Physical Ecology Lab

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.

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.

It’s also possible, under the right circumstances, to get a reverse Magnus effect. For more on that, check out this video and Smith’s analysis. (Image credit: top – M. Spiske, others – N. Sakib and B. Smith; research credit: N. Sakib and B. Smith, pdf)

We’re celebrating the Olympics with sports-themed fluid dynamics. Learn how surface roughness affects a volleyball serve, see the wingtip vortices of sail boats, and find out how to optimize rowing oars. And don’t forget to come back next week for more!

The sleek hulls of racing boats are designed to minimize drag, but there’s optimization to the oars as well. Mathematical models – and the history of rowing – indicate that shorter oars are more ideal for the sprint-style races seen in the Olympics. Shorter oars may be less efficient at transferring energy, but they’re easier to move quickly, and an athlete’s higher stroke rate more than makes up for the loss of efficiency. (Note that the advantage only holds for sprint events; in endurance events, a longer oar is preferable because holding a high stroke rate for a long time is difficult.)

Physicists have taken this a step further by building a mathematical model that predicts the optimal oar length for a given athlete, based on their height, strength, and other characteristics. They validated their modeling with a robotic rowboat. They note, however, that the effects are really only useful for elite rowers. Amateurs are better served by learning proper technique than they are by using an optimal length oar. (Image credit: J. Calabrese; research credit: R. Labbé et al.; via APS Physics)

We’re celebrating the Olympics with sports-themed fluid dynamics. Learn how surface roughness affects a volleyball serve, see the wingtip vortices of sail boats, and find out about the physics of surfing. And don’t forget to come back next week for more!

Surfing is making its Olympic debut this year with a shortboard competition held at Shidashita Beach, with the event’s timing determined by weather and wave quality. The fluid dynamics involved in surfing could easily fill their own series of posts, so we’ll just scratch the surface here. Check out the video embedded below for a nice overview.

We sometimes think of waves as enormous walls of water moving on the ocean, but the truth is that individual water particles move very little when a wave passes. Instead waves are a method of transferring energy through the water, and surfers harness this energy while negotiating a delicate balance of forces between gravity, buoyancy, and hydrodynamics.

So how do surfers catch a wave? After all, anyone who’s been to the beach or in a wave pool knows that waves can easily pass without carrying you along with them. To ride a wave, surfers orient themselves in the direction the wave is traveling, then they paddle to bring their velocity close that of the incoming wave. Their surfboard helps by providing a large surface for the water to push, accelerating the surfer as the wave approaches. The longer and larger a surfboard is, the less speed the surfer themself has to provide. This is one reason it’s easier to catch a wave on a longboard than on a shortboard. But shortboards — like those used by competitors in the Tokyo Olympics — are far more maneuverable, allowing surfers more freedom in the moves they choose to make as they ride. (Image credit: B. Selway; video credit: TED-Ed; see also M. Grissom and Science Connected)

Recent controversies over swimsuit technologies haven’t damped the creativity of Speedo’s marketing staff. They recently unveiled Fastskin 4.0, a futuristic concept designed for the swimmers of 2040*. They’ve envisioned a custom-made, biodegradable, self-powered swimsuit that looks like a superhero’s costume. Some of the technologies strike me as extremely pie-in-the-sky, but a few of them have at least some basis in reality. Of particular interest to us, of course, are the Dynamic Flow Zones and the Shark Skin Boosters, two features intended to minimize drag and boost speed.

The Dynamic Flow Zones seem to be part of a built-in exoskeleton around the swimmer’s midriff, and they are apparently inspired by the underbelly of whales. At least one study shows that similar ridges on whale sharks help reduce flow separation on their bodies, but — given the vastly different swim styles of a human and a whale shark — it’s unclear to me that these structures would help a human swimmer. It also seems as though their helpfulness would be strongly dependent on what stroke the swimmer was using.

As for the Shark Skin Boosters, a shark’s skin does, in fact, helps its speed and agility. Individual denticles on the shark can (passively) bristle when flow near the skin tries to reverse direction. The adaptation helps them shut down flow separation before it happens, thereby maintaining flow control and low drag. Additionally, studies of 3D-printed shark skin have shown that the right texture can provide a speed boost. It would take some work to figure out just the right texture to adapt the shark’s ability to a human swimmer, but this is one feature of Fastskin 4.0 that isn’t just science fiction. (Image and video credit: Speedo; via Gizmodo)

*To be 100% clear, this product does not exist and likely never will.

Join us all this week for more Olympic-themed fluid dynamics!