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  • Capturing SLS

    Capturing SLS

    NASA’s recent full-scale ground test of their Space Launch System (SLS) rocket was notable for more than just the engine. It was an opportunity to use a new high dynamic range, high speed camera prototype,

    HiDyRS-X, to capture the rocket’s exhaust in detail never seen before. Usually the extreme brightness of the rocket exhaust makes it impossible to see any structure in the flow without completely obscuring the ground equipment. With this camera, however, engineers can see how the engine, exhaust, and surroundings all interact. Be sure to check out the full video. I particularly like watching how the rocket’s exhaust entrains dust and sand from the ground nearby.  (Image credit: NASA, source; submitted by Chris P. and Matt S.)

  • Soap Film Wakes

    Soap Film Wakes

    Soap films can create remarkable flow visualizations when illuminated with monochromatic (single color) light. Each of the photos above shows a flow moving from left to right with a small object near the left creating an obstruction. In the top two images, the objects are cylinders; in the lower one it’s a flat plate tilted at 45 degrees. All of the objects shed vortices as the flow moves past. These vortices alternate in direction – the first spins clockwise, the next counter-clockwise, then clockwise again and so on. This pattern is known as a von Karman vortex street and can even show up in the atmosphere! (Image credit: D. Araya et al.)

  • Rio 2016: The Swimming Pool Controversy

    Rio 2016: The Swimming Pool Controversy

    Statistical analysis suggests possible current in the Rio Olympics swimming pool

    Several news outlets, beginning with The Wall Street Journal, are reporting that the swimming pool in Rio may have had a current that biased athletes’ performances. This is based on a statistical analysis of athlete performances across the meet, conducted by Indiana University’s Joel Stager and his coworkers. According to WSJ, Stager et al. analyzed times of athletes in the preliminary, semifinal, and final races of the 50m, 800m, and 1500m events and found consistent evidence that swimmers in the higher numbered lanes swam faster when moving toward the starting block and swimmers in the lower numbered lanes swam faster when moving toward the turn end of the pool. A separate analysis by Barry Revzin at Swim Swam came to similar conclusions about the direction and magnitude of lane effect in Rio.

    Past questions about lane bias

    This is not the first time questions have been raised about a current-induced bias in competition pools. In fact, Stager and his colleagues published an analysis in 2014 that suggested a similar bias in the pool used for the 2013 World Championships in Barcelona. That pool was a temporary pool built specifically for the competition by Myrtha Pools and was disassembled immediately after, before Stager et al.’s analysis was published.

    A more recent paper by Stager and his colleagues found that lane bias seems to be more prevalent in temporary pools than in permanent ones. The Rio Olympics pool, like the 2013 Worlds pool, is a temporary pool also built by Myrtha Pools.

    Myrtha Pools responds to the criticism 

    Myrtha responded to both WSJ and Swim Swam by sharing videos (1, 2) of their current test, which was conducted before the competition and on Day 3 of competition. The videos show a floating object in one of the outside lanes; neither video shows any noticeable movement of the object.

    Fluid dynamics and swimming pool design

    Competitive swimming pools are complicated recirculating systems that can contain special structures intended to minimize interactions between competitors. Myrtha has built many special event pools in recent years, including ones where the results did not show a bias. According to their website, Myrtha has fluid dynamicists on staff and uses computational fluid dynamics (CFD) to analyze pool performance during design, although they only show examples of freeform pools – not competition pools.

    In fact, I have found remarkably few CFD analyses of swimming pools in the literature. Most papers seem to focus on distribution of disinfectants in pools or in predicting evaporation rates – both practical problems but ones with limited relevance to this particular question.

    So, is there a current in the Rio pool?

    It’s tough to say with certainty that there is a current in Rio’s pool. The performance analyses by Stager et al. and by Revzin do show anomalies in the times of athletes in Rio based on their swim lane, and they show that those anomalies do not exist in many other recent competitions.

    I also do not think Myrtha’s current test constitutes evidence of a lack of current. Their floating object is only indicative of conditions at the air-water interface. Swimmers ride lower in the water and spend significant time completely underwater. Lane markers may also damp any flow effects near the surface.

    I think introducing dye underwater in the pool would do more to reveal any flow that may exist, and this would be a worthwhile test to conduct prior to the deconstruction of the Rio Olympic pool. Additionally, it would be wonderful to see a CFD analysis of the swimming pool, but this would require significant detail about the pool’s design (inlet and outlet locations, etc.) some of which is likely proprietary information.

    Neither dye visualization nor CFD simulation will change the results of this competition, but it may help reveal underlying issues in temporary pool designs so that any bias can be avoided in future competitions.

    (Image credit: Rio City Government)

    Special thanks to @MicahJGreen for bringing this story to my attention and to Dave B. for his assistance.

  • Rio 2016: Long Jump

    Rio 2016: Long Jump

    Long jump, like many track and field events, is affected by fluid dynamics in subtle ways. Both wind speed and altitude can modify a jumper’s performance – first, by changing the maximum speed they reach in their sprint, and second, through aerodynamic drag while in flight. Air resistance accounts for roughly 10% of a sprinter’s energy expenditure. A slight tailwind gives an athlete a minor boost in speed that can translate into a more significant increase in jump distance. On the other hand, though, a headwind of the same magnitude has an even stronger negative effect on performance.

    The other factor, altitude, comes into play through air density. The official Olympic record for the long jump was set by Bob Beamon in the 1968 Mexico City Games. The high altitude of Mexico City results in an air density that’s only 75% of that at sea level. That’s tougher on athletes in terms of oxygen levels, but it’s a big reduction in the overall drag they face, resulting in both a higher sprinting speed and less aerial drag. This is part of why Beamon’s jump stood as a world record for well over 20 years! (Image credits: AP Photo; AFP/GettyImages; Reuters)

    Previously: Can dimpled shoes help runners?; the unusual aerodynamics of the javelin; the physics of the discus

    Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!

  • Rio 2016: Synchro Swimming and Water Polo

    Rio 2016: Synchro Swimming and Water Polo

    Both synchronized swimming and water polo require competitors to hold themselves stable above the water’s surface without touching the pool’s bottom. One of the basic techniques for doing so in both sports is known as the eggbeater kick, shown above. The eggbeater kick is very similar to the motion for the breaststroke’s kick, but it’s performed upright and with alternating leg motions, sweeping a clockwise circle with the left leg and a counterclockwise one with the right.

    A swimmer typically stays afloat due to a buoyant force equal to the weight of the volume of water the swimmer displaces. Rising further out the water means reducing the buoyant force, so the swimmer must generate other forces to counter their weight. The eggbeater kick does this two ways. First, as the swimmer sweeps their foot around, it acts like a hydrofoil, generating lift that holds the swimmer up. Second, other parts of the kick cycle force water downward, which, by Newton’s third law, pushes the swimmer up. 

    Keeping a wide stance and sweeping the legs alternately allows the athlete to balance the horizontal forces their motions create while keeping the upward forces generated relatively constant. This gives them a stable, arms-free platform that’s a foundation for everything else their sport requires.  (Image credits: GettyImages; The Studio WLV, source)

    Previously: How buoyancy helps swimmers

    Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!

  • Rio 2016: Track Cycling

    Rio 2016: Track Cycling

    Track cycling is a sport where speed is everything. As much as 90% of the resistance a rider has to overcome is aerodynamic drag. To minimize drag, riders wear form-fitting one-piece skinsuits and wear special, streamlined helmets. They have aerodynamic bikes (unique left-side-drive ones, if you’re the Team USA women) and ride with their arms close together and thrust in front of them to minimize the frontal area they expose. 

    Even the tactics of racing rely heavily on aerodynamics. In events like team pursuit cyclists stay extremely close to the wheel of the rider ahead of them in order to remain in that rider’s slipstream and experience less drag. When switching off the position of lead rider, cyclists use the curvature of the track to help them move to the back of the paceline quickly to minimize the time spent outside of their teammate’s draft. (Image credits: GettyImages via the Guardian and the IOC, source)

    Previously: Cycling’s aero equipment helps them beat the clock; the effect of cyclists being followed by a motorbike or car

    Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!

  • Rio 2016: Whitewater Sports

    Rio 2016: Whitewater Sports

    The whitewater rapids of canoe slalom have their origins in mountain streams. Today the sport’s Olympic venues are artificial rivers, specially designed to provide world-class rapids whatever the geography of the host city. Rio’s course, like London’s, is reconfigurable; its features are controlled by the placement of Lego-like plastic blocks.

    A key part of the course’s design process was building a small-scale physical model of the course. To maintain the dynamics of the rapids at a smaller physical scale, engineers used a concept called similitude. Surface waves like rapids are a function of the flow’s inertia and the effects of gravity, a ratio that’s captured in the dimensionless Froude number. To match the small-scale model to the real flow, engineers scaled the features of the real course down such that the Froude number stayed the same between the model and the full-scale course. As seen in the animations above, this meant that the model had the same general flow features as the final course, letting engineers and designers test and fine-tune features before construction. Learn more about the model and its construction in these two videos. (Image credits: kayaker – Getty Images; model comparisons – J. Pollert, source)

    Previously: Physics of rowing; why that octopus kite looks so real

    Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!

  • Rio 2016: Swimming

    Rio 2016: Swimming

    Strange as it seems, elite swimmers are faster when swimming underwater than they are at the surface. So much so, in fact, that they’re restricted to being underwater only 15 m after a dive or turn. To see just how stark a difference this makes, check out this crazy video.  (I know, right?!)

    To understand how this is possible, it helps to look at the three types of drag a swimmer experiences: pressure drag, skin friction, and wave drag. Pressure drag is probably the most familiar; it’s the drag that comes from the swimmer’s shape and how the fluid moves around it. Skin friction is the drag that comes from viscous friction between the swimmer and the water. The final type, wave drag, comes from the energy expended to create waves at the surface of the water. As you might expect, energy that goes into splashing is energy that isn’t going into propulsion.

    When swimming at the surface, swimmers experience a lot of wave drag. At least one experiment showed that wave drag accounted for most of a surface swimmer’s drag. In contrast, at a depth of more than 0.5 m, a swimmer’s wave drag is virtually negligible. The submersion does come at the cost of higher skin friction (since more of the swimmer is in contact with the water), but there is also more opportunity for useful propulsion since both sides of a kick can move water (and not air.) Bonus read for those interested in more: Is the fish kick the fastest stroke yet? (Image credits: AP; B. Esposito)

    Previously: what makes a pool fast?

    Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!

  • Rio 2016: Cycling

    Rio 2016: Cycling

    Today marks the official start of the 2016 Summer Olympics in Rio. Here at FYFD we’ll be celebrating by taking a look at how fluid dynamics affects Olympic sports. You can check out our previous series on the London Olympics here. Since this weekend features the men’s and women’s cycling road races, we’ll get started with cycling!

    In road cycling, equipment and race strategy are all built around aerodynamic efficiency. It’s understood that following a car or motorbike gives a cyclist an unfair advantage, and officials can be quick to punish infractions. What the rules don’t account for, though, is the advantage a cyclist gets when they’re followed by a motorbike (or car). These vehicles are significantly larger than a cyclist, and when they are trailing a cyclist, they have a significant upstream effect. Essentially the higher pressure traveling ahead of the motorbike will counter the low pressure region immediately behind the cyclist. The result is that the cyclist, despite being in front, experiences less drag than they would if the motorbike weren’t there.

    The difference isn’t tiny either: if a motorbike follows a rider at a distance of 0.5 m for just 1 km, the rider saves more than 2 seconds. When events can be won or lost by fractions of a second, those gains are significant. (Image credits: DCMS; B. Blocken et al., GettyImages, Reuters; research credit: B. Blocken et al.; submitted by Marc A.)

  • Sharks Swimming Sideways

    Sharks Swimming Sideways

    Like many sharks, the great hammerhead shark is negatively buoyant, meaning that, absent other forces, it would sink in water. To compensate, sharks generate lift with their pectoral (side) fins to offset their weight. Their dorsal (top) fin is used to generate the horizontal forces needed for control and turning. However, both captive and wild great hammerhead sharks tend to swim rolled partway onto their sides. The reason for this unusual behavior is hydrodynamic – it is more efficient for the shark. Unlike other species, the great hammerhead has a dorsal fin that is longer than its pectoral fins. By tipping sideways, the shark effectively creates a larger lifting span and is able to induce less drag than when it swims upright. Models show that swimming on their sides requires ~8% less energy than swimming upright! (Image credit: N. Payne et al., source)

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