“The Flow II” film by Bose Collins and colleagues features a ferrofluid, a magnetically-sensitive liquid made up of a carrier fluid like oil and many tiny, ferrous nanoparticles. Although ferrofluids are known for many strange behaviors, their most distinctive one is the spiky appearance they take on when exposed to a constant magnetic field. This peak-and-valley structure is known as the normal-field instability. It’s the result of the fluid attempting to follow the magnetic field lines upward. Gravity and surface tension oppose this magnetic force, allowing the fluid to be drawn upward only so far until all three forces balance. (Video credit: B. Collins et al.)
Tag: fluid dynamics
Typhoon Neoguri
Astronaut Reid Wiseman has been posting photos of Typhoon Neoguri in his Twitter feed this week. From our perspective on the ground, it’s easy to forget how three-dimensional the typhoons and hurricanes in our atmosphere are. But Wiseman’s photos capture the depth in the storm, especially the depression of the eye. From the top, the typhoon looks much like a vortex in a bathtub, or what’s more formally known as a free surface vortex. To understand why a vortex dips in the middle, imagine a container of water on a rotating plate. As the water is spun, its interface with the air takes on a paraboloid shape. Two external forces are acting on the fluid: gravity in the downward direction and a centrifugal force in the radial direction. The free surface of the fluid adopts a shape that is always perpendicular to the combination of these two forces. This ensures that the pressure along the free surface is a constant. (Photo credits: R. Wiseman 1,2,3)
Giant Bubbles
In their latest video, Gavin and Dan of The Slow Mo Guys demonstrate what giant bubbles look like in high-speed video from birth to death. Surface tension, which arises from the imbalance of intermolecular forces across the soapy-water/air interface, is the driving force for bubbles. As they move the wand, cylindrical sheets of bubble film form. These bubble tubes undulate in part because of the motion of air around them. In a cylindrical form, surface tension cannot really counteract these undulations. Instead it drives the film toward break-up into multiple spherical bubbles. You can see examples of that early in the video. The second half of the video shows the deaths of these large bubble tubes when they don’t manage to pinch off into bubbles. The soap film tears away from the wand and the destructive front propagates down the tube, tearing the film into fluid ligaments and tiny droplets (most of which are not visible in the video). Instead it looks almost as if a giant eraser is removing the outer bubble tube, which is a pretty awesome effect. (Video credit: The Slow Mo Guys)
Waterspouts
Waterspouts are commonly thought of as tornadoes over water, but this is only partially true. Some waterspouts do begin as tornadoes, but waterspouts are more commonly non-tornadic or fair-weather in origin. These non-tornadic waterspouts form when cold, dry air moves over warm water. As the warm, moist air rises, entrainment and conservation of angular momentum cause the air nearby to begin rotating. The spout does not actually suck water up from the surface. Instead, the humid rising air cools and the water vapor condenses, forming the cloud wall of the spout. Waterspouts are typically very short-lived and last 5 to 10 minutes before the inflowing air cools and the vortex weakens and dissipates. (Photo credit: U.S. Navy/K. Wasson)
Blood Flow Simulations
Though we may not often consider it, our bodies are full of fluid dynamics. Blood flow is a prime example, and, in this video, researchers describe their simulations of flow through the left side of the heart. Beginning with 3D medical imaging of a patient’s heart, they construct a computational domain – a meshed virtual heart that imitates the shape and movements of the real heart. Then, after solving the governing equations with an additional model for turbulence, the researchers can observe flow inside a beating heart. Each cycle consists of two phases. In the first, oxygenated blood fills the ventricle from the atrium. This injection of fresh blood generates a vortex ring. Near the end of this phase, the blood mixes strongly and appears to be mildly turbulent. In the second phase, the ventricle contracts, ejecting the blood out into the body and drawing freshly oxygenated blood into the atrium. (Video credit: C. Chnafa et al.)
Tip Vortex
Smoke released from the end of a test blade shows the helical pattern of a tip vortex from a horizontal-axis wind turbine. Like airplane wings, wind turbine blades generate a vortex in their wake, and the vortices from each blade can interact downstream as seen in this video. These intricate wakes complicate wind turbine placement for wind farms. A turbine located downstream of one of its fellows not only has a decreased power output but also has higher fatigue loads than the upstream neighbor. In other words, the downstream turbine produces less power and will wear out sooner. Researchers visualize, measure, and simulate turbine wakes and their interactions to find ways of maximizing the wind power generated. (Photo credit: National Renewable Energy Laboratory)
Fireworks Taking Off
Aerial fireworks are essentially semi-controlled exploding rockets. Here Discovery Channel shares high-speed video of fireworks taking off. The turbulent billowing exhaust on the ground is reminiscent of other rocket launches. The tube-launched firework clip is a great example of an underexpanded nozzle. The pressure of the gases in the tube is higher than the ambient air, so when the gases escape, the exhaust fans out to equalize the pressure. And, finally, the explosion that propels the colorful chemicals outward forms jets that can affect the final form of the display. To my American readers: Happy 4th of July! And be safe! (Video credit: Discovery Slow-Down)
Soap Film Grass
In the summer months, a breeze can set long grasses waving in an impressive display. Similar behaviors are seen in aquatic plants during tides. Researchers simulate the behavior in two-dimensions using a flowing soap film and nylon filaments. Flow visualization reveals the strong differences between flow above and between the grass. Vortices recirculate between the filaments at speeds much slower than the flow overhead. The instantaneous interaction of the high-speed freestream, the unsteady vortices, and the resistance of the grass results in familiar synchronous waves of grain. (Video credit: R. Singh et al.)
Reader Question: Wave Vortex
Reader unquietcode asks:
I saw this post recently and it made me wonder what’s going on. If you look in the upper right of the frame as the camera submerges, you can see a little vortex of water whirring about. Even with the awesome power of the wave rolling forward a little tornado of water seems able to stably form. Any idea what causes this phenomenon?
This awesome clip was taken from John John Florence’s “& Again” surf video. What you’re seeing is the vortex motion of a plunging breaking wave. As ocean waves approach the shore, the water depth decreases, which amplifies the wave’s height. When the wave reaches a critical height, it breaks and begins to lose its energy to turbulence. There are multiple kinds of breaking waves, but plungers are the classic surfer’s wave. These waves become steep enough that the top of the wave overturns and plunges into the water ahead of the wave. This generates the vortex-like tube you see in the animation. Such waves can produce complicated three-dimensional vortex structures like those seen in this video by Clark Little. Any initial variation in the main vortex gets stretched as the wave rolls on, and this spins up and strengthens the rib vortices seen wrapped around the primary vortex. (Source video: B. Kueny and J. Florence)
Measuring Wind Speed by Satellite
Weather modeling and forecasting in recent decades have benefited enormously from the availability of more data. For example, satellites now measure wind speeds over the open ocean, instead of data simply coming from isolated ships and buoys. The satellites do this by measuring the roughness of the ocean using radar or GPS signals bounced off the ocean surface. From this researchers can construct a map of wave height and direction like the one in the animation above. For a large body of water, waves are primarily generated by wind shearing the water at the interface. The waves we see are a result of the Kelvin-Helmholtz instability between the wind and ocean. Because this is a well-known behavior, it is possible to connect the waves we observe with the wind conditions that must have generated them. (Image credit: ESA; animation credit: Wired; submitted by jshoer)
