In this video astronaut Don Pettit demonstrates some interesting laminar flow effects using a water film in microgravity. By using a film, fluid motion is essentially confined to two dimensions. This is important because it prohibits the development of turbulence, which is a purely three-dimensional phenomenon. Doing the experiment in microgravity allows Pettit to leave the experiment for a long period of time without buoyant effects or similar disturbances. When he first stirs the film, the tracer particles show some signs of what looks like turbulent mixing, but soon the film rotates uniformly with streaks of gray caused by different concentrations of tracer particles. Pettit notes that he allowed the film to rotate overnight and it eventually all turned milky white. This is the effect of molecular diffusion of the tracer particles; without turbulence, the only way for mixing to occur is through the random motion of molecules. See more of Pettit’s Saturday Morning Science videos for additional microgravity fluid mechanics.
Category: Phenomena
Seeing the Invisible
Schlieren photography is a common experimental flow visualization technique, especially in supersonic flows (where it enables one to see shock waves). Here the Science Channel’s “Cool Stuff: How It Works” show explains the technique and shows some examples from everyday life.
Vibrating Oobleck
[original media no longer available]
This video explores some of the non-Newtonian behaviors of oobleck when shaken. The pattern across the surface once the vibrations start is called Faraday waves, a type of nonlinear standing wave that forms once a critical vibrational frequency is passed and the flat surface of the fluid becomes unstable. Toward the end of the video, the frequency of the vibrations is increased until “finger-like protrusions” form. This is a behavior exhibited by shear-thickening non-Newtonian fluids.
Wavy Vortices
Shown above is the flow between two concentric cylinders (Taylor-Couette flow). In the laminar regime, the velocity profile between the two cylinders is linear. As the rate of rotation of the inner cylinder increases, the flow develops toroidal vortices known as Taylor vortices, seen in the video above after 9 seconds or so. This is a fluid instability exhibited by transitional flow. Increasing the rotational rate further can result in wavy Taylor vortex flow. At high enough speeds, the flow will become completely turbulent.
Superfluid Dripping
This high-speed video shows superfluid helium dripping and breaking up. Although superfluid has no viscosity, this does not prevent the Plateau-Rayleigh instability from breaking the helium into droplets once the mass of the liquid is too great for surface tension to contain.
Tears of Wine
Tears of wine are caused by the Marangoni effect, in which a gradient in surface tension causes mass flow. The water in the wine has a higher surface tension than the alcohol in the wine, causing the wine to be drawn away from regions of higher alcohol concentration. #
Turbulence Near the Wall
This photo shows a flow visualization of a turbulent boundary layer at Mach 2.8. The direction of flow is from right to left. In nature, the boundary layer between a surface and a fluid is usually turbulent but impossible to see. The visualization represents an instantaneous snapshot of the flow. Turbulence is known for its intermittency–its strong variation in time–a characteristic that is clear just from comparing the two snapsnots. #
Kelvin-Helmholtz Instability
The Kelvin-Helmholtz instability occurs when velocity shear is present in a single fluid or when two different fluids have a velocity difference across their interface. As shown in this numerical simulation, the instability produces a fractal-like pattern of eddies turning over on themselves. The Kelvin-Helmholtz instability is commonly found in nature between cloud layers. #
ETA: It looks like animated GIFs may not work with Tumblr. Be sure to click on the picture to see the animation on Wikipedia.
Physics Tattoos
This is a man with great commitment to fluid dynamics. He writes:
This, on my leg, is the incompressible form of the conservation of mass equation in a fluid, also known as the continuity equation. When people ask what it means, I say it defines flow. Sometimes I say it means you should have studied more physics, but that is only when I am feeling like being funny. What it means in more detail is that, for an incompressible fluid, the partial derivative of the velocity of the fluid in the three spatial dimensions must sum to zero. It therefore concisely states the fundamental nature of a fluid. #
(via physicsphysics)
The Leidenfrost Effect
The Leidenfrost effect occurs when a liquid comes in contact with a mass significantly hotter than the liquid’s boiling point. Upon contact, a thin layer of the liquid will be vaporized, forming a lubricating gas layer that temporarily insulates the hot mass from the cold liquid. This effect is responsible for water skittering across a hot plate as well as protecting the hands of many a professor from a dunk in liquid nitrogen at the front of a classroom.
reblogged from fyeahchemistry:
(Thanks for the submission, singbird-sing!)
