The Faraday instability forms when a fluid interface is vibrated. This high-speed video shows the differences in the shapes formed by a vibrated fluid interface when the two fluids are miscible–capable of mixing–and when they are immiscible–like oil and water. Note how the miscible interface breaks down quickly into turbulence, but the immiscible interface maintains a complex shape.
Tag: turbulence
Turbulent Phytoplankton Eddies
Where warm and cold ocean currents collide, turbulent eddies form and pull up valuable nutrients from the ocean floor. Massive phytoplankton blooms ensue, effectively providing natural flow visualization for the process. #
Thermal Convection
This video turbulent convection in a vertical channel. Buoyancy and the density variations caused by small differences in temperature are what drive the behavior.
Starting a Rocket
This computational fluid dynamics (CFD) simulation shows the start-up of a two-dimensional, ideal rocket nozzle. Starting a rocket engine or supersonic wind tunnel is more complicated than its subsonic counterpart because it’s necessary for a shockwave to pass completely through the engine (or tunnel), leaving supersonic flow in its wake. Here the situation is further complicated by turbulent boundary layers along the nozzle walls. (Video credit: B. Olson)
Microgravity Water Films
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.
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.
Bristling Scales Give Sharks Speed
The shortfin mako shark is one of the ocean’s fastest and most agile hunters, thanks in part to flexible scales along its body. As water flows around the shark’s body, the scales bristle to angles in excess of 60 degrees. This causes turbulence in the boundary layer along the shark’s body and prevents boundary layer separation which would otherwise increase the shark’s drag. In this respect, the scales serve much the same purpose as dimples on a golf ball. (Abstract, National Geographic article) #
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. #
Effects of Viscosity
[original media no longer available]
Today’s video demonstrates the effect of viscosity, which measures a fluid’s resistance to deformation. On the left is a column of highly viscous fluid; the fluids become less viscous as one moves right. When a jet of dye is released into the highly viscous fluid, the jet is very slow to penetrate, whereas, in the rightmost column, the dye expands quickly into a turbulent jet. Between these extremes, we see a laminar dye jet entering the liquid. The mushroom-like shape the laminar jet takes is the result of the Rayleigh-Taylor instability, which occurs when a denser fluid is on top of a lighter fluid in a gravitational field.
Three Flows in One
These plumes of smoke demonstrate the three types of fluid flow: laminar, transitional, and turbulent. At the bottom of the photo, the plumes are smooth and orderly, as is typical for laminar flow. At the top, the smoke’s movement is chaotic and intermittent, full of turbulent eddies. Between these two stages, the flow is in transition; there is still some semblance of order to it, but disturbances in the plume are getting amplified and breaking down into turbulence.
Photo credit: J. Russo
