FYFD

Tag: football

  • The Knuckleball

    The Knuckleball

    For more than a century, athletes have used the zigzagging path of a knuckleball to confound their opponents. Knuckleballing is best known in baseball but appears also in volleyball, soccer, and cricket. It occurs when the ball has little to no spin. The source of the knuckleball’s confusing trajectory, according to a new study, is the unsteadiness of the lift forces around the ball. As the ball flies, tiny variations occur in the flow on either side, causing small variations to the lift as well. Using experiments and numerical models, the researchers established that this white noise in the lift forces is sufficient to cause knuckleball-like path changes.

    They were also able to explain why some sports see the knuckleball effect and others don’t. The wavelength of the deviations – the distance between a zig and a zag – is relatively long, so knuckleballing can only be noticed if the distance the ball flies is long enough for the deviation to be apparent. Additionally, the side-to-side motion is largest when flow on the ball is transitioning from laminar to turbulent flow, so knuckleballing also requires a very particular (and usually low) initial speed. (Image credit: L. Kang; research credit: B. Texier et al.; submitted by @1307phaezr)

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    Fluids Round-Up

    Time for another fluids round-up! Here’s some of the best fluid dynamics from around the web:

    – Band Ok Go filmed their latest music video in microgravity, complete with floating, splattering fluids. Here they describe how they did it. Rhett Allain also provides a write-up on the physics.

    – Scientists are trying to measure the impact of airliners’ contrails on climate change. (pdf; via @KyungMSong)

    – Researchers observing the strange moving hills on Pluto suspect they may, in fact, be icebergs.

    – The best angle for skipping a rock is 20-degrees. Related: elastic spheres skip well even at higher angles. (via @JenLucPiquant)

    – Fluid dynamics and acoustics have some fascinating overlaps. Be sure to check out “The World Through Sound” series at Acoustics Today, written by Andrew “Pi” Pyzdek, who also writes one of my favorite science blogs.

    – Over at the Toast, Mallory Ortberg explores the poetry of the Beaufort wind scale.

    – Could dark matter be a superfluid? (via @JenLucPiquant)

    – Understanding the physics of the perfect pancake is helping doctors treat glaucoma. (submitted by Maria-Isabel)

    – Van Gogh’s “Starry Night” shows swirling skies, but just how turbulent are they? (submitted by @NathanMechEng)

    – The physics (and fluid dynamics!) of throwing a football – what’s the best angle for a maximum distance throw? (submitted by @rjallain)

    (Video credit: Ok Go)

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  • American Football Aerodynamics

    American Football Aerodynamics

    Like many sports balls, the American football’s shape and construction make a big difference in its aerodynamics. Unlike the international football (soccer ball), which undergoes significant redesigns every few years thanks to the World Cup, the American football has been largely unchanged for decades. The images above come from a computational fluid dynamics (CFD) simulation of a spiraling football in flight. Although the surface is lightly dimpled, the largest impact on aerodynamics comes from the laces and the air valve (just visible in the upper right image). Both of these features protrude into the flow and add energy and turbulence to the boundary layer. By doing so, they help keep flow attached along the football longer, which helps it fly farther and more predictably. For more, check out the video of the CFD simulation. (Image credits: CD-adapco; via engineering.com)

  • The Magnus Effect in Football

    The Magnus Effect in Football

    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)

  • Brazuca

    Brazuca

    Since 2006, Adidas has unveiled a new football design for each FIFA World Cup. This year’s ball, the Brazuca, is the first 6-panel ball and features glued panels instead of stitched ones. It also has a grippy surface covered in tiny nubs. Wind tunnel tests indicate the Brazuca experiences less drag than other recent low-panel-number footballs as well as less drag than a conventional 32-panel ball. Its stability and trajectory in flight are also more similar to a conventional ball than other recent World Cup balls, particularly the infamous Jabulani of the 2010 World Cup. The Brazuca’s similar flight performance relative to a conventional ball is likely due to its rough surface. Like the many stitched seams of a conventional football, the nubs on the Brazuca help trip flow around the ball to turbulence, much like dimples on a golf ball. Because the roughness is uniformly distributed, this transition is likely to happen simultaneously on all sides of the ball. Contrast this with a smooth, 8-panel football like the Jabulani; with fewer seams to trip flow on the ball, transition is uneven, causing a pressure imbalance across the ball that makes it change its trajectory. For more, be sure to check out the Brazuca articles at National Geographic and Popular Mechanics, as well as the original research article. (Photo credit: D. Karmann; research credit: S. Hong and T. Asai)

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    London 2012: Soccer Aerodynamics

    Corner kicks and free kicks are tough to defend in football (soccer for Americans) because the ball’s trajectory can curve in a non-intuitive fashion. Known as the Magnus effect, the fluid dynamics around a spinning ball cause this curvature in the flight path. When an object spins while moving through the fluid, it drags the air near the surface with it. On one side of the spinning ball, the motion opposes the direction of freestream airflow, causing a lower relative velocity, and on the opposite side, the spin adds to the airflow, creating a higher velocity. According to Bernoulli’s principle, this causes a lower pressure on the side of the ball spinning with the flow and a higher pressure on other side. This difference in pressure results in a force acting perpendicular to the direction of travel, causing the unexpected curvature in the football’s path. In the case of the corner kick above, the player kicks the ball from the right side, imparting an anti-clockwise spin when viewed from above. As the ball travels past the goal, air is moving faster over the side nearest the goal and slower on the opposite side. The difference in velocities, and thus pressures, creates the sideways force that drives the ball into the goal even without touching another player. The same effect is used in many other sports to complicate play and confuse opponents. In tennis and volleyball, for example, topspin is used to make the ball drop quickly after passing the net.

    ETA: Check out this other great example of a free kick sent in by reader amphinomos.

    FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out some of our previous posts including the flight of a javelin, how divers reduce splash, and what makes a racing hull fast.