Tag: submission

Deforming Soap Films
It’s the time of year when new Gallery of Fluid Motion videos start popping up online. We’ve already featured several and no doubt there will be more to come. Today’s post is a submission from Saad Bhamla, who gave this introduction to the work:
Soap bubbles occupy the rare position of delighting and fascinating both young children and scientific minds alike. Sir Isaac Newton, Joseph Plateau, Carlo Marangoni and Pierre-Gilles de Gennes, not to mention countless others, have discovered remarkable results in optics, molecular forces and fluid dynamics from investigating this seemingly simple system.
This video is a compilation of curiosity-driven experiments that systematically investigate the surface flows on a rising soap bubble. From childhood experience, we are familiar with the vibrant colors and mesmerizing display of chaotic flows on the surface of a soap bubble. These flows arise due to surface tension gradients, also known as Marangoni flows or instabilities. In this video, we show the surprising effect of layering multiple instabilities on top of each other, highlighting that unexpected new phenomena are still waiting to be discovered, even in the simple soap bubble.
As illustrated in the video, raising a bubble beneath the soap film moves surfactants in the film, which causes local differences in surface tension. Any time a difference in surface tension exists, fluid will flow from areas of low surface tension to ones with higher surface tension. This is called the Marangoni effect. On a soap bubble, this is visible in the chaotic swirling colors we see. In this system, Bhamla and his co-author found that by raising the bubble in steps, they could “freeze” the Marangoni-induced patterns created by the previous motion. (Video credit and submission: S. Bhamla et al.)
October 30, 2015
Wave Tank
A new wave tank facility opening at the University of Edinburgh promises new capabilities to simulate ocean wave behavior. The circular 25m diameter wave tank is lined with 168 wave makers and is equipped with 28 submerged flow-drive units. Together, these allow the tank to simultaneously simulate nearly any wave type as well as tidal currents up to 1.6 m/s. The facility is intended for 1/20th scale modeling; projected to full-size, this means that the tank is capable of making waves representative of 28 m high ocean waves and tidal currents in excess of 12 knots. It’s expected to be particularly valuable in the development and testing of wave and tidal motion generators for clean energy. For more, see BBC News and FloWave’s own website. (Image credit: Brightspace/BBC News; submitted by srikard)
June 6, 2014
Reader Question: Does Flow Viz Alter Flow?
gorbax asks:
I’ve been wondering for a while, actually, how do we know when the method of flow visualization doesn’t actually alter the flow of a fluid itself?
This is a great question and one that fluid dynamicists have to deal with all the time. Ideally, we’d love to measure everything we want from a flow at all points at all times without doing anything to affect it. In reality, however, that just doesn’t happen. Some measurement techniques are less intrusive than others, but just about everything risks having some effect. This raises two questions: 1) How small can we make that effect? and 2) Do we even care if we’re affecting the flow?
With regards to the first, the onus is typically on the experimentalist to show that whatever visualization technique he/she uses is not significantly affecting the flow. For something like particle image velocimetry, which requires seeding the flow with particles, this means selecting particles that follow the flow rather than changing it and considering carefully how and where to seed the flow such that any added vorticity from the injection does not alter the flow significantly. Checking for this can be done many ways, for example with comparisons to other measurement techniques (with and without seeding) or by comparing to simulation.
The second question–do we care?–is also a significant consideration. Because the purpose of flow visualization is often to get a qualitative feel for the flow field rather than quantitative information, it is often not a significant concern if there is some slight effect from the visualization technique. This can often be the case with smoke-wire and dye visualizations where we just want to see what’s going on.
Finally, there are some instances of flow visualization which are completely unobtrusive to the flow. Schlieren photography and infrared thermography are two examples. Both are optical techniques that act from a distance and take advantage of extant flow properties to make certain features visible. The real key is knowing what technique(s) will work for the flow you have and will give you the information you want. After that, it’s all about proper and thorough execution. (Photo credits: N. Vandenberge et al., T. Omer, M. Canals, P. Danehy et al., A. Wilkens et al., W. Saric et al.)
June 11, 2013
Titan’s Vortex
The timelapse animation above shows a swirling vortex above the south pole of Saturn’s moon Titan. It completes a full rotation in about nine hours, significantly quicker than the 16-day rotation of the moon. The vortex appears to demonstrate open cell convection, in which air sinks at the center of the cell and and rises at the edges to form clouds along the cell edges. For the most part the dense haze of Titan’s atmosphere prevents scientists from seeing what goes on beneath the clouds, but Titan is thought to have weather cycles similar to Earth’s, except featuring methane rather than water. (Photo credit: NASA, Cassini; submitted by Adam L)
ETA: This theme sometimes dislikes displaying .GIF images. If you don’t see the animation, click here.
July 17, 2012
Fire Tornado
An artificial fire tornado makes for fascinating viewing. The box fans are positioned around a central firepit such that they impart the angular velocity needed to create a vortex. I’ve actually seen an even bigger live demonstration than this one at a fluid dynamics conference. Do not try this yourself. Fire tornadoes occur in nature, too: take a look at how they form. (submitted by acervant)
July 9, 2012
How Mosquitoes Fly in the Rain
One might think that rainfall would keep the mosquitoes away, but it turns out that rain strikes don’t bother these little pests much. Because the insect is so small and light compared to a falling raindrop, the water bounces off instead of splashing. This results in a relatively small transfer of momentum, although the mosquito does get deflected quite strongly. Researchers estimate that the insects endure accelerations up to 300 times that of gravity, which is more than 10 times what a human can withstand. (Video credit: A. Dickerson et al; submitted by Phillipe M.)
June 12, 2012
Fixing Potholes with Oobleck
Shear-thickening non-Newtonian fluids like oobleck become more viscous as force is applied to them. This behavior causes them to form finger-like structures when vibrated, makes it good liquid armor, and even enables people to run across a pool of it without sinking. Now undergraduates at Case Western Reserve University have found a new use for such fluids: pothole filling. They have created a pothole patch that consists of a waterproof bag filled with a dry solution that, when mixed with water, creates a non-Newtonian fluid capable of flowing to take the shape of the pothole but resisting a car tire like a solid. They cover the patch with a layer of black fabric so that drivers don’t avoid the patch. See the video above for a demonstration and ScienceNOW for more. (submitted by aggieastronaut)
April 16, 2012
Reynolds Stress
[original media no longer available]
From reader jessecaps who hung it on the office door. I expect this joke will make sense to very few but as someone who once dabbled in turbulence, I could not resist.
March 14, 2012