This image shows a composite X-ray (red, green, and blue) and optical (gold) view of the supernova remnant Cassiopeia A, located about 11,000 light years away. At the heart of this supernova remnant is a neutron star. After ten years of observations, astronomers have found a 4% decline in the temperature of this neutron star, which cannot be accounted for in current theory. Two research teams have independently found that this cooling could be due to the star converting the neutrons in its core into a superfluid. As the neutron superfluid is formed, neutrinos are emitted; this decreases the energy in the star and causes more rapid cooling. See Wired for more. #
Month: February 2011
Godspeed, Discovery!
The space shuttle, despite three decades of service, remains a triumph of engineering. Although it is nominally a space vehicle, fluid dynamics are vital throughout its operation. From the combustion in the engine to the overexpansion of the exhaust gases; from the turbulent plume of the shuttle’s wake to the life support and waste management systems on orbit, fluid mechanics cannot be escaped. Countless simulations and experiments have helped determine the forces, temperatures, and flight profiles for the vehicle during ascent and re-entry. Experiments have flown as payloads and hundreds of astronauts have “performed experiments in fluid mechanics” in microgravity. Since STS-114, flow transition experiments have even been mounted on the orbiter wing. The effort and love put into making these machines fly is staggering, but all things end. Godspeed to Discovery and her crew on this, her final mission!
Swimming Sandfish Lizards
Sandfish lizards can “swim” through granular flows like sand using an undulating, sinusoidal motion. Having studied this motion, engineers have built a robot that swims similarly through large glass beads and have now created a numerical simulation of the physics that matches the measured forces on the swimmer to within 8%. This type of flow is, in some respects, tougher than actual fluids because individual particles have to followed, while in most of fluid mechanics, we can use the continuum assumption to treat a liquid or gas as a continuous medium. #
Dancing Droplets
When a droplet falls onto a larger pool of the same liquid, it briefly sits on a layer of air that prevents coalescence. When that air drains away, the coalescence cascade–in which the droplet breaks into progressively smaller droplets until fully absorbed–begins. But if you vibrate the pool of liquid, the droplet bounces, effectively injecting more air between it and the pool. This prevents coalescence. What’s really neat here is that the researchers demonstrate this effect with arrays of droplets dancing in formation.
Shock Waves
Flow visualization really can be considered a form of art. Though we fluid mechanicians are looking for physics, we’re quite aware of the beauty of what we study. The clips in this video mostly show transient shockwave behavior, including lots of shock reflection and even a few instabilities. It’s unclear what the speeds are, aside from faster than sound; the medium is air.
Rafting for Rocks
Another look at the science behind the roaming rocks of Death Valley.
Rocket Launch Phenomena
The launch of the Solar Dynamics Observatory (SDO) last year provided a rarely seen glimpse of how shock waves affect the atmosphere during launch, but only recently have researchers explained the white column that seemed to follow SDO toward orbit. Simulations indicate that the shock waves from the rocket aligned the ice crystals in the atmosphere into an array of spinning tops. Individual crystals precess as a result of the rocket passing; the column is part of a larger oval that would have been visible had the ice crystals covered a larger range. See Wired for more. #
Reader Question: Rotor Ships
lazenby asks:
Can you explain how the magnus effect makes rotor ships move?
When a spinning body is placed in a flow, the body experiences a force perpendicular to the direction of the flow. This is called the Magnus effect and is, for example, why baseballs, soccer balls, and tennis balls veer from the path we expect them to take. To understand why a spinning body experiences this force, take a look at the streamlines around a rotating cylinder.
In this picture, the flow goes from left to right and the cylinder is spinning in the clockwise direction. The red dots represent the stagnation points of the flow. Air over the top of the cylinder gets accelerated by the spinning, shown here by the narrowing of space between streamlines. On the underside of the cylinder, the surface is moving in the opposite direction of the air, which decelerates the flow. We know from Bernoulli that this means there is low pressure on the top of the cylinder and high pressure on the bottom. As a result, the cylinder experiences a upward force – lift! You can explore the effect of rotation on the streamlines yourself using this neat demo from Wolfram.
Rotor ships, invented in the 1920s, used this effect for ship’s propulsion. They used a regular motor to begin moving, and, once they had some wind, used motors to spin giant cylinders on the deck. As the rotors spun, the ships were pushed in a direction perpendicular to the wind. They could apparently tack 20-30 degrees into the wind while conventional ships could only manage 45 degrees. Unfortunately, so much energy was required to spin the rotors that the design was pretty inefficient and never caught on.
Hello, New Folks!
Lots of new faces around here at FYFD, so thanks, everyone, for spreading the word! As a reminder, you’re welcome to submit post ideas if you see something neat online and you can also ask any questions you have related to fluid mechanics, and I’ll do my best to answer them or find someone who can! I also respond to comments on Twitter, if you prefer.
Hole-Punch Clouds
These hole-punch clouds seen over Myrtle Beach, SC were probably caused by three aircraft flying in military formation. When airplanes pass through supercooled water vapor, the acceleration of air over the wing causes a pressure drop that can flash-freeze the water vapor, resulting in a localized snow shower. See National Geographic for more. #
