Fluorescent oil sprayed onto a model in the NASA Langley 14 by 22-Foot Subsonic Wind Tunnel glows under ultraviolet light. Airflow over the model pulls the initially even coat of oil into patterns dependent on the air’s path. The air accelerates around the curved leading edge of the model, curling up into a strong lifting vortex similar to that seen on a delta wing. At the joint where the wings separate from the body those lifting vortices appear to form strong recirculation zones, as evidenced by the spiral patterns in the oil. Dark patches, like those downstream of the engines could be caused by an uneven application of oil or by areas of turbulent flow, which has larger shear stress at the wall than laminar flow and thus applies more force to move the oil away. Be sure to check out NASA’s page for high-resolution versions of the photo. (Photo credit: NASA Langley/Preston Martin; via PopSci)
Month: September 2013
Shock Trains
In compressible flows, shock waves are singularities, a tiny distance across which the density, temperature, and pressure of a fluid change suddenly and discontinuously. In this video, there is a wedge at the top and bottom of the frame and a Pitot probe roughly in the center. Flow is left to right and is initially subsonic. Once Mach 6 flow is established in the wind tunnel, a series of shock waves and expansion fans appear as light and dark lines in this schlieren video. Oblique shocks extend from the sharp tip of each wedge and interfere to create a normal shock in front of the Pitot probe. The air that passes through the normal shock is subsonic to the right of the shock, whereas air that goes through the oblique shocks remains supersonic. The fainter lines further to the right are weaker shock waves and expansion fans that reflect off the walls and probe. They exist to continue turning the airflow around the probe and to equalize conditions between different regions. (Video credit: C. Mai et al.)
Beach Cusps
Beach cusps are arc-like patterns of sediment that appear on shorelines around the world. Cusps consist of horns, made up of coarse materials, connected by a curved embayment that contains finer particles. They are regular and periodic in their spacing and usually only a few meters across. A couple of theories exist as to how cusps form, but once they do, they are self-sustaining. When an incoming wave hits a horn, the water splits and diverts. The impact of the wave on the horn slows the water, causing it to deposit heavy, coarse particles on the horns while finer sediment gets carried up to the embayment before the wave flows back outward. (Photo credit: L. Tella; inspired by E. Wiebe)
The Reynolds Number Illustrated
The dimensionless Reynolds number is a key concept in fluid dynamics, allowing scientists to distinguish regimes of flow between differing geometries and even different fluids. This video gives a great primer on the subject by examining the physics of swimming for a sperm versus a sperm whale. The Reynolds number is essentially a ratio between inertial forces (driven by velocity and size) and viscous forces, and its value can indicate how important different effects are. Sperm and other microbes live at very small Reynolds numbers, meaning that viscosity dominates as the force they must overcome to move. For more on the low Reynolds number world, check out how brine shrimp swim and what happens if a microbe tries to flap its tail. (Hint: it goes nowhere, and this is why.) (Video credit: A. Bhatia/TED Ed; via Jennifer Ouellette)
Reader Question: Oceans Meeting?
Reader favoringfire asks:
Hi! Maybe you can help me: I’ve seen a pic revolving around Tumblr from the Danish city of Skagen showing the Baltic and North sea meeting. Where they meet the ocean is two very distinct hues of blue–what captions say are “two opposing tides with different densities.” Tides? Currents w/different temps often are often diff color from one another. But can “tides” be of different “densities???”
After some searching, I think the photo above is probably the one you’ve seen represented as where the Baltic and North Seas meet. It turns out, however, that it’s not. It’s a photo from an Alaskan cruise taken by Kent Smith. Fluid dynamically, though, it’s still very interesting! What we see here is a sharp gradient between regions with very different densities. One side contains lots of freshwater from rivers fed by melting glaciers, which creates a very different density from the general seawater.
It’s not true, however, that the two won’t mix. This border is not a static phenomenon but one that is ever-changing due to currents and the diffusion of one fluid into another. In a sense, this photo is very much the sea-level version of photos like these which show the massive scale of sediment transport and nutrient mixing that occur in our oceans.
(Photo credit: K. Smith)
Bouncing Atop a Pool
When slowed down, everyday occurrences, like a drop of water falling into a pool, can look absolutely extraordinary. When a falling drop has low momentum, it doesn’t simply disappear into the puddle. It sits on the surface, separated from the main pool by a very thin layer of air. Given time, the air drains away and the droplet cascades its way into the pool via smaller and smaller droplets. By vibrating the surface, the droplet bounces, with each bounce refreshing the layer of air that separates it from the main pool. Minute Lab’s video does a great job of explaining the process from beginning to end, accompanied with wonderful video of each step in action. For even more mind-boggling, check out how these bouncing droplets can demonstrate quantum mechanical behaviors. (Video credit: Minute Laboratory; submitted by Pascal)
Fluids Round-up – 21 September 2013
First off, I’d like to give a special shout-out to FYFD’s friends at Pointwise, who were kind enough to invite me for a visit this week. For any readers looking for CFD grid-generation software, check them out; they are a fantastic bunch and very good at what they do.
My thanks again to everyone who donated this week to help get me to the APS conference. The campaign is still open if anyone wants to get in on the FYFD wallpapers and stickers on offer to donors. As a reminder, any funds beyond conference costs will go toward improving FYFD, including getting equipment to make FYFD videos. On to the fluids round-up!
- Wired takes us behind the scenes of the creation of Games of Thrones’ dragons. Believe it or not, the VFX team actually did digital simulations of the dragons flying in a wind tunnel.
- Nature dissects whether a submarine at relativistic speeds sinks or floats. (via io9) Note that Nature article says the submarine is in water but the original paper simply says that the submarine is immersed in a fluid and makes no account for the compressibility (or lack thereof) of that fluid.
- Add some excitement to your day with liquid-nitrogen-induced explosions from Distort (via io9).
- Flow Viz shows off a great picture of condensation-induced flow visualization on an airplane wing.
- Check out this awesome video of vibrating lycopodium powder from Susi Sie. (via io9)
- National Geographic considers whether Hawaii’s molasses spill is more or less environmentally damaging than an oil spill.
- Finally, our lead image shows a natural visualization of flow around a kayaker. The foam atop the water forms when air and water mix with the gas produced by decomposing leaves. The photo by Lucas Gilman appeared in Outside Magazine earlier this summer. (via Flow Visualization)
(Photo credit: L. Gilman)
Other Ig Nobel Fluids
To round out our series on fluid dynamics in the Ig Nobel Prizes (which are not the same thing as the actual Nobel Prizes), here are some of the other winners. Last year Mayer and Krechetnikov won for a study on coffee sloshing when people walk. We’ve mentioned the pitch-drop experiment measuring the viscosity of an extremely viscous fluid a couple times; Mainstone and Parnell won a 2005 Ig Nobel for that (on-going) work. Another 2005 prize went to Meyer-Rochow and Gal for calculating the pressures involved in penguin defecation. (Yes, seriously.) A avian-related award was also handed out to B. Vonnegut for estimating tornado wind speeds by their ability to strip a chicken of its feathers. And, finally, for those looking to interest undergraduate lab students in mathematics and fluid dynamics, I suggest following the lead of 2002 winner A. Leike who demonstrates laws of exponential decay with beer head. (Photo credit: S. Depolo)
Ig Nobel Fluids: Shower Curtain Science
Nearly everyone has faced the frustration of a shower curtain billowing inwards to stick to one’s leg. Various explanations have been offered to explain the effect, but David Schmidt won the 2001 Ig Nobel Prize in Physics for a numerical simulation suggesting that the spray of droplets from the shower head drives a horizontal vortex whose axis of rotation is perpendicular to the shower curtain. Since vortices have a low-pressure region in their core, this weak shower vortex has the power to suck a light curtain inward, much to the chagrin of the shower’s occupant. Of course, a heavier or weighted shower curtain will help avoid the effect. This post is part of a series on fluids-related Ig Nobel Prizes. (Photo credit: W. Taylor; research credit: D. Schmidt)
Ig Nobel Fluids: Swimming in Syrup
Does a person swim faster in water or syrup? One expects the more viscous syrup would offer a swimmer greater resistance, but, at the same time, it could also provide more to push against. Gettelfinger and Cussler put this to a test experimentally with competitive and recreational swimmers in a pool of water and in one with a fluid measuring roughly twice the viscosity of water. Their results showed no significant change in swimming speed. When you consider that human swimming is highly turbulent, however, the result makes sense. In fluid dynamics, the dimensionless Reynolds number represents a ratio between inertial forces and viscous forces in a flow. The researchers estimate a Reynolds number of a typical human in water at 600,000, meaning that inertial effects far outweigh viscous effects. In this case, doubling the viscosity only reduces the Reynolds number by half, leaving it still well inside the turbulent range. Thus, swimming in syrup has little effect on humans. The Mythbusters also tackled this problem, with similar conclusions. This is a continuation of a series on fluids-related Ig Nobel Prizes. (Photo credit: Mythbusters/Discovery Channel; research credit: B. Gettelfinger and E. L. Cussler, winners of the 2005 Ig Nobel Prize in Chemistry)
