Pull a bit of a long chain out of a container, and you’ll quickly find the beads take on a life of their own, siphoning out of the jar while leaping and looping in the air. Some of the dynamics are clear – the ever-growing free end of the chain has weight enough to pull the rest of the chain out, much like the pressure difference that drives a siphon. But a lot of the rest of the dynamics are unclear and have generated a lot of discussion. It turns out that the same behavior is observed for chain laid out on a horizontal surface (video links on the right of that page) and even the dynamics of that simpler version of the problem are complex. All part of the beauty inherent in Newton’s second law. (Video credit: Steve Mould/Earth Unplugged; Research credit: J. A. Hanna et al.; submitted by Elin R)
Search results for: “art”
Studying Coughs
Bioaerosols–tiny airborne fluid droplets generated by coughing or sneezing–are a major concern for the spread of contagions like influenza. It may be possible, however, to mitigate some of these effects by manipulating biological fluid properties. The video above shows an experimental model of a cough, complete with the generation of bioaerosols from some fake human lung mucus. Contrast this with a cough where the model’s mucus has been treated to increase its viscoelasticity. The treated mucus generates substantially fewer droplets during a cough. The results suggest that drugs that increase viscoselasticity of biofluids may help stem the spread of disease. (Video credit: K. Argue et al.; research credit: M. D. A. Hasan et al.)
Reader Question: Non-Coalescing Droplets
Reader ancientavian asks:
I’ve often noticed that, when water splashes (especially as with raindrops or other forms of spray), often it appears that small droplets of water skitter off on top of the larger surface before rejoining the main body. Is this an actual phenomenon, or an optical illusion? What causes it?
That’s a great observation, and it’s a real-world example of some of the physics we’ve talked about before. When a drop hits a pool, it rebounds in a little pillar called a Worthington jet and often ejects a smaller droplet. This droplet, thanks to its lower inertia, can bounce off the surface. If we slow things way down and look closely at that drop, we’ll see that it can even sit briefly on the surface before all the air beneath it drains away and it coalesces with the pool below. But that kind of coalescence cascade typically happens in microseconds, far too fast for the human eye.
But it is possible outside the lab to find instances where this effect lasts long enough for the eye to catch. Take a look at this video. Here Destin of Smarter Every Day captures some great footage of water droplets skittering across a pool. They last long enough to be visible to the naked eye. What’s happening here is the same as the situation we described before, except that the water surface is essentially vibrating! The impacts of all the multitude of droplets create ripples that undulate the water’s surface continuously. As a result, air gets injected beneath the droplets and they skate along above the surface for longer than they would if the water were still. (Video credit: SuperSloMoVideos)
Glinting Off Waves
Sunglint on the ocean surface can sometimes reveal different patterns in wave conditions. In the satellite photo above, we see the Canary Islands with wavering silvery wakes stretching to the southwest. The predominant wind direction over the islands is from the northeast. The rocky islands act as a wind-break, redirecting the flow and shadowing the ocean in their wake from much of it. As a result, fewer waves are stirred up in the islands’ wakes, thereby changing the local surface reflection properties and making this image possible. (Photo credit: NASA Earth Observatory)
Fluids Round-up – 23 June 2013
Time for another round-up! Here are the recent fluidsy links I’ve collected:
- A new study on Mars suggests that dry ice may be forming gullies in dunes in a fashion akin to the Leidenfrost effect. Personally, I’m reminded of Death Valley’s roaming rocks.
- A recent episode of It’s Okay to Be Smart explains what wind is.
- xkcd’s What If blog explores what would happen if you row a boat on different fluids such as mercury, bromine, and liquid helium (for you superfluid aficionados).
- Those who love microfluidics may want to follow Physics in Drops for some small-scale fluid fun.
- Not explicitly fluid dynamical, but this video of a peregrine falcon chasing a downhill mountain biker has some great examples of aerodynamics and the in-flight agility of birds.
- For the Android users among you, be sure to check out Fleya, a multi-touch, real-time fluids simulator. (via Jeremy M/Flow Visualization)
(Photo credit: Fixed Point Code)
Simulating a Curveball
Spinning an object in motion through a fluid produces a lift force perpendicular to the spin axis. Known as the Magnus effect, this physics is behind the non-intuitive behavior of football’s corner kick, volleyball’s spike, golf’s slice, and baseball’s curveball. The simulation above shows a curveball during flight, with pressure distributions across the ball’s surface shown with colors. Red corresponds to high pressure and blue to low pressure. Because the ball is spinning forward, pressure forces are unequal between the top and bottom of the ball, with the bottom part of the baseball experiencing lower pressure. As with a wing in flight, this pressure difference between surfaces creates a force – for the curveball, downward. (Video credit: Tetra Research)
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.)
Visualization Via Temperature
One downside to many flow visualization techniques, like those using dye, smoke, or particles, is the difficulty of dealing with their aftermath. You can only introduce so much of them into a wind or water tunnel before it’s necessary to shutdown and clean everything. One alternative is to use temperature, as shown in the video above. By simply introducing a warmer fluid and using an IR camera, it’s possible to accomplish many of the same effects without the mess. (Video credit: A. Khandekar and J. Jacob; submitted by J. Jacob)
Visualizing F-18 Flow
Flow visualization techniques are helpful outside of wind and water tunnels, too. The photo above comes from the F-18 High Alpha Research Vehicle (HARV) program in which techniques like smoke and dye visualization were used in-flight to visualize airflow around an F-18 at large angles of attack. During flight a glycol-based liquid dye was released from tiny holes along the plane’s forebody, creating the pattern seen here later on the ground. This particular test corresponded to about 26 degrees angle of attack. (Photo credit: NASA Dryden)
“Levitating Water”
Al Seckel, a cognitive neuroscientist and expert on illusions, created this “Levitating Water” installation, in which multiple streams of water appear as a series of levitating droplets thanks to a strobing light. The well-timed strobe lighting tricks the brain into seeing many different falling droplets as the same, nearly stationary droplet. The effect is similar to the one created by vibrating a stream of falling water. (Video credit: wunhanglo)
