Schlieren photography is a common method of visualizing shock waves in wind tunnel experiments, but it’s much harder to pull off for aircraft in the sky. This video from NASA shows off some stunning work out of NASA Dryden capturing schlieren video of shock waves from a F-15B aircraft at Mach 1.38. You’ll notice that shock waves extend off the nose, wings, tail, and other parts of the airplane and extend well beyond the camera’s field of view. It’s these shock waves hitting the ground level that causes distinctive sonic booms. These tests are part of NASA’s on-going research into minimizing the effects of sonic boom so that civilian supersonic flight over land is feasible in the future. When the U.S. government shutdown ends, you’ll be able to learn more about this work at NASA Dryden’s GASPS page. (Video credit: NASA Dryden)
Search results for: “wind tunnel”
Oil Flow Viz
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)
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.)
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)
Ski Jumping Aerodynamics
Last summer we featured fluid dynamics in the Summer Olympics and there’s more to come for Sochi. Winter athletes like ski jumper Sarah Hendrickson are hard at work preparing, which can include time in wind tunnels, as shown here. There are two main diagnostics in tests like these: drag measurements and smoke visualization. The board Hendrickson stands on is connected to the tunnel’s force balance, which allows engineers to measure the differences in drag on her as she adjusts equipment and positions. This gives a macroscopic measure of drag reduction, and reduced drag makes the skier faster on the snow and lets her fly longer in the jump. The smoke wand provides a way to visualize local flow conditions to ensure flow remains attached around the athlete, which also reduces drag. (Video credit: Red Bull/Outside Magazine; submitted by @YvesDubief)
Vortex Street in the Clouds
Most objects are not particularly aerodynamic or streamlined. When air flows over such bluff bodies, they can shed regular vortices from one side and then the other. This periodic shedding creates a von Karman vortex street, like this one stretching out from Isla Socorro off western Mexico. From the wind’s perspective, the volcanic island forms a blunt disruption to the otherwise smooth ocean. This vortex shedding is seen at smaller scales, as well, in the wind tunnel, in soap films, and in water tunnels. If you’ve ever been outside on a windy day and heard the electrical lines “singing” in the wind, that’s the same phenomena, too. With the right crosswind, radial bicycle spokes will buzz for the same reason as well! (Photo credit: MODIS/NASA Earth Observatory)
Fluids Round-up – 25 May 2013
Sometimes I come across cool links and stories about fluid dynamics that don’t quite fit into a typical FYFD post, but I’d like to start sharing those semi-regularly with round-up posts. Here’s some fun stuff I’ve seen lately:
- We’ve talked before about penguins and fluid dynamics, but this IOP talk from Helen Czerski is a great take on how they use fluids to escape predators.
- Over at FlowViz, Richard has a great video demonstrating deep versus shallow water waves side-by-side.
- io9 shows off the vibrational modes of mercury as well as explaining why birds can be good at flying or swimming but not both.
- Giant paint explosions (specifically here) may be fun to watch but the dangers are not for squeamish viewers.
- Dylan G sent us this 512K real-time fluid particle simulation that’s a fun distraction.
- xckd’s Randall Munroe answers all kinds of physics questions at his What If? blog, but occasionally there are some fluids ones. Two of my favorites consider a hypersonic steak and what happens if the glass is literally half full.
- Finally, for the cyclists and triathletes among you, the recent news that bicycle manufacturer Specialized has built its own wind tunnel has generated lots of discussion of aerodynamics in sport. Mark Cote (@MITAeroBike) and Chris Yu (@chrisyuinc) of Specialized’s aero R&D have been answering questions on Slowtwitch, Twitter, and in live chats.
And, yes, that last Specialized video chat includes an FYFD shout-out about 49 minutes in. 🙂
(Photo credit: Specialized)
Effects of Hills on Flow
Hills and other topology can have interesting and complex effects on a flowfield. With the FAITH experiment, NASA has been investigating an axisymmetric model hill using a combination of experimental methods. The video above shows flow visualization over the hill in a water channel using dye injection both upstream and downstream of the model. They’ve also done wind tunnel tests with oil-flow visualization, particle-image velocimetry, pressure sensitive paint and other measurement techniques. There are nice photos of some of these by Rob Bulmahn. By combining qualitative and quantitative flow measurement techniques, the researchers are able to capture many different aspects of the flow, which can then be shared and compared with other groups’ works. (Video credit: NASA Ames Research Center)
Lift on a Paper Plane
In this still image from a student experiment, smoke visualization shows the formation of a vortex over the wing of a paper airplane during a wind tunnel test. This wing vortex is mirrored on the opposite wing, though there is no smoke to show it. At high angle of attack, the delta-wing shape of the traditional paper air plane creates these vortices on the upper surface, which helps generate the lift necessary to keep the plane aloft. (Photo credit: A. Lindholdt, R. Frausing, C. Rechter, and S. Rytman)
Hummingbirds Singing with their Tail Feathers
Aeroelastic flutter occurs when fluid mechanical forces and structural forces get coupled together, one feeding the other. Usually, we think of it as a destructive mechanism, but, for hummingbirds, it’s part of courtship. When a male hummingbird looks to attract a mate, he’ll climb and dive, flaring his tail feathers one or more times. As he does so, air flow over the feathers causes them to vibrate and produce noise. Researchers studied such tail feathers in a wind tunnel, finding a variety of vibrational behaviors, including a tendency for constructive interference–in other words two feathers vibrating in proximity is much louder than either individually. For more, check out the original Science article or the write-up at phys.org. (Video credit: C. Clark et al.)