This image, taken from a direct numerical simulation, shows turbulence in a stably stratified flow in which lighter fluid sits atop a denser fluid. In the image lighter colors represent denser fluid. Turbulence is created by the shear forces caused when the lighter fluid on top moves faster than the denser fluid on the bottom; however the stable stratification will tend to counteract or stabilize the turbulence. Note the vast variety and detail of the scales involved in turbulence; this is what makes it such a difficult process to simulate and model. (Image credit: G. Matheou and D. Chung, NASA/JPL-Caltech)
This numerical simulation shows the variation of salinity in the Atlantic Ocean near the mouth of the Amazon River over the course of 36 months. The turbulent mixing of the fresh river water and salty ocean shifts with the ebb and flooding of the river. Salt content causes variations in ocean water density, which can strongly affect mixing and transport properties between different depths in the ocean due to buoyancy. Understanding this kind of flow helps predict climate forecasts, rain predictions, ice melting and much more. (Video credit: Mercator Ocean)
When conditions are just right, the low pressure at the center of a wingtip vortex can drop the local temperature below the dew point, causing condensation to form. Here vortices are visible extending from the tips of the propellers in addition to the wingtip. Because of the spinning of the propeller and the forward motion of the airplane, the prop vortices extend backwards in a twisted spiral that will quickly break down into turbulence. The same behavior can be observed with helicopter blades. (Photo credit: benurs)
Diffusion of ink in water + Lego minifigs = an awesome example of fluid mechanics as art. (Photo credit: Alberto Seveso; via io9; thanks to Jennifer for the link!)
From reader jessecaps who hung it on the office door. I expect this joke will make sense to very few but as someone who once dabbled in turbulence, I could not resist.
While we typically think about boundary layers as a small region near the surface of an object—be it airplane, golf ball, or engine wall—boundary layers can be enormous, like the planetary boundary layer, the part of the atmosphere directly affected by the earth’s surface. Shown above is a flow visualization of the boundary layer in an urban area; note the models of buildings. In these atmospheric boundary layers, buildings, trees, and even mountains act like a random rough surface over which the air moves. This roughness drives the fluid to turbulent motion, clear here from the unsteadiness and intermittency of the boundary layer as well as the large variation in scale between the largest and smallest eddies and whorls. In the atmosphere, the difference in scale between the largest and smallest eddies can vary more than five orders of magnitude.
Flow visualization in a water tunnel shows what the flow around a line of traffic looks like. Note the progressively more turbulent flow around each car as it sits in the wake of the car before it. Turbulent flow is usually associated with increased drag forces, but because turbulence can actually help prevent flow separation it is sometimes desirable as a method for decreasing drag. In the case of these cars drafting on one another, it is clear that the cars further back in the line cause less effect on the fluid—and thus have less drag to overcome—than the front car. (Photo credit: Rob Bulmahn)
Smoke visualization, illuminated by a laser sheet, shows a 2D slice from an axisymmetric jet as it breaks down to turbulence. The flow is laminar upon exiting the nozzle, but the high velocity at the edge of the jet and low velocity of the surrounding air causes shear that leads to the Kelvin-Helmholtz instability. This instability leads to the formation of small vortices that grow as they are advected downstream until they are large enough to interrupt the jet and it breaks down into fully turbulent flow. (Video credit: B. O. Anderson and J. H. Jensen)
Mixing of surface waters with deeper ocean currents brings together the minerals and nutrients used by phytoplankton, resulting in gorgeous swirls of color in the ocean. These phytoplankton blooms are most common in the spring and summer, and while lovely, can be harmful to other marine life, either through the production of toxins or by depleting the waters of oxygen. Because the phytoplankton move according to the wind and waves, they can also form a sort of natural flow visualization. (Photo credit: ESA)
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This spectacular high-speed video shows a dove in flight. Note how its wings flex through its stroke and the way the wings rotate over the course of the downstroke and reversal. There is incredible beauty and complexity in this motion. The change in wing shape and angle of attack is what allows the bird to maximize the lift it generates. Note also how the outer feathers flare during the downstroke. This promotes turbulence in the air moving near the wing, which prevents separated flow that would cause the dove to stall. (See also: how owls stay silent. Video credit: W. Hoebink and X. van der Sar, Vliegkunstenaars project)
(Source: vliegkunstenaars.nl)
Celebrating the physics of all that flows. 