We humans generally use fixed wings for flight, but in nature, flapping flight dominates. As an animal flaps, it extends or draws in its wings during key points of the cycle in order to change its aerodynamics. But this control can be more than just a matter of stretching their wings. Recent work on bats shows that they can fine-tune the stiffness of their wings’ membrane using tiny, hair-thin muscles. Each muscle is too slight to change a wing’s shape on its own, but by firing synchronously–tensing on the downstroke and relaxing on the upstroke–the bat can manipulate its membrane stiffness and thereby affect its wing shape. Moreover, the timing of the muscles’ action changes with flight speed, suggesting that the bats are actively controlling their aerodynamics during flight. (Video credit: Swartz-Breuer lab/Brown University; via Futurity; submitted by Boris M)
Tag: fluid dynamics
The Airbag’s Inflation
Airbags have become a standard safety feature for automobiles. As the Slow Mo Guys demonstrate in the video above, the bags inflate incredibly quickly–less than 1/25th of a second! The incredible speed of the system’s deployment is what keeps the car’s occupants from slamming into the hard surfaces of the wheel or dashboard. But this only works if the passenger is far enough away that the airbag is inflated before they contact it. Because the bag inflates so quickly, it does so with enormous force, like the airbag in the video flinging the glass of water. When a car registers a crash, it sparks the ignitor of a solid-propellant inflator, initiating a chemical reaction that produces the nitrogen gas that fills the airbag. This is essentially the same process as a solid-propellant rocket. (Video credit: The Slow Mo Guys)
Barchan Dunes
Crescent-shaped barchan dunes are common on both Earth (top image) and Mars (bottom image). They form in areas where the wind comes predominantly from one direction. As the wind blows, it deposits sand on the gently sloping windward face of the dune. The leeward face of the dune is steeper; its shape is set by the sand’s angle of repose–essentially the steepest angle the sand can maintain without an avalanche. Barchan dunes are very mobile, moving between one and a hundred meters per year. They have also been seen moving through one another or moving along in formation. (Image credits: Google Earth, NASA/JPL/University of Arizona)
Inside a Water Blob
This new video from the Space Station shows once again that astronauts have the most fun job on–or off–the planet. In it, the Expedition 40 crew members submerge a GoPro camera in a microgravity water blob. Here on Earth, we’re used to surface tension being a minor or secondary force with most fluids we experience daily. This is because gravity often provides the overwhelming effect. But in microgravity, those effects are absent, and forces like surface tension and adhesion dominate water’s behavior. This both why the crew can make such a large water sphere hold together, and why one astronaut eventually gets his hands stuck in the sphere. (Video credit: NASA; submitted by jshoer)
“Milky WaY”
Photographer Paulo Stagnaro uses milk and food coloring in his series “Milky WaY”. Despite the simple ingredients, the photos illustrate the enormous variety of shape and form in fluid dynamics. Surface tension, diffusion, and intentional mixing create abstract and ephemeral portraits of fluid motion. For similar work, see Pery Bruge’s art or just try browsing through FYFD’s “fluids as art” tag for more examples of science and art intersecting. (Photo credit: P. Stagnaro; submitted by Stephanie M.)
Von Karman Vortex Streets
The wake of a cylinder is a series of alternating vortices shed as the flow moves past. This distinctive pattern is known as a von Karman vortex street. The speed of the flow and the size of the cylinder determine how often vortices are shed. Incredibly, this pattern appears at scales ranging from the laboratory demo all the way to the wakes of islands. Von Karman vortex streets can even be seen from space. (Image credit: R. Gontijo and W. Cerqueira, source video)
Turbulence and Star Formation
Galaxy clusters are objects containing hundreds or thousands of galaxies immersed in hot gas. This gas glows brightly in X-ray, as seen in the Perseus (top) and Virgo (bottom) clusters above. Over time, the gas near the center of the clusters should cool, generating many new stars, but this is not what astronomers observe. New research suggests turbulence may prevent this star formation. The supermassive black holes near the center of these galaxy clusters pump enormous amounts of energy into their surroundings through jets of particles. Those jets churn the gas of the cluster, generating turbulence, which ultimately dissipates as heat. It is this turbulent heating astronomers think counters the radiative cooling of the gas, thereby keeping the gas hot enough to prevent star formation. You can read more about the findings in the research paper. (Image credits: NASA/Chandra/I. Zhuravleva et al.; via io9)
Iridescent Clouds
Look up at the clouds on the right day and you may catch a glimpse of a rainbow-like phenomenon known as cloud iridescence. These colors occur when sunlight is diffracted through small water droplets or ice crystals. For the effect to be apparent, the cloud must be optically thin, meaning that most of the rays of sunlight must pass through only a single droplet or ice crystal. This means the effect is usually visible only near the edges of clouds or as new clouds are forming. You can see more photos of the phenomenon here, and there’s a great video where cloud iridescence makes an appearance during a rocket launch in this previous entry. (Photo credit and submission: C. Havlin)
The Hidden Complexities of the Simple Match
Striking a match and blowing it out seems rather simple to the naked eye. But with high-speed video and schlieren photography, the act takes on new complexity. Schlieren photography is an optical technique that is incredibly sensitive to changes in density, which makes it a prime choice for visualizing flows with temperatures variations or shock waves. Here it shows the hot gases generated as the match is lit. Once the match ignites, the flow calms somewhat into a gently rising plume of exhaust and hot air. When someone enters the frame to blow out the match, the frame rate increases to capture what happens next. The flow field around the match becomes very complex as the air and flame interact. The range of length scales in the flow increases, from scales of several centimeters down to those less than a millimeter. This complexity and range of sizes is a hallmark of turbulence. (Video credit: V. Miller et al.)
FYFD at APS DFD 2014
I’m excited to announce that I will be attending the American Physical Society Division of Fluid Dynamics meeting in San Francisco next month. This year I will be co-teaching a workshop on communicating science to general audiences. Here’s the lowdown:
Scientist-Reporter Workshop: How to tell your science story
Want to share your research with the public? Five experts lead a workshop on ways to draw attention to your research. Join science journalist Flora Lichtman, whose work has appeared on NPR, and in The New York Times and Popular Science; Jason Bardi, writer and Director of Media Services at AIP; Nicole Sharp, creator of “F! Yeah Fluid Dynamics”; Rachel Levy, creator of “Grandma got STEM” and biomechanic David Hu for a workshop on disseminating your research to news outlets.
To learn more or register to attend, check out: http://apsdfd2014.stanford.edu/?q=content/special-events
We’ll have a follow-on to the workshop at Sunday night’s banquet. For those who can’t make it to the conference in person, never fear: we will be taking to the Internet, too. More on that at a later date.
So who all is coming?
