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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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)

Reader 3d-time asks:

Hi, there is a guy, at my college, who is doing a master’s degree thesis in turbulence. He says he uses fractals and computational methods. Can you explain how fractals can be used in fluid dynamics?

That’s a good question! Fractals are a relatively recent mathematical development, and they have several features that make them an attractive tool, especially in the field of turbulence. Firstly, fractals, especially the Mandelbrot set shown above, demonstrate that great complexity can be generated out of simple rules or equations. Secondly, fractals have a feature known as self-similarity, meaning that they appear essentially the same regardless of scale. If you zoom in on the Mandelbrot set, you keep finding copy after copy of the same pattern. Nature, of course, doesn’t have this perfect infinite self-similarity; at some point things break down into atoms if you keep zooming in. But it is possible to have self-similarity across a large range of scales. This is where turbulence comes in. Take a look at the turbulent plume of the volcanic eruption in the photo above. Physically, it contains scales ranging from hundreds of meters to millimeters, and these scales are connected to one another by their motion and the energy being passed from one scale to another. There have been theories suggested to describe the relationship between these scales, but no one has yet found a theory truly capable of explaining turbulence as we observe it. Both the self-similarity and the complex nature of fractals suggest they could be useful tools in finally unraveling turbulence. In fact, Mandelbrot himself wrote several papers connecting the two concepts. Perhaps your friend will help find the next hints!  (Image credit: U.S. Geological Survey, Wikimedia)

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)

Photographers Cassandra Warner and Jeremy Floto produced the "Clourant" series of high-speed photographs of colorful liquid splashes. The artists took special care to disguise the origin of splashes, making them appear like frozen sculptures. The photos are beautiful examples of making fluid effects and instabilities. Many of them feature thin liquid sheets with thicker rims just developing ligaments. In other spots, surface tension has been wholly overcome by momentum’s effects and what was once ligaments has exploded into a spray of droplets. (Photo credit: C. Warner and J. Floto; submitted by jshoer; via Colossal)

Like many sports, the gameplay in football can be strongly affected by the ball’s spin. Corner kicks and free kicks can curve in non-intuitive ways, making the job of the goalie much harder. These seemingly impossible changes in trajectory are due to airflow around the spinning ball and what’s known as the Magnus effect. In the animation above, flow is moving from right to left around a football. As the ball starts spinning, the symmetry of the flow around the ball is broken. On top, the ball is spinning toward the incoming flow, and the green dye pulls away from the surface. This is flow separation and creates a high-pressure, low-velocity area along the top of the ball. In contrast, the bottom edge of the ball pulls dye along with it, keeping flow attached to the ball for longer and creating low pressure. Just as a wing has lift due to the pressure difference on either side of the wing, the pressure imbalance on the football creates a force acting from high-to-low pressure. In this case, that is a downward force relative to the ball’s rightward motion. In a freely moving football, this force would curve its trajectory to the side. (GIF credit: SkunkBear/NPR; original video: NASA Ames; via skunkbear)

Like many sports, the gameplay in football can be strongly affected by the ball’s spin. Corner kicks and free kicks can curve in non-intuitive ways, making the job of the goalie much harder. These seemingly impossible changes in trajectory are due to airflow around the spinning ball and what’s known as the Magnus effect. In the animation above, flow is moving from right to left around a football. As the ball starts spinning, the symmetry of the flow around the ball is broken. On top, the ball is spinning toward the incoming flow, and the green dye pulls away from the surface. This is flow separation and creates a high-pressure, low-velocity area along the top of the ball. In contrast, the bottom edge of the ball pulls dye along with it, keeping flow attached to the ball for longer and creating low pressure. Just as a wing has lift due to the pressure difference on either side of the wing, the pressure imbalance on the football creates a force acting from high-to-low pressure. In this case, that is a downward force relative to the ball’s rightward motion. In a freely moving football, this force would curve its trajectory to the side. (GIF credit: SkunkBear/NPR; original video: NASA Ames; via skunkbear)

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)

These photos are shadowgraphs of a hydrogen flame exploding inside a balloon. The shadowgraph optical technique highlights density and temperature variations through their effect on a fluid’s refractive index. Here we see that the hydrogen flame has a strong cellular structure and is more turbulent than a methane flame. The cellular structure is a sign of an instability in the curved flame front. The instability and accompanying cellular appearance are a result of the complicated transport and reaction of fuel and oxidizer inside the flame. (Photo credits: P. Julien et al.)

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)

The Marangoni effect is generated by variations in surface tension at an interface. Such variations can be temperature-driven, concentration-driven, or simply due to the mixing between fluids of differing surface tensions as is the case here. The pattern in the image above formed after a dyed water droplet impacted a layer of glycerin. The initial impact of the drop formed an inner circle and outer ring. This image is from 30 seconds or so after impact, after the Marangoni instability has taken over. The higher surface tension of the water pulls the glycerin toward it, resulting in a flower-like pattern. (Photo credit: E. Tan and S. Thoroddsen)

Chemical Bouillon’s art often mixes chemistry and fluid dynamics. Here dense UV dyes falling through a less dense fluid form long strings with mushroom-like caps or tree-like branches. (For reference, gravity is pointing up relative to the video frame in most clips.) This behavior is related to the Rayleigh-Taylor instability that deforms interfaces and causes mixing between unstably stratified fluids.  (Video credit: Chemical Bouillon)