The fluid dynamics of a commercial-quality blender amount to a lot more than just stirring. Here high-speed video shows how the blender’s moving blades create a suction effect that pulls contents down through the middle of the blender, then flings them outward. This motion creates large shear stresses, which help break up the food, as well as turbulence that can mix it. But if you watch carefully, you’ll also see tiny bubbles spinning off the blades. These bubbles, formed by the pressure drop of fluid accelerated over the arms of the blades, are cavitation bubbles. When they collapse, or implode, they create localized shock waves that further break up the blender’s contents. This same effect is responsible for damage to boat propellers and lets you destroy glass bottles. (Video credit: ChefSteps; via Wired; submitted by jshoer)
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Tuning Fork Fluids
This high-speed video shows a liquid crystal fluid vibrating on a tuning fork. As the surface moves, tiny jets shoot upward, sometimes with sufficient energy that the fluid column is stretched beyond surface tension’s ability to keep it intact, resulting in droplet ejection. The jets and surface waves create a mesmerizing pattern of fluid motion. (Video credit: J. Savage)
Turbulent Flames
The flames surrounding a burning tree stump flicker and billow in this image from photographer Serdar Ozturk. The chaotic motion of the flames is indicative of turbulence, a state of fluid flow known for its many scales. Note the range of lengthscales and structures in the fire. In turbulent flows, kinetic energy cascades from large scales, like the width of the top of the plume, down to the small scales, which may be even smaller than the wisps of flame at the edges of the fire. At the largest scales, the structures and behaviors we observe are all flow- and geometry-dependent, but theory predicts that, at the smallest scales, all turbulent flows look the same. (Photo credit: trashhand/Serdar Ozturk)
Humpback-Inspired Turbine Blades
The bumps–or tubercles–on the edge of a humpback whale’s fins have important hydrodynamic effects on its swimming. Here dye is used to visualize flow over a hydrofoil with tubercle-like protuberances–a sort of artificial whale fin. Dye released from the peaks and troughs of the protuberances flows straight back in a narrow line before breakdown to turbulence. But the dye released from ports on the shoulders of the protuberances twists and spirals into vortices. At angle of attack, these vortices are stronger. They may help keep flow from separating on the upper side of a whale’s fin. (Photo credits: SIDwilliams, H. Johari)
Dye Flow
Fluid flow near a surface–inside the boundary layer–can often be unstable. This image shows one possible instability, formed when a cylinder is rotated back and forth about its longitudinal axis. This oscillation and the curvature of the cylinder destabilize flow in the boundary layer, forming vortices that line the cylinder. This particular behavior is called a Görtler instability. To visualize it, threads soaked in fluorescing dye have been embedded into slits in the cylinder. The cylinder is oscillated in a water tank and ultraviolet light is used to fluoresce the dye for the image. (Photo credit: Miguel Canals/University of Hawaii)
Fishbones
When two liquid jets collide, they can form an array of shapes ranging from a chain-like stream or a liquid sheet to a fishbone-type structure of periodic droplets. This series of images show the collision of two viscoelastic jets–in which polymer additives give the fluids elasticity properties unlike those of familiar Newtonian fluids like water. The jet velocities increase with each image, changing the behavior from a fluid chain (a and b); to a fishbone structure (c and d); to a smooth liquid sheet (e); to a fluttering sheet (f and g); to a disintegrating ruffled sheet (h), and finally a violently flapping sheet (i and j). The behavior of such jets is of particular interest in problems of atomization, where it can be desirable to break an incoming stream of liquid up into droplets as quickly as possible. (Photo credit: S. Jung et al.)
Laser-Induced Fluorescence
As demonstrated in the video above, lasers can be used to excite molecules into a higher energy state, which will decay via the emission of photons, causing the medium to glow. This laser-induced fluorescence is utilized in several techniques for measurements in fluid dynamics, including planar laser-induced fluorescence (PLIF) and molecular tagging velocimetry (MTV). In these techniques a flow is usually seeded with a fluorescing material–nitric oxide is popular for super- and hypersonic flows–and then lasers are used to excite a slice of the flow field. The resulting fluorescence can be used for both qualitative and quantitative flow measurements. Here are a couple of examples, one in low-Reynolds number flow and one in combustion. (Video credit: L. Martin et al./UC Berkeley)
Jump in a Lake
Ever wonder what would happen if every person on earth jumped into a lake at the same time? Wonder no more! Physicist Rhett Allain breaks it down over at Dot Physics.
Liquid Sculptures
Artist Corrie White uses dyes and droplets to capture fantastical liquid sculptures at high-speed. The mushroom-like upper half of this photo is formed when the rebounding jet from one droplet’s impact on the water is hit by a well-timed second droplet, creating the splash’s umbrella. In the lower half of the picture, we see the remains of previous droplets, mixing and diffusing into the water via the Rayleigh-Taylor instability caused by their slight difference in density relative to the water. There’s also a hint of a vortex ring, likely from the droplet that caused the rebounding jet. (Photo credit: Corrie White)
Stalling
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At high angles of attack, the flow around the leading edge of an airfoil can separate from the airfoil, leading to a drastic loss of lift also known as stall. Separation of the flow from the surface occurs because the pressure is increasing past the initial curve of the leading edge and positive pressure gradients reduce fluid velocity; such a pressure gradient is referred to as adverse. One way to prevent this separation from occurring at high angle of attack is to apply suction at the leading edge. The suction creates an artificial negative (or favorable) pressure gradient to counteract the adverse pressure gradient and allows flow to remain attached around the shoulder of the airfoil. Suction is sometimes also used to control the transition of a boundary layer from laminar to turbulent flow.