Month: July 2021

Tokyo 2020: Optimizing Oar Length

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!
July 30, 2021
Tokyo 2020: Surf Physics
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)
July 29, 2021
Tokyo 2020: Future Swim Tech
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!
July 28, 2021
Tokyo 2020: Sailing Physics
At first glance, sailboats don’t look much like an airplane, but physics-wise, they’re closely related. Both the sail and hull of a sailboat act like wings turned on their side. Just as with airplane wings, the driving force for a sail comes from a difference in pressure across the two sides of the sail. The same effects applied to the hull and its keel (the wing-like extension that sits below the hull) provide the force that keeps a sailboat from slipping sideways as it cuts a path through wind and water.
Like airplane wings, sailboats also generate tip vortices: one from the top of the sail, the other from the bottom of the keel. Those vortices are typically invisible, but in foggy weather, like in the photo below, you can see the tracks they leave behind. (Image credits: top – Ludomił; bottom – D. Forster; research credit: B. Anderson; submitted by Lluís J.)
Follow along all this week and next as we celebrate the Olympics with sports-themed fluid dynamics.
July 27, 2021
Tokyo 2020: Volleyball Aerodynamics
Like footballs and baseballs, the trajectory of a volleyball is strongly influenced by aerodynamics. When spinning, the ball experiences a difference in pressure on either side, which causes it to swerve, per the Magnus effect. But volleyball also has the float serve, which like the knuckleball in baseball, uses no spin.
In this case, how the ball behaves depends strongly on the way the ball is made. Some volleyballs use smooth panels, while others have surfaces modified with dimples or honeycomb patterns, and researchers found that these subtle changes make a big difference in aerodynamics. A float serve’s trajectory is unpredictable because the ball will swerve whenever air near the surface of the ball on one side goes turbulent or separates. And without spin to influence that transition, everything comes down to the ball’s speed and its surface.
Researchers found that volleyballs with patterned surfaces transition to turbulence at lower speeds, which makes their behavior more predictable overall. But players who want to maximize the unpredictability of their float serve might prefer smooth-paneled balls, which don’t make the transition until higher speeds. (Image credit: game – Pixabay, volleyballs – U. Tsukuba; research credit: S. Hong et al., T. Asai et al.; via Ars Technica)
Stick around all this week and next for more Olympic-themed fluid physics!
July 26, 2021
“The Goblet of Fire”
Sometimes the mundane events of life hide extraordinary phenomena. This award-winning photograph by Sarang Naik shows yellow-brown spores streaming off a mushroom during monsoon season. The plume is abstract and beautiful; you could easily mistake it for the flames of an Olympic torch. But common as they are, the lowly mushroom hides interesting depths. To get their spores to travel further, mushrooms actually generate their own breezes! (Image credit: S. Naik; via Big Picture Competition)
With the Olympics kicking off today, FYFD will follow our usual tradition of Olympic-themed posts for the next couple weeks, so be sure to come back each day for the latest featured sport!
July 23, 2021
Pump Problems
Pumps are a critical piece of infrastructure, but to keep them operating, engineers have to account for several potential pitfalls. In this Practical Engineering video, Grady discusses some of the common fluid dynamical effects that can destroy a pump and its performance. As you’ll see in the video, a lot of the challenges boil down to keeping air out of the pump. Since air and water are vastly different in their density and compressibility, most pumps cannot handle both of them at the same time. Pumps need to be primed to displace any air inside them and allow them to develop the suction needed to pump water. On the other hand, too much suction can create cavitation, which damages pump parts. And, finally, the intake systems for pumps have to be designed to keep air from getting sucked in. If nothing else, having too much air in the lines reduces the pump’s efficiency. (Image and video credit: Practical Engineering)
July 22, 2021
Contact-Line Dissipation
In the confines of a narrow tube, a flow’s energy gets dissipated in two places: inside the bulk fluid and along the contact line. The former is standard for all flows; viscosity acts like internal friction in the fluid and dissipates a flow’s kinetic energy into heat. Contact line dissipation is trickier. While it isn’t hard to imagine that a moving contact line would dissipate energy, it’s been unclear just how much energy the contact line eats up.
To answer that question, researchers performed a novel experiment using an extremely narrow capillary tube, initially filled with air. By dipping one end of a horizontal tube in an oil reservoir, they sucked some oil into the tube. Then they set the oil-filled end of the tube against a water reservoir, causing it to suck up water. The oil slug then moves along the tube at a constant speed, which enables the team to separate out the two sources of dissipation. They found that contact-line dissipation accounted for a surprisingly large amount of the overall dissipation — between 20 and 50 percent, depending on the length of the oil slug! (Image credit: N. Sharp; research credit and submission: B. Primkulov et al.)
July 21, 2021
Mud Pots
Mud pots, or mud volcanoes, form when volcanic gases escape underlying magma and rise through water and earth to form bubbling mud pits. I had the chance to watch some at Yellowstone National Park a few years ago and they are bizarrely fascinating. In this Physics Girl video, Dianna recounts her adventures in trying to locate some mud pots in southern California and explains the geology that enables them there. And if you haven’t seen it yet, check out her related video on the only known moving mud puddle! (Image and video credit: Physics Girl)
July 20, 2021
Devising Greener Chemistry
Not all microfluidic devices use tiny channels to pump and mix fluids. Some, like the Vortex Fluidic Device (VFD), conduct their microfluidic mixing in thin films of fluid. The VFD is essentially a tube spinning at several thousand RPM that can be tilted to various angles. Coriolis forces, shear, and Faraday instabilities in the thin fluid film create a complex microfluidic flow field that’s excellent for mixing, crystallization, and processing of injected chemicals. One rather notorious application of this device was unboiling an egg, a feat for which the researchers won an Ig Nobel Prize. But other, more practical applications abound, including a waste-free method for coating particles. (Image and research credit: T. Alharbi et al.; video credit: Flinders University; via Cosmos; submitted by Marc A.)

July 19, 2021