud, they are as yet officially unrecognized. Despite their dramatic appearance, these clouds are not associated with storms. Instead, they’re thought to form in a process similar to mammatus clouds, where wind shear at the cloud level causes undulations to form. This wave-like structure is especially visible in the photo above thanks to a low sun angle illuminating the underside of the clouds. (Image credit: W. Priester; via APOD)
Coastal Upwelling
Cool temperatures and abundant nutrients make the waters off the western coast of North America especially biologically productive. This image is a composite of satellite data highlighting large phytoplankton blooms in the California Current. This current runs southward along the coastline, and, like other eastern boundary currents, it experiences strong upwelling, or rising of colder, nutrient-rich waters from lower depths. The upwelling is driven in part by Earth’s rotation. As the earth spins, Coriolis effects push the California Current out from the coast, allowing deeper waters to rise and fill the void. The cooler water provided by the upwelling is a major factor in the moderated climate along the West Coast. (Image credit: NASA/N.Kuring; via NASA Earth Observatory)
Crown Splash Sealing
A sphere falling into water generates a spectacular crown
splash at the surface. The object’s impact ejects a thin sheet of fluid
that rises vertically. The air pulled down into the cavity by the
sphere’s passage makes the air pressure inside the sheet lower than the
ambient air pressure on the exterior of the sheet. This pressure
difference is part of what draws the crown inward to seal the cavity. As
the splash collapses inward and seals, the liquid sheet starts to
buckle and wrinkle, leaving periodic stripes around the closing neck.
This so-called buckling instability occurs when the radius of the neck
collapses faster than the vertical speed of the splash. For more, see
the research paper or this award-winning video. (Image credit: J. Marston et al., source)
Auroras From Space
NASA has released a jaw-dropping new compilation of Earth’s auroras viewed from the International Space Station. It’s available in up to 4K resolution, and I heartily recommend watching it fullscreen at the highest resolution you can comfortably manage. (To paraphrase: this is ultra high definition – it’s better resolution than real life!) I don’t think I’ve ever seen aurora footage that so clearly showed the fluid behavior of auroras when viewed from space. This flow-like quality is to be expected since the auroras occur due to ionized particles from the solar wind exciting atoms in our upper atmosphere in a magnetohydrodynamic dance that never gets too old to watch. (Video credit: NASA; via Gizmodo)
Boston area FYFDers: I’m giving a talk at Harvard tomorrow afternoon on science communication – Wed. April 20th, 4pm, Maxwell Dworkin, G115.
Pinning a Drop
The shape of a droplet sitting on a surface depends, in part, on its surface tension properties but also on the nanoscale roughness of the surface. Small variations in the height and shape of the surface will change the area a drop contacts as well as the contact angle the edge of the drop makes with the surface. If the contact line between the drop and surface stays the same as a droplet evaporates into the surrounding gas or dissolves into the surrounding liquid, then we say the drop is pinned. A pinned drop’s contact angle will decrease as the drop’s volume decreases. This strains the ability of the nanoscale roughness to keep the drop’s edge pinned. As individual points of contact fail, the drop’s edge may jump inward to a new contact point. This set of discrete jumps between pinned states is called a stick-jump or stick-slip mode. (Image credit: E. Dietrich et al., source; see also: E. Dietrich et al. 2015)
Shock Waves in Flight
This week NASA released two new images of the shock waves surrounding T-38C jets in free flight. They’re the result of NASA’s new adaptations of the schlieren photography technique, which has let scientists visualize shock waves (in the lab, at least) for more than a century. To celebrate, I thought it would be fun to demonstrate some of the data engineers can extract from images like the one above. So I’m going to show you how to calculate how fast this plane was flying!
Shock waves depend a lot on geometry. This is not too surprising, really, since shock waves are nature’s way of quickly turning the air because there’s an object in the way. This leads to a very powerful observation, though: the angle of a shock wave depends on the geometry of the object and the Mach number of the flow. (The Mach number is the ratio of an object’s speed to the local speed of sound, so an object moving at Mach 1 is moving at the speed of sound.)
The reverse observation is also true: if we can measure the angle of a shock wave from a known geometry, then we can calculate the Mach number. Now, I don’t have any special information about the geometry of a T-38, so most of the shock waves in this picture can’t tell me much quantitatively.
But, it turns out, I don’t need to know anything about the geometry of the plane to figure out its Mach number. That’s because that very first shock wave over on the right is coming off a sharp probe mounted over the airplane’s nose. The probe is sharp enough, in fact, that I can treat it as though it’s a tiny point disturbance. That means that rightmost shock wave is a special kind of shock known as a Mach wave, and its geometry depends solely on the Mach number. It’s a pretty simple equation, too:
So, all I have to do is fire up some software like GIMP or ImageJ and estimate the angle of that first shock wave.
I came up with an estimate of about 77 degrees for the shock wave angle, which gives Mach 1.026 for the plane’s speed. Keep in mind that a) I’m using a grainy photo; and b) I have no information about the plane’s orientation relative to the camera. Nevertheless, NASA’s caption reports that this plane was moving at Mach 1.05 in the picture. My quick and dirty estimate is only off by 2%!
Of course, engineers are interested in a lot more than estimating an aircraft’s speed from these photos. With a little more geometry information, they can gather a lot of useful data from these images. One of the goals for the new photography technique is to help study new aircraft designs that generate weaker shock waves and quieter sonic booms. (Original images: NASA)
Fluids Round-up
Time for another look at some of the best fluids content out there. It’s the fluids round-up – with a special focus this week on oceans!
– Ryan Pernofski spent two years filming the ocean in slow motion with his iPhone to make the short film “Slowmocean” seen above. It’s a gorgeous ode to the beauty of breaking waves.
– Oceans with higher salinity than Earth’s could drive global circulation that would make exoplanets more hospitable to life.
– Speaking of alien oceans that could harbor signs of life, there’s discussion afoot of how future missions to icy moons like Europa or Enceladus could collect samples from plumes ejected from beneath the ice.
– Wind and waves make harsh, erosive environments. This photo essay from SFGate shows how greatly the sands of Pacifica shift over time. (submitted by Richard)
Bonuses:
– New research explores how Martian mountains may have been carved out by the wind.
– Ever listened to an orchestra made from ice? You should! Learn about Tim Linhart, who builds and maintains ice instruments. (submitted by ashketchumm)
– MIT has demonstrated a new 3D-printing technique that allows for printing liquid and solid parts simultaneously, allowing would-be creators to rapid-prototype hydraulically-driven robotics.
Even more bonus bonus!
– ICYMI, the new FYFD video made Gizmodo!
If you’re a fan of FYFD, please consider becoming a patron. As a bonus, you’ll get access to this weekend’s planetary science webcast!
(Video credit: R. Pernofski; via Flow Visualization; Pluto image credit: NASA/APL)
The Brazil Nut Effect
The Brazil nut effect is a common name for the phenomenon where large particles tend to rise to the top of a mixture when it’s shaken. It’s also the subject of the latest FYFD video, which you can see above.
I’ve seen other mentions of the topic previously, but when I started researching the literature, I discovered that the Brazil nut effect was much more complicated than I’d thought! Hopefully, you’ll find the results as interesting as I did. And if you want to dig further, there are links to the papers I used over on YouTube.
Filming was also interesting this time around. I tried out stop-motion animation for the first time. It takes so much patience! But I think the results are so cute. (Image and video credit: N. Sharp/FYFD)
