In the 1920s, the world saw a new sort of marine propulsion, ships with one or more tall, smokeless cylinders. These Flettner rotors, named for their inventor, would spin in the wind, generating lift to propel the boat, much as a sail would. (The difference is that the rotor uses the Magnus effect.)
The market crash that kicked off the Great Depression spelled an end to the rotorship, but the idea is getting revived as industries search for greener forms of ship propulsion. Although the Flettner rotor still uses fuel (to spin the rotor), it can complete a voyage on only a small fraction of the fuel needed for conventional propulsion. (Image credit: Getty Images; via PopSci)
Like many sports that feature balls, spin plays a big role in tennis. By imparting a topspin or backspin to a tennis ball, players can alter the ball’s trajectory after a bounce and, using the Magnus effect to alter lift around the ball, change how it travels through the air. For example, a ball hit with backspin can dive just after the net, forcing an opponent to scramble after it. How much spin a player can impart depends on the speed of the racket’s head. Competitive rackets are carefully engineered — in terms of weight, string tension, and frame stiffness — to translate the kinetic energy of a player’s swing into the ball. But aerodynamics also play a role: new rackets designed to minimize drag hit the market 15-20 years ago, promising drag reductions up to 24% compared to previous rackets. That gives a player more swing speed and higher spins at a lower energy cost. (Image credit: C. Costello)
Spin, or the lack thereof, plays a major role in many sports — including tennis, golf, football, baseball, volleyball, and table tennis — because it affects whether flow stays attached around a ball, as well as how much lift or side force a ball gets. A ball’s spin doesn’t stay constant, however. During flight, a ball’s spin decays at a rate proportional to its initial spin and velocity. Researchers have found that a ball’s moment of inertia, flow regime, and surface roughness all affect that decay, but which factor is the most significant varies by ball and by sport.
Whether a ball bounces while spinning also matters. For compliant balls on a non-compliant surface — think tennis balls on a court — a bounce can actually change how much a ball spins. During impact, a tennis ball can: slide, decreasing its tangential velocity while increasing its topspin; roll, where the ball’s tangential velocity matches the tangential velocity of the surface; or over-spin, where the ball spins faster than it rolls. For a given impact angle and velocity, researchers found that stiffer and/or lighter balls were more likely to over-spin. Within tennis’s allowable range of ball stiffness and mass, manufacturers could create tennis balls that over-spin far more than conventional ones, creating another opportunity for deceptive tactics in the sport. (Image credit: J. Calabrese; research credit: T. Allen et al.)
For a long time, people thought baseball aerodynamics were simply a competition between gravity and the Magnus effect caused when a ball is spinning. But the seams of a baseball are so prominent that they, too, have a role to play. Here’s a baseline image of flow around a non-spinning baseball:
An non-spinning baseball with a straight, unaltered wake.
As in our previous post on golf, the colors indicate the direction of vorticity but don’t matter much to us here. What’s important is that the wake behind the ball is straight, indicating that there is no additional force beyond gravity and drag acting on the ball. Contrast this to the spinning baseball below:
Flow around a baseball spinning clockwise.
This ball is spinning in a clockwise motion, which causes flow to separate from the ball earlier on the advancing (bottom) side and later on the retreating (top) side. As a result, the wake is tilted downward. This indicates an upward force on the ball, caused by the Magnus effect.
But what if the seams fall in a place where they affect the flow? Here’s another baseball that’s not spinning:
Flow around a non-spinning baseball with a seam-shifted wake caused by early separation on the top surface of the baseball.
Notice that seam sitting just past the widest point on the top of the baseball. Flow around that wide point (called the shoulder) is very sensitive to disturbances essentially because the boundary layer is just barely hanging on to the ball. The blue arrow marks where the boundary layer separates from the ball on the top, which takes place earlier than the flow separation on the bottom, marked by the red arrow. As a result, the wake of the ball is tilted upward, indicating a downward force on the ball. The researchers who first proved this effect call it a seam-shifted wake, and it turns out to be a very common effect in baseball. They’ve got a great blog dedicated to baseball aerodynamics where you can learn tons more if you’re interested. (Image credit: top – Pixabay, others – B. Smith; research credit: B. Smith; see also Baseball Aerodynamics)
Today wraps up our Olympic coverage, but if you missed our earlier posts, you can find them all here.
Golf returned to the Olympics in 2016 in Rio and is back for the Tokyo edition. Golf balls — with their turbulence-promoting dimples — are a perennial favorite for aerodynamics explanations because, counterintuitively, a dimpled golf ball flies farther than a smooth one. But today we’re going to focus on a different aspect of golf aerodynamics, namely, what happens when a golf ball is spinning. Here’s an animation showing the difference between flow around a non-spinning golf ball and flow around a golf ball spinning at 3180 rpm. Both balls are moving to the left at 30 m/s.
Animation toggling between a non-spinning and spinning golf ball moving at 30 m/s.
The colors in this image indicate the direction of vorticity (which is unimportant for us at the moment). What matters are the blue and red arrows, which mark where flow is leaving the surface of the golf ball, in other words, where the wake begins. For the non-spinning golf ball, flow leaves the ball at the same streamwise position on both sides of the ball. This gives a symmetric wake that is neither tilted upward nor downward.
On the spinning ball, though, the blue arrow on top of the ball moves backward, indicating that separation occurs later. On the lower surface, the red arrow moves forward, so separation happens earlier. These shifts cause the golf ball’s wake to tilt downward, which — by Newton’s Third Law — tells us that the ball is experiencing an upward force. This is known as the Magnus effect, and it plays a big role in soccer, volleyball, tennis, and any other sports with spinning balls.
Skipping stones across water has fascinated humans for millennia, but incredibly, we’re still uncovering the physics of this game today. A recent paper built and experimentally validated a mathematical model of a spinning, skipping disk. The authors found that, in order to skip, a stone needs to generate upward acceleration greater than 3.8 times gravity.
To get that lift, the stone needs both the Magnus effect and the gyro effect. The Magnus effect is an aerodynamic force generated by an object spinning in a fluid that curves it away from its direction of travel — it’s what curves a corner kick into the goal in a soccer match. The gyro — or gyroscopic — effect also has to do with spinning, but it’s a result of conservation of angular momentum. Essentially, when you try to shift the axis that a rotating object spins around, there’s a force that resists that change. (The classic demo for this uses a spinning bicycle wheel.)
In stone skipping, the gyro effect helps stabilize the stone’s bounce and, if it’s spinning fast enough, keeps its direction of travel straight. Once the stone’s spinning slows, the Magnus effect can start to curve its trajectory. (Image credit: B. Davies; research credit: J. Tang et al.; via Physics World; submitted by Kam-Yung Soh)
This short film for the 2016 Gallery of Fluid Motion features Montana State University students experiencing fluid dynamics in the classroom and in their daily lives. As in her previous film (which we deconstructed), Shanon Reckinger aims to illustrate some of our everyday interactions with fluids. This time identifying individual phenomena is left as an exercise for the viewer, but there are hints hidden in the classroom scenes. How many can you catch? I’ve labeled some of the ones I noticed in the tags. (Video credit: S. Reckinger et al.)
Many sports use spherical balls, but the small size and weight of a table tennis ball makes it the one where aerodynamics have the strongest effect. Spin also plays a big role in the game by creating asymmetry in the flow around the ball.
Consider a table tennis ball with topspin, meaning that its upper surface is rolling in the direction of travel. That means that air flowing over the top of the ball is moving in the opposite direction as the ball’s surface. This will tend to make the flow separate from the ball at its widest point.
On the other side, the ball’s surface is spinning in the same direction as the air flow. This helps hold the air to the surface so that it follows the curve of the ball longer and doesn’t detach until well after the ball’s widest point. As a result of both these effects, air flowing around the ball experiences a net upward force, which in turn pushes the table tennis ball downward. This is known as the Magnus effect, and it plays a significant role in many sports. (Image credits: GettyImages; AFP)
We play with fluid dynamics all the time, though we don’t always think of it as such. Here are a few ways it shows up in the ways we play:
Aerodynamics This is the study of air moving past an object. Whether you’re throwing a paper plane, flying a kite, or riding a bike, aerodynamics has an impact on what you’re doing.
Lift
Skipping a rock won’t work unless its impact generates some lift, but we see lift in lots of other places, too, from birds and planes to racecars and sailboats.
Magnus Effect
The Magnus effect relates to lift forces on a spinning object. It can affect the way a frisbee flies, but we see it a lot in ball-related sports, too. The flight of golf balls, volleyballs, baseballs, and soccer balls can all be significantly impacted by the Magnus effect. Check out these videos for a primer on the Magnus effect and the reverse Magnus effect.
Bubbles
Everybody loves playing with bubbles. But they may have more of a impact than you realize, whether it’s in making the foam on your latte, enhancing the aroma of your champagne, or making your joints pop.
Tune in all week for more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)
A good soccer player can kick the ball from the corner of the field into the goal thanks to the Magnus effect. But if you’ve ever tried to play soccer with a smooth ball, you may have noticed that sometimes the ball bends the wrong way! This is the reverse Magnus effect and it’s caused when the boundary layers on either side of the ball switch from turbulent to laminar flow at different times. Dianna Cowern explains (with a little help from yours truly) in the video above. Want to learn more about how roughness affects boundary layers? Check out our companion video on FYFD’s YouTube channel. (Video credit: D. Cowern/Physics Girl)
