Convection is a major driver in many flows in nature. In this film, the UCLA Spinlab demonstrates buoyant convection caused by a local heat source. They deposit dye on a submerged, continuously heated plate, then observe as the dye slowly rises with the heated (lower density) fluid. The surface forms a cap for the rising dye, which then spreads horizontally. Qualitatively similar flows can be seen in nature over volcanic eruptions or in thunderstorms when clouds reach the troposphere or a capping inversion. Be sure to check out the rest of the Spinlab’s videos. (Video credit: UCLA Spinlab; submitted by Jon B.)
Tag: physics
Jumps in Stratified Flows
One of the factors that complicates geophysical flows is that both the atmosphere and the ocean are stratified fluids with many stacked layers of differing densities. These variations in density can generate instabilities, trap rising or sinking fluids, and transmit waves. The animations above show flow over two ridges with dye visualization (top), velocity (middle), and contours of density (bottom). The upstream influence of the left ridge creates a smooth, focused flow that quickly becomes turbulent after the crest. The jet rebounds as a turbulent hydraulic jump before slowing again upstream of the second ridge. Like the first ridge, the second ridge also generates a hydraulic jump on the lee side. Clearly both stratification and the local topography play a big role in how air moves over and between the ridges. If prevailing winds favor these kinds of flows, it can help generate local microclimates. (Image credit and submission: K. Winters, source videos)
Rain-spread Pathogens
Like humans, plants can spread pathogens to one another. Although scientists had observed correlations between rainfall and the spread of diseases among plants, this study is one of the first to look at the fluid dynamics of leaf and rainfall interaction. When a raindrop hits a leaf, it doesn’t simply splash as it would against an immobile surface. The impact of the drop deforms the leaf, and the plant’s rebound significantly affects the trajectory and size of the resulting droplets. Depending on factors like the leaf’s stiffness, a large drop, carrying many pathogens, may rebound and splatter onto a neighboring leaf. Other leaves tend to catapult out many smaller droplets, which may fly farther afield but carry fewer pathogens. For more, check out the press release or the original research paper. (Video credit: Massachusetts Institute of Technology; research credit: Bourouiba Research Group)
Kitchen Fluid Dynamics
The kitchen is a fantastic place to witness the everyday beauty of fluid dynamics. Daria Khoroshavina and Olga Kolesnikova capture these delectable cooking-related GIFs on their Buttery Planet Tumblr. From pouring cream to drizzling syrup, there are countless examples of fluids in daily life. Check out their site for more awesome images and be sure to keep your eyes open for great examples of fluid behavior in your day-to-day life. (Image credits: Buttery Planet; via Colossal)
Get Your Own Space Coffee Cup
A few weeks ago, we reported on the espresso machine NASA and the ESA sent to astronauts aboard ISS. The Capillary Beverage Experiment, known colloquially as the “Space Coffee Cup”, is an accompanying project that aims to use our understanding of fluid behavior in microgravity to design an open cup that simulates earthbound drinking experiences. As you can see above, astronauts are already enjoying drinks with it. The cup’s special shape is optimized so that surface tension can replace the role gravity plays in drinking on Earth. Where we pour drinks on Earth, the cup wicks liquid to the spout using surface-tension-driven capillary action. Right now there are only a handful of 3D printed cups on-orbit and here on Earth, but the company that designed them wants to manufacture glass versions for use here on the ground. So if you’d like your own space coffee cup, be sure to check out their Kickstarter campaign! (Video credit: IRPI LLC; image credit: NASA/IRPI LLC; Kickstarter project link)
ETA: To those who have been asking, that’s European astronaut Samantha Cristoforetti, who is (clearly) a Star Trek fan. I believe she’s doing a tribute to Captain Janeway’s coffee. (Black.)
Coalescence Cascade
The simple coalescence of a drop with a pool is more complicated than the human eye can capture. Fortunately, we have high-speed cameras. Here a droplet coalesces by what is known as the coalescence cascade. Because it has been dropped with very little momentum, the droplet will initially bounce, then seem to settle like a bead on the surface. A tiny film of air separates the drop and the pool at this point. When that air drains away, the drop contacts the pool and part–but not all!–of it coalesces. Surface tension snaps the remainder into a smaller droplet which follows the same pattern: bounce, settle, drain, partially coalesce. This continues until the remaining droplet is so small that it can be coalesced completely. (Image credit: Laboratory of Porous Media and Thermophysical Properties, source video)
Re-lighting a Candle
When you blow out a candle, you can re-light the wick using the smoke trail left behind. This is a topic we’ve discussed before, but I’m thrilled to finally see the process in true high-speed, thanks to the Slow Mo Guys. The plume that rises from the extinguished candle is an atomized mixture of fuel (wax) and air. When you bring a new combustion source–the match–close enough, that mixture ignites and the flame spreads downward back to the wick. (Video credit: The Slow Mo Guys)
3D Printing Fluids
Most flows vary in three spatial dimensions and time. In experimental fluid dynamics, the challenge is measuring as much of this information as possible. For those who use computational fluid dynamics to study flows, their simulations provide massive amounts of data and the challenge comes in visualizing and processing that data in a useful way. Unless you can find and analyze the important aspects of the simulation results, they’re just a bunch of numbers. As computers have advanced, the size and complexity of simulation results has increased, too, making the task even more difficult. Using technologies like virtual reality projections (above) or 3D printing (below) allow researchers to interact with flow information in completely new but intuitive ways, hopefully leading to new insights into the data.
(Video credit: M. Stock; photo credit: K. Taira et al.)
** The 3D-printed vortices are an image I took of a poster at the APS DFD Gallery of Fluid Motion in 2013, but I’m missing the researchers’ names. If you know whose poster these were from, please let me know (fyfluids [at] gmail [dot] com) so that I can update the credits accordingly. Thanks to Shervin for helping me find the right lab to credit!
Laser-Induced Fluorescence
One of the challenges of experimental fluid dynamics is capturing information about a flow that varies in three spatial dimensions and time. Experimentalists have developed many techniques over the years–some qualitative and some quantitative–all of which can only capture a small portion of the flow. The photos above are a series of laser-induced fluorescence (LIF) images of an airfoil at increasing angles of attack. The green swirls are from an added chemical that fluoresces after being excited with a laser. In this case, the technique is providing flow visualization, showing how flow over the upper surface of the airfoil shifts and separates as the angle of attack increases. The technique can also be used, however, to measure velocity, temperature, and chemical concentration. (Image credit: S. Wang et al.)
How Loud Can Sound Get?
Sound and acoustics often intersect with fluid dynamics. Most of the sounds we experience are pressure waves traveling through air. In this video, Joe of It’s Okay To Be Smart takes a closer look at sound: what it is; how we measure it; and just how loud a sound can get. For air at sea level, the loudest possible sound is 194 dB. Add any more energy and it distorts the pressure wave from what we recognize as sound into what’s known as a shock wave. (Video credit: It’s Okay To Be Smart/PBS Digital Studios)
