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)
Category: Phenomena
Homemade Hybrid Rocket Engine
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)
Formula 1 Aerodynamics
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Computational fluid dynamics (CFD) and the advent of supercomputing have forever changed the way engineers design. Here the use of CFD in the design of Formula 1 racing cars is discussed. Although CFD is used by many companies in place of wind tunnel testing, each method has its advantages. CFD provides information about all flow quantities at all points in the flow but can only do so with an accuracy dependent on the grid and models used. It remains impossible to solve the equations of motion exactly for any problem of practical application because the computational cost is simply too high; instead software packages like FLUENT utilize turbulence models that approximate the physics. Wind tunnel testing, on the other hand, is physically accurate but typically yields only limited data and flow quantities due to the difficulty of instrumentation. (Video credit: BBC News; submitted by carhogg)
(Source: /)Wingtip Vortices
Any finite length wing produces wingtip vortices–potentially intense regions of rotational flow downstream of the wing’s ends. These vortices are associated both with the production of lift on the wing and with unavoidable induced drag. The tabletop demonstration above shows the region of the vortices’ influence and how strong the rotation is there. Note also that the two vortices have opposite rotational senses–the left side induces a clockwise rotation, whereas the right side induces an anti-clockwise rotation. The larger an aircraft, the stronger and longer lasting its vortices; this can be a source of danger for smaller aircraft passing through the wake. If a pilot crosses one wingtip vortex and overreacts to compensate, crossing the second counter-rotating vortex can cause even greater damage.
Didgeridoo Soap Bubble
This high-speed video shows a soap bubble being blown via didgeridoo, a wind instrument developed by the Indigenous Australians. The oscillations of the capillary waves on the surface of the bubble vary with the frequency of note being played. High frequency notes excite small wavelengths, whereas lower notes create large wavelength oscillations. For more fun, check out what you can do with didgeridoos in space. (submitted by Christopher B)
When Fluids Behave Like Solids
Many common fluids–like air and water–are Newtonian fluids, meaning that stress in the fluid is linearly proportional to the rate at which the fluid is deformed. Viscosity is the constant that relates the stress and rate of strain, or deformation. The term non-Newtonian is used to describe any fluid whose properties do not follow this relationship; instead their viscosity is dependent on the rate of strain, viscoelasticity, or even changes with time. A neat common example of a non-Newtonian fluid is oobleck, a mixture of cornstarch and water that is shear-thickening, meaning that it is resistant to fast deformations. Like the cornstarch-based custard in the video above, these fluids react similarly to a solid when struck, resisting changing their shape, but if deformed slowly, they will flow in the manner of any liquid.
Creating Lava
In Syracuse, NY, artists and scientists work together to study volcanic flows by melting crushed basalt in a special furnace before releasing the lava into the parking lot. This particular flow is very prone to boiling behavior, likely because of the cold air and ground temperatures (less than 0 C). The outer layers of rock cool quickly, leaving bubble-shaped chambers which hotter lava can fill before melting out. (via It’s Okay To Be Smart; submitted by @jpshoer)
Fireball in Slow Motion
The high-speed video above shows an atomized spray of flammable liquid being ignited using a lighter. It was filmed at 10,000 fps and is replayed at 30 fps. Although uncontained, this demonstration is similar to the combustion observed inside of many types of engines. Automobiles, jet engines, and rockets all break their liquid fuel into a spray of droplets to increase the efficiency of combustion. The turbulence of the flames dances and swirls, with small-scale motions close to the sprayed droplets and larger-scale motions around the vaporized fuel. This variation in size of the scales of motion is a hallmark feature of turbulence and can be used to characterize a flow.
How to Escape a Whitewater Hole
One of the perils of whitewater sports is getting stuck in what paddlers call a “hole” or a “hydraulic”. This river feature forms just downstream of large obstacles like rocks or low-level dams. As water pours over the obstacle and into its shadow, the flow forms a recirculating vortex-like zone. Immediately next to the obstacle, water is pulled upstream toward the obstacle and then down toward the bottom of the river. This makes the hydraulic very dangerous and hard to escape. Note in the video how the raft is held in place by the upstream motion of the water at the surface of the hydraulic. The rafters are preventing their craft from flipping over by weighing down the side experiencing the upward flow of the vortex. Escaping a hydraulic usually requires getting near its edge, where its current is weaker. If swimming, the best way to escape is to swim toward the bottom of the river and then downstream with the current of the hydraulic rather than against it at the surface.
Astro Puffs
Microgravity continues to be a fascinating playground for observing surface tension effects on the macroscale without pesky gravity getting in the way. Here astronaut Don Pettit has created a sphere of water, which he then strikes with a jet of air from a syringe. Initially, the momentum from the jet of air creates a sharp cavity in the water, which rebounds into a jet of water that ejects one or more satellite drops. Surface waves and inertial waves (inside the water sphere) reflect back and forth until the fluid comes to rest as a sphere once more. Note how similar the behavior is to the pinch-off of a water column. Both effects are dominated by surface tension, but on Earth we can only see this behavior with extremely small droplets and high-speed cameras! (Video credit: Don Pettit, Science Off the Sphere)
