When two jets of a viscous liquid collide, they can form a chain-like stream or even a fishbone pattern, depending on the flow rate. This video demonstrates the menagerie of shapes that form not only with changing flow rates but by changing how the jets collide – from a glancing impingement to direct collision. When just touching, the viscous jets generate long threads of fluid that tear off and form tiny satellite droplets. At low flow rates, continuing to bring the jets closer causes them to twist around one another, releasing a series of pinched-off droplets. At higher flow rates, bringing the jets closer to each other creates a thin webbing of fluid between the jets that ultimately becomes a full fishbone pattern when the jets fully collide. The surface-tension-driven Plateau-Rayleigh instability helps drive the pinch-off and break-up into droplets. (Video credit: B. Keshavarz and G. McKinley)
Tag: APS DFD 2013
Measuring Wind Turbines with Snowfall
One of the challenges in large-scale wind energy is that operating wind turbines do not behave exactly as predicted by simulation or wind tunnel experiments. To determine where our models and small-scale experiments are lacking, it’s useful to make measurements using a full-scale working turbine, but making quantitative measurements in such a large-scale, uncontrolled environment is very difficult. Here researchers have used natural snowfall as seeding particles for flow visualization. The regular gaps in the flow are vortices shed from the tip of the passing turbine blades. With a searchlight illuminating a 36 m x 36 m slice of the flow behind a wind turbine, the engineers performed particle image velocimetry, obtaining velocity measurements in that region that could then be correlated to the wind turbine’s power output. Such in situ measurements will help researchers improve wind turbine performance. (Video credit: J. Hong et al.)
APS DFD etc.
It’s time! The American Physical Society’s Division of Fluid Dynamics meeting opens in Pittsburgh tomorrow morning. It promises to be a very busy few days. Most of that activity will probably not be immediate apparent here on FYFD, but I encourage you to follow along on @fyfluiddynamics, where I’ll be giving a running commentary.
For those attending APS, I have two talks:
- “F*** Yeah Fluid Dynamics: Lessons in online outreach” – Sunday, 5:37pm, Rm 306/307
- “Discrete surface roughness effects on a blunt hypersonic cone in a quiet tunnel” – Tuesday, 1:31pm, Rm 326
I’m looking forward to the chance to meet in person as well, so keep an eye out (I’ll have FYFD stickers on my nametag) and be sure to say hello! There’s been some interest in an informal FYFD get-together, too, so keep an eye on Twitter for that.
Finally, I want to extend my thanks again to all the donors who made it possible for me to get to APS this year. I deeply appreciate your generosity. Special thanks are due to Pointwise, Symscape, and @cenyree for their outstanding support of FYFD! Thank you all again. – Nicole
Avoiding Splashback
Here’s a likely Ig Nobel Prize candidate from the BYU SplashLab: a study of splashing caused by a stream of fluid entering a horizontal body of water or hitting a solid vertical surface. In other words, urinal dynamics. The researchers simulated this activity using a stream of water released from a given height and angle and observed the resulting splash with high-speed video. They found a stream falls only 15-20 centimeters before the Plateau-Rayleigh instability breaks it into a series of droplets, and that this is the worst-case scenario for splash-back. The video above shows how a stream of droplets hits the pool, creating a complex cavity driven deeper with each droplet impact. Not only does each impact create a splash, the cavity’s collapse does as well. Similarly, when it comes to solid surfaces, they found that a continuous stream splashes less. They’ve also put together a helpful primer on the best ways to avoid splash-back. (Video credit: R. Hurd and T. Truscott; submitted by Ian N., bewuethr, John C. and possibly others)
For readers attending the APS DFD meeting, you can catch their talk, “Urinal Dynamics,” Sunday afternoon in Session E9 before you come to E18 for my FYFD talk.
The Cheerios Effect and Tiny Swimmers
Anyone who has eaten a bowl of Cheerios is familiar with the way solid objects floating on a liquid surface will congregate. This is a form of capillary force driven by the wetting of the particles, surface tension, and buoyancy. Using ferromagnetic particles and a vertical magnetic field, one can balance capillary action and lock the particles into a fixed configuration relative to one another. By adding a second, oscillating magnetic field, it’s possible to make the beads dance and swim together. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: M. Hubert et al.)
Self-Propelled Droplets
Leidenfrost drops hover and move above hot surfaces on a thin layer of their own vapor. Over a flat surface, this vapor flows radially out from under the droplet, but creating rachets in the surface forces the vapor to flow in a single direction. The vapor then acts like exhaust, generating propulsion in the droplet and making it roll. How quickly the drop moves depends both on the droplet’s size and the rachets’ aspect ratio. For a given length, deeper rachets propel a drop faster than their shallower counterparts. The droplet’s size also affects the thrust with different scalings depending on the drop’s initial size. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: A. G. Marin et al.)
Shaping and Levitating Droplets
Opposing ultrasonic speakers can be used to trap and levitate droplets against gravity using acoustic pressure. Changes to field strength can do things like bring separate objects together or flatten droplets. The squished shape of the droplet is the result of a balance between acoustic pressure trying to flatten the drop and surface tension, which tries to pull the drop into a sphere. If the acoustic field strength changes with a frequency that is a harmonic of the drop’s resonant frequency, the drop will oscillate in a star-like shape dependent on the harmonic. The video above demonstrates this for many harmonic frequencies. It also shows how alterations to the drop’s surface tension (by adding water at 2:19) can trigger the instability. Finally, if the field strength is increased even further, the drop’s behavior becomes chaotic as the acoustic pressure overwhelms surface tension’s ability to hold the drop together. Like all of this week’s videos, this video is a submission to the 2103 Gallery of Fluid Motion. (Video credit: W. Ran and S. Fredericks)
Fluid Juggling
It’s that time of the year – the 2013 APS Division of Fluid Dynamics meeting is not far off, and entries to this year’s Gallery of Fluid Motion are starting to appear. This week we’ll be taking a look at some of the early video submissions, beginning with one that you can recreate at home. This video demonstrates a neat interaction between a slightly-inclined liquid jet and a lightweight ball. The jet can stably support–or, as the authors suggest, juggle–the ball under many circumstances, as seen in the video. Initially, the jet impacts near the bottom of the ball and then spreads into a thin film over the surface. This decrease in thickness between the jet and the film is accompanied by an increase in speed due to conservation of mass. That velocity increase in the film corresponds to a pressure decrease because of Bernoulli’s principle. This means that there is a region of higher pressure where the jet impacts the ball and lower pressure where the film flows around the ball. Just as with airflow over an airfoil, this generates a lift force that holds the ball aloft. (Video credit: E. Soto and R. Zenit)
Thank You!
I have the best readers in the world. Seriously, everyone one of you is amazing. In less than 23 hours, you have blown past the goal I set. I will be going to the APS Division of Fluid Dynamics meeting thanks to you. THANK YOU!
For those of you reading who will be at APS, I plan to do my utmost to be available to grab a coffee between sessions, hang out, discuss research, talk outreach, go out to dinner – whatever! For those of you who won’t be there, I want to share as much of the experience as possible with you through social media. Prepare to be inundated at the end of the November. Without all of you, I wouldn’t be at APS, and I’d like everyone who contributed to have a chance to enjoy the experience.
Per IndieGoGo’s terms, the campaign will remain open until its October 11th deadline. Any contributions I receive above and beyond my APS costs, I plan to set aside for improvements to FYFD. The reader survey indicated lots of you would like me to make my own videos, and I aim to. Extra funds will first go toward equipment for that purpose.
Thank you again to each and every one of you, whether you contributed your money or helped spread the word. I appreciate everything you’ve done for me and will continue striving to bring the best of fluid dynamics to FYFD every weekday. Thank you all!
Help FYFD Get to APS DFD 2013
Readers, I need your help! Funding for my project got cancelled prematurely thanks to sequester-induced budget cuts and my research group no longer has the funds to send me to the American Physical Society’s Division of Fluid Dynamics meeting where I am scheduled to give two talks, one about FYFD and one about my research. APS’s DFD meeting is the big fluid dynamics conference of year, where thousands of researchers, professionals, and students come together to present their work. It’s always a major source of beautiful, interesting, and exciting photos and videos for FYFD. I’m asking you to help me raise the $2000 I need to attend. Watch the video, check out the perks available for donors over at IndieGoGo, and please help me spread the word by reblogging, retweeting, etc. Thank you!
