Over at Smarter Every Day, Destin has a new video, this time about how fish eat, which involves some pretty awesome physics. Instead of accelerating their entire body to close the distance to prey, fish thrust their jaws forward. As they do, they open their mouth, expanding the volume there and lowering the pressure. This causes water to flow into their mouth, pulling the prey with it. But the water has momentum, which would push the fish backward. To prevent this, the fish then opens its gills, allowing the water to rush back out while trapping the prey in its mouth. Be sure to check out Destin’s video so that you can see the process in high-speed. (Video credit: Smarter Every Day)
Videos
“The Flow II”
“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.)
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)
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.)
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.)
Specialized’s Win Tunnel
Awhile back, I mentioned that bike manufacturer Specialized had built their own wind tunnel to test cycling equipment. In this video, they provide a walk-through of their facility. Although there are features unique to this tunnel and its intended purpose, much of what Chris and Mark describe is standard for any subsonic wind tunnel. The story begins upstream in the inlet and contraction, where air is pulled into the tunnel. Honeycomb flow straighteners direct the incoming air, followed by a series of mesh screens. These screens break up any turbulent eddies, which helps smooth and laminarize the flow. The test section is where measurements occur, whether on cyclists or other models. This part of the tunnel is usually equipped with many sensors and specialized equipment, like the balance shown. These allow researchers to measure quantities like force, velocity, pressure, and/or temperature. Then the wind tunnel widens gradually in a diffuser, which slows down the air and helps prevent disturbances from propagating upstream. Finally, the fans at the back provide the source of low-pressure that drives the air flow. (Video credit: Specialized Bicycles; submitted by J. Salazar)
Steam Hammer
The steam hammer phenomenon–and the closely related water hammer one–is a violent behavior that occurs in two-phase flows. Nick Moore has a fantastic step-by-step explanation of the physics, accompanied by high-speed footage, in the video above. Pressure and temperature are driving forces in the effect, beginning with the high-temperature steam that first draws the water up into the bottle. As the steam condenses into the cooler water, the steam’s pressure drops, drawing in more water. Eventually it drops low enough that the incoming water drops below the vapor pressure. This triggers some very sudden thermodynamic changes. The drop in pressure vaporizes incoming water, but the subsequent cloud cools rapidly, which causes it to condense but also drops the pressure further. Water pours in violently, cavitating near the mouth of the bottle because the acceleration there drops the local pressure below the vapor pressure again. The end result is a flow that’s part-water, part-vapor and full of rapid changes in pressure and phase. As you might imagine, the forces generated can destroy whatever container the fluids are in. Be sure to check out Nick’s bonus high-speed footage to appreciate every stage of the phenomenon. (Video credit and submission: N. Moore)
Turbojet Engines
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GE has a great new video with a straightforward explanation of the turbojet and the turbofan engines. The simplest description of the engines–suck, squeeze, bang, blow–sounds like a euphemism but it’s fairly accurate. The engines draw in air, compress it by making it flow through a series of small rotating blades, add fuel and combust the mixture, pull out energy through a turbine, and then blow the high-speed exhaust out the back to generate thrust. The thrust is key because it’s the force that overcomes drag on the plane and also generates the speed needed to create lift. There are two ways to significantly increase thrust: a) increase the mass flow rate of air through the engine, and/or b) increase the exhaust velocity. The turbojet engine draws in smaller amounts of air but generates very high exhaust velocities. The turbofan is today’s preferred commercial aircraft engine because it can generate thrust more efficiently at the desired aircraft velocity. The turbofan essentially has a turbojet engine in its center and is surrounded by a large air-bypass. Most of the air passing through the engine flows through the bypass and the fan. This increases its velocity only slightly, but it means that the engine accelerates much larger amounts of air without requiring much larger amounts of fuel. As an added bonus, the lower exhaust velocities of the turbofan engine make it much quieter in operation. (Video credit: General Electric)
Jupiter Timelapse
This timelapse video shows Jupiter as seen by Voyager 1. In it, each second corresponds to approximately 1 Jupiter day, or 10 Earth hours. Be sure to fullscreen it so that you can appreciate the details. The timelapse highlights the differences in velocity (and even flow direction!) between Jupiter’s cloud bands. It is these velocity differences that create the shear forces which cause Kelvin-Helmholtz instabilities–the series of overturning eddies–seen between the bands. Earth also has bands of winds moving in opposite directions, but there are fewer of them and the composition of our atmosphere is such that they do not make for such a dramatic naked eye view of large-scale fluid dynamics. (Video credit: NASA/JPL/B. Jónsson/I. Regan)
