Computational fluid dynamics and supercomputers can produce some stunning flow visualizations. Above are examples of turbulence, the Rayleigh-Taylor instability, and the Kelvin-Helmholtz instability. Be sure to check out LCSE’s website for more; they’ve included wallpapers of some of the most spectacular ones. (Photo credits: Laboratory for Computational Science and Engineering, University of Minnesota, #)
When a water balloon pops in microgravity, waves propagate from the initial point of contact and the final point of contact (where the balloon skin peels away). As these waves come inward toward one another, the water is compressed from its original potato-like shape into a pancake-like one. In most cases, surface tension will provide a damping force on this oscillatory motion, eventually making the water into a sphere. On Earth, in contrast, a water balloon seems to hold its shape after popping. This is because the effect of gravity on the water is much larger than the effect of the propagating waves. This is one reason that it is useful to have a laboratory in space! Without a microgravity environment, it is much harder to study and observe secondary and tertiary-order forces on a physical event. (Video credit: Don Pettit, Science Off The Sphere)
High viscosity silicon oil is sandwiched between two circular plates. As the upper plate is lifted at a constant speed, air flows in from the sides. The initially circular interface develops finger-like instabilities, due to the Saffman-Taylor mechanism, as the air penetrates. Eventually the fluid will completely detach from one plate. (Photo credit: D. Derks, M. Shelley, A. Lindner)
Physics students are often taught to ignore the effects of air on a projectile, but such effects are not always negligible. This video features several great examples of the Magnus effect, which occurs when a spinning object moves through a fluid. The Magnus force acts perpendicular to the spin axis and is generated by pressure imbalances in the fluid near the object’s surface. On one side of the spinning object, fluid is dragged with the spin, staying attached to the object for longer than if it weren’t spinning. On the other side, however, the fluid is quickly stopped by the spin acting in the direction opposite to the fluid motion. The pressure will be higher on the side where the fluid stagnates and lower on the side where the flow stays attached, thereby generating a force acting from high-to-low, just like with lift on an airfoil. Sports players use this effect all the time: pitchers throw curveballs, volleyball and tennis players use topspin to drive a ball downward past the net, and golfers use backspin to keep a golf ball flying farther. (Video credit: Veritasium)
This stunning National Geographic photo contest winner shows an F-15 banking at an airshow and a array of great fluid dynamics. A vapor cloud has formed over the wings of the plane due to the acceleration of air over the top of the plane. The acceleration has dropped the local pressure enough that the moisture of the air condenses. Some of this condensation has been caught by the wingtip vortices, highlighting those as well. Finally, the twin exhausts have a wake full of shock diamonds, formed by a series of shock waves and expansion fans that adjust the exhaust’s pressure to match that of the ambient atmosphere. (Photo credit: Darryl Skinner/National Geographic; via In Focus; submitted by jshoer)
Here astronaut Andre Kuipers demonstrates fluid dynamics in microgravity. A roughly spherical droplet of water acts as a lens, refracting the image of his face so that it appears upside down. The air bubble inside the droplet refracts the image back to our normal perspective again. (Photo credit: Andre Kuipers, ESA; via Bad Astronomy)
This photo collage shows vortices shed off a backward-facing step. The flow is left to right. Here the flow is visualized using dye released in water. Initially, the vortex forms near the bottom of the step in the recirculation zone. Because flow over the top of the vortex is much faster than the flow beneath the vortex, a low pressure zone forms over the vortex and gradually draws it up toward the top of the step. Eventually the vortex will rise to the point where the upstream flow pushes it downstream and the process begins anew. (Photo credit: Andrew Carter, University of Colorado)
In this video, Ben Krasnow details and demos a small hybrid rocket engine he built in his workshop. Hybrid rockets utilize propellants that are two different states of matter, in this case gaseous oxygen as the oxidizer and solid acrylic as the fuel. Krasnow’s verbal explanation of a convergent-divergent nozzle, used to accelerate flow to supersonic speeds is not quite right. In reality, a compressible fluid like air reaches the sonic point (i.e. Mach 1) at the narrowest point of the nozzle, also called the throat. The divergent portion of the nozzle causes the compressible fluid to expand in volume, which drops the temperature and pressure while the velocity increases beyond the speed of sound.
Krasnow says he did no calculations for his rocket, but I decided to have a little fun by doing some myself. Supersonic flow through the nozzle is only achieved if the flow is choked, meaning that the mass flow rate through the nozzle will not increase if the downstream pressure is decreased further relative to the upstream pressure. For Krasnow’s rocket, the downstream pressure is atmospheric pressure (14.7 psi) and the upstream pressure is provided by the oxygen canister, which he notes was at most 80 psi. Fortunately, the upstream pressure necessary to choke the nozzle is only 27.8 psi, so even with the ball valve partially closed, Krasnow’s rocket is definitely capable of supersonic speeds.
The Mach number achievable by any given supersonic nozzle is related to the ratio of the nozzle throat to its exit diameter (#). Krasnow gives the throat diameter as ¼-inch and the exit diameter as 5/8-inch. This means that the Mach number at the exit of the nozzle, assuming choked supersonic flow, is about Mach 3.4. (Video credit: Ben Krasnow; via Universe Today; submitted by jshoer)
High-speed video reveals the complexity of fluid instabilities leading to atomization–the breakup of a liquid sheet into droplets. A thin annular liquid sheet is sandwiched between concentric air streams. As the velocity of the air on either side of the liquid sheet varies, shear forces cause the sheet to develop waves that result in mushroom-like shapes that break down into ligaments and droplets. Quick breakup into droplets is important in many applications, most notably combustion, where the size and dispersal of fuel droplets affects the efficiency of an engine. (Video credit: D. Duke, D. Honnery, and J. Soria)
A viscous fluid inside a horizontally rotating circular cylinder forms a shark-tooth-like pattern along the fluid’s free surface. This is one of several patterns observed depending on the fluid’s viscosity and surface tension and the rotational rate of the cylinder. (Photo credit: S. Thoroddsen and L. Mahadevan; for more, see Thoroddsen and Mahadevan 1996 and 1997)