Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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The hummingbird has long been admired for its ability to hover in flight. The key to this behavior is the bird’s capability to produce lift on both its downstroke and its upstroke. The animation above shows a simulation of hovering hummingbird. The kinematics of the bird’s flapping—the figure-8 motion and the twist of the wings through each cycle—are based on high-speed video of actual hummingbirds. These data were then used to construct a digital model of a hummingbird, about which scientists simulated airflow. About 70% of the lift each cycle is generated by the downstroke, much of it coming from the leading-edge vortex that develops on the wing. The remainder of the lift is creating during the upstroke as the bird pulls its wings back. During this part of the cycle, the flexible hummingbird twists its wings to a very high angle of attack, which is necessary to generate and maintain a leading-edge vortex on the upstroke. The full-scale animation is here. (Image credit: J. Song et al.; via Wired; submitted by averagegrdy)

The hummingbird has long been admired for its ability to hover in flight. The key to this behavior is the bird’s capability to produce lift on both its downstroke and its upstroke. The animation above shows a simulation of hovering hummingbird. The kinematics of the bird’s flapping—the figure-8 motion and the twist of the wings through each cycle—are based on high-speed video of actual hummingbirds. These data were then used to construct a digital model of a hummingbird, about which scientists simulated airflow. About 70% of the lift each cycle is generated by the downstroke, much of it coming from the leading-edge vortex that develops on the wing. The remainder of the lift is creating during the upstroke as the bird pulls its wings back. During this part of the cycle, the flexible hummingbird twists its wings to a very high angle of attack, which is necessary to generate and maintain a leading-edge vortex on the upstroke. The full-scale animation is here. (Image credit: J. Song et al.; via Wired; submitted by averagegrdy)

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.)

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)

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 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 rotatingThe 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)

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.)

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)

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)

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.)

Asker tarastarr1 Asks:
For the (awesome) wave gif and explanation, I think the asker was wondering about that little branch-like projection you can see in the top-middle part of the gif right after the camera submerges. Your explanation of the wave is great, but now I'm also wondering: if the wave is so powerful, how can that little jet form?
fuckyeahfluiddynamics fuckyeahfluiddynamics Said:

I think you’re probably right about the original question. I actually didn’t even notice that tiny vortex until after the post went up today! I think the little vortex is probably similar to the rib vortices I referenced at the end of the last post. If there happened to be some small localized rotation in the water initially, the wave’s passage would stretch it out. Stretching a vortex causes it to spin faster, exactly the way that an ice skater pulling her arms in causes her to spin faster - conservation of angular momentum! In that situation, the wave’s passage actually strengthens the vortex rather than destroying it.