In Olympic high-diving, athletes leap from a maximum of 10 meters above the water. Although the force of their water impact is substantial, it’s small enough that they can enter the water head first. For cliff divers — who may jump from 27 meters! — the impact force is too great to risk a head-first entry, so they enter the water feet first. But this does not eliminate their risk of injury.
As the diver’s body enters the water, each leg creates its own cavity, and the proximity of the two cavities generates a repulsive force. If the diver isn’t prepared to resist that force, it will force their legs apart, potentially injuring them. (Image and research credit: T. Guillet et al.)
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.
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)
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!
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.)
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.)
Mars‘s northern pole is capped by a spiral-like pattern of deep troughs that are covered by carbon dioxide ice in winter but visible from orbit in summer. A new study posits that the spiral formed by wind erosion, exposing layer after layer of Martian geology.
The center of Mars’s polar cap is higher in the center than toward the edges, so katabatic winds — cold, dense flows beginning at high elevation — rush down from the pole. But because Mars spins, the Coriolis force causes those winds to flow in an anti-clockwise spiral. As those winds encounter depressions perpendicular to their path, they generate vortices that erode the depression. Eventually, a depression deepens, merges with other depressions, and forms a trough. According to this theory, the clockwise spiral of the troughs is a direct result of the katabatic winds flowing across them. Head over to Bad Astronomy or check out the original paper for more. (Image credit: ESA/DLR/FU Berlin/J. Cowart; research credit: J. Rodriguez et al.; via Bad Astronomy; submitted by Kam-Yung Soh)
In a crowded room, it can be hard to pick out the one conversation you want to hear. This so-called “cocktail party problem” is one animals have to contend with, too, when a noisy landscape can obscure the calls of potential mates. American green tree frogs have a clever solution to the problem: inflating their lungs to dampen out other frog species’ calls.
This method works because frogs have a direct anatomical connection between their lungs and their eardrums. Researchers found that when these frogs inflate their lungs, there’s a pronounced drop in their sensitivity to sound in the 1.4 – 2.2 kHz frequency band. That frequency range falls between the green tree frog’s peak mating call frequencies, but it coincides with the frequencies of other frogs living in the same regions. So rather than using their lungs to make themselves louder, these clever amphibians use them to make other frogs quieter! (Image credit: B. Gratwicke; research credit: N. Lee et al.; via Physics Today)
Springtails are tiny hexapods often found near water, where they execute their superpower: backflipping off the water’s surface. When standing on the water, the springtail’s hydrophilic claws protrude beneath the water surface and give it traction. But its spring-loaded furcula is hydrophobic, so when it snaps down it strikes the water without breaking through. The impact propels the springtail upward and sets it spinning at an incredible rate — Smith saw up to 290 backflips a second! (Image and video credit: Ant Lab/A. Smith)
Venus is a thoroughly unpleasant place thanks to its hellish temperatures and acidic clouds, but a new study adds another wrinkle to our strange sister planet: Venus’s day varies by up to 21 minutes in length. This peculiar factoid is the result of 15 years spent monitoring Venus’s rotation via radar. Previous attempts to pin down the exact length of Venus’s day produced differing answers; those disagreements make more sense in light of the new study, where individuals measurements of Venus’s rotation rate could differ by 3 minutes just from one (Earth) day to the next!
So why does Venus’s rotation rate change so dramatically? Venus’s atmosphere is massive — 100 times more massive than Earth’s — and it spins incredibly fast. The upper layers of Venus’s atmosphere can complete a rotation in 4 Earth days, while the solid ground requires 243 Earth days. As the atmosphere spins and sloshes, some of its angular momentum gets transferred to the ground, changing the planet’s rotation rate. (Image credit: NASA/JPL-Caltech; research credit: J. Margot et al.; via AGU Eos; submitted by Kam-Yung Soh)
