Vapor cones typically appear around aircraft flying in the transonic regime–near, but still below, the speed of sound. Air moving over the vehicle accelerates and decelerates as it moves around different parts of the plane; if it didn’t, the plane couldn’t generate lift and wouldn’t fly. When the local flow accelerates past the speed of sound, the accompanying drop in pressure and temperature can be enough to for conditions to fall below the dew point, causing the condensation we see. At the back of the airplane, a shock wave decelerates the airflow back to subsonic speeds and raises local conditions back above the dew point, thereby truncating the cone. (Image credit: C. Caine)
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Flow Around a Delta Wing
Colorful streaks of dye wrap like ribbons along the leading edge of a delta wing. At an angle of attack, this triangular wing forms a set of vortices that run along its edge, providing much of the low pressure–and therefore lift–on the upper surface of the wing. In contrast, the red streaks of dye in the middle of the wing demonstrate clean, laminar flow. Highly swept delta wings are popular for aircraft traveling at supersonic speeds, but they can also work well subsonically, as shown here. For more incredible and beautiful examples of flow visualizations by Henri Werlé, check out his 1974 film Courants et couleurs. (Photo credit: H. Werlé; via eFluids)
Hummingbird Hovering
Hummingbirds have a unique way of flying among birds. By flapping in a figure-8 motion, they generate lift on both the upstroke and the downstroke, which enables them to fly forward, backward, and even hover for extended periods. Such mid-air acrobatics are necessary for a species that feeds on flower nectar. What is especially impressive about the birds, though, is how they hold up even in adverse conditions like wind or rain. By placing birds in a wind tunnel and filming with high-speed video, researchers can see how hummingbirds maintain their feeding position even in 20 mph (32 kph) winds. By fanning out their tail feathers like a rudder, they can control their body orientation despite turbulent gusts. Not even rain stops them. The birds will periodically shake themselves dry, much like a dog if a dog could manage to fly while shaking itself. (Video credit: Deep Look; submitted by entropy-perturbation)
Cloud Formation
Clouds are so ubiquitous here on Earth that it’s easy to take them for granted. But there’s remarkable complexity in the mechanics of their formation. This great video from Minute Earth steps through the processes of evaporation and condensation that drive basic cloud formation. After evaporation, buoyancy lifts warm, moist air upward. That warm air expands and cools until it reaches an altitude where water droplets can condense onto dust particles in the atmosphere. These droplets form the wispy cloud we see. Turbulence mixes these droplets and helps them collide and grow. Interestingly, although we understand the basic process of cloud formation, relatively little is understood about the details, and the subject is still very much an area of active research. (Video credit: Minute Earth; via io9)
Snowy Deserts
Windblown snow bears a certain resemblance to desert sands or a Martian landscape. Many of the same aeolian processes–like erosion, transport, and deposition–take place in each. The animation above shows an example of suspension, where fine snowflakes are lifted and carried along near the ground. Larger snowflakes may bounce or skip along the surface in a process called saltation. For more, check out some of the crazy things snow does or learn about how dunes form. (Image credit: Redemption Designs, source video)
A Toast!
When you lift a glass of champagne or sparkling wine at midnight tonight, your nose and mouth will be greeted by a plethora of aromas, flavors, and sensations propagated by the tiny bubbles in the drink. Carbon dioxide dissolved in the wine gathers in a stream of tiny bubbles that rise at the center of the glass. (The bubbles form at the center because champagne glasses are often etched in a ring there to provide nucleation points where the bubbles can grow.) This stream of rising bubbles generates vortical motion in the glass that helps carry the carbon dioxide to the surface, where it is released when the bubbles burst. In the tall, thin champagne flute these vortices mix the entire contents of the glass, but, in a wider coupe, the vortices are confined to the center, leaving a stiller region along the glass’s edges. For those who find that a freshly poured flute of champagne stings their noses–a side effect of the high gaseous carbon dioxide concentration just after decanting–the wider coupe lowers the concentration at the glass’s lip and may provide a more pleasant experience for toasting the new year. (Image credit: F. Beaumont et al.)
Transonic Flow
In the transonic speed regime the overall speed of an airplane is less than Mach 1 but some parts of the flow around the aircraft break the speed of sound. The photo above shows a schlieren photograph of flow over an airfoil at transonic speeds. The nearly vertical lines are shock waves on the upper and lower surfaces of the airfoil. Although the freestream speed in the tunnel is less than Mach 1 upstream of the airfoil, air accelerates over the curved surface of airfoil and locally exceeds the speed of sound. When that supersonic flow cannot be sustained, a shock wave occurs; flow to the right of the shock wave is once again subsonic. It’s also worth noting the bright white turbulent flow along the upper surface of the airfoil after the shock. This is the boundary layer, which can often separate from the wing in transonic flows, causing a marked increase in drag and decrease in lift. Most commercial airliners operate at transonic Mach numbers and their geometry is specifically designed to mitigate some of the challenges of this speed regime. (Image credit: NASA; via D. Baals and W. Corliss)
Frisbee Physics, Part 2
Yesterday we discussed some of the basic mechanics of a frisbee in flight. Although frisbees do generate lift similarly to a wing, they do have some unique features. You’ve probably noticed, for example, that the top surface of a frisbee has several raised concentric rings. These are not simply decoration! Instead the rings disrupt airflow at the surface of the frisbee. This actually creates a narrow region of separated flow, visible in region B on the left oil-flow image. Airflow reattaches to the frisbee in the image after the second black arc, and the boundary layer along region C remains turbulent and attached for the remaining length of the frisbee. Keeping the boundary layer attached over the top surface ensures low pressure so that the disk has plenty of lift and remains aerodynamically stable in flight. A smooth frisbee would be much harder to throw accurately because its flight would be very sensitive to angle of attack and likely to stall. (Image credits: J. Potts and W. Crowther; recommended papers by: V. Morrison and R. Lorentz)
Frisbee Physics
Frisbees are a popular summertime toy, but they involve some pretty neat physics, too. Two key ingredients to their long flight times are their lift generation and spin. A frisbee in flight behaves very much like a wing, generating lift by flying at an angle of attack. This angle of attack and the curvature of the disk rim cause air to accelerate over the top of the leading edge. Airflow over the top of the disk is faster than that across the bottom; thus, pressure is lower over the top of the frisbee and lift is generated. Aerodynamic lift and drag aren’t enough to keep the frisbee aloft long, though. Spin matters, too. If the frisbee is launched without spin, gravity acts on it through its center of mass, but lift and drag act through a point off-center because lift tends to be higher on the front of the disk than the back. This offset between gravitational forces and aerodynamic forces creates a torque that tends to flip the frisbee. By spinning the frisbee, the thrower gives it a high angular momentum acting about its spin axis. Now instead of flipping the disk, the torque caused by the offset forces just tips the angular momentum vector slightly. Physically, this is known as spin stabilization or gyroscopic stability. Tomorrow we’ll take a closer look at airflow over the frisbee. (Image credit: A. Leibel and C. Pugh, source video; recommended papers by: V. Morrison and R. Lorentz)
What’s in a Splash?
A droplet falling onto a solid, dry surface seems like a simple situation, one that would be easy to understand. But splashes can be unpredictable. Velocity, viscosity, and surface tension all play clear roles, but the surrounding air also has an impact – drop the air pressure low enough and a droplet won’t splash. A new paper has tackled the problem, producing a mathematical model in agreement with experimental results. So why do some drops splash and others don’t? When a drop falls, its momentum flattens it into a pancake shape while surface tension struggles to hold it together. The spreading edge, called the lamella, can pull away from the surface. When it does, a pocket of high pressure forms beneath it due to lubrication effects, and the faster airflow over the top of the lamella creates a suction effect. This is analogous to a wing producing lift. Like the momentum that spread the droplet, the lift force pulls the lamella and ejecta sheet further up and outward, overcoming the restoring force of surface tension and tearing the droplet apart. For more on the effect, check out the research paper or this Inside Science article. (Video credit: G. Riboux and J. Gordillo; via Inside Science)