#2016gofm

Editor’s note: Starting tomorrow I’ll be on a trip that takes me out of range of the Internet until next week. Regular posts are queued up and should post as usual, but we’ll all have to trust Tumblr to handle everything because I won’t be able to check. Thanks!

A bubble rising through water can get caught on an aerophilic (air-attracting) fiber. The bubble will then adhere to the fiber and be guided to the surface by it. In the poster above, the image is a composite photo of such a bubble every 40 milliseconds. Once captured by the fiber, the bubble first accelerates and then reaches a terminal velocity, indicated by the equal spacing of the bubble photos toward the right end of the picture. The terminal velocity strikes a balance between buoyancy, which pulls the bubble upward, and skin friction between the bubble and the water, which acts like drag on the bubble. At the terminal velocity, these forces are equal; neither is able to speed up or slow down the bubble. (Image credit: H. de Maleprade et al.)

When a water droplet hits a frozen surface, what happens depends significantly on the temperature of the substrate. At relatively high temperatures (-20 degrees C), the droplet freezes without any cracking (upper left). As the surface gets colder, drops begin to crack. At first the cracks are relatively large and unstructured (upper right), but at lower temperatures, they grow in a network of smaller cracks with more distinctive structure (lower left). Cold temperatures can also affect the contact line where water, air, and substrate meet. This can cause droplets to splash even as they’re freezing (lower right). (Image credit: V. Thievenaz et al.; see also E. Ghabache et al.)

One of the challenges in engineering and operating wind turbines is that full-scale turbines rarely behave as predicted in smaller-scale laboratory experiments and simulations. One way to reconcile these differences (and discover what our experiments and simulations are missing) is to take the experiments out into the field. One research group has done this by using snowfall to visualize the flow around wind turbines. In this video, they share some of their observations, which include interactions of tip vortices with one another and with the vortex from the tower. My favorite part starts around 1:50 where you can observe tip vortices leap-frogging one another behind the wind turbine! (Video credit: Y. Liu et al.)

We wrote earlier this year about the discovery of a new type of fire whirl -- the blue whirl -- but now the authors have published video of the blue whirl in action! The blue whirl was discovered while investigating the use of fire whirls to more efficiently burn off oil spilled atop water. A tightly spinning yellow fire whirl produces less soot than a non-vortex burn; the blue whirl is even more efficient, producing little to no soot at all. Much remains to be learned about this new type of fire vortex, but in the meantime, enjoy some high-speed video of the blue whirl, particularly from 1:50 onward. (Video credit: M. Gollner et al.)

A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s -- it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video.  (Image credit and submission: A. Sauret et al., source)

This short film for the 2016 Gallery of Fluid Motion features Montana State University students experiencing fluid dynamics in the classroom and in their daily lives. As in her previous film (which we deconstructed), Shanon Reckinger aims to illustrate some of our everyday interactions with fluids. This time identifying individual phenomena is left as an exercise for the viewer, but there are hints hidden in the classroom scenes. How many can you catch? I’ve labeled some of the ones I noticed in the tags. (Video credit: S. Reckinger et al.)