FYFD

Tag: swimming

  • Breaking the Wave Speed Limit

    Breaking the Wave Speed Limit

    Whirligig beetles are small surface swimming insects. As they race across the water surface, they create both visible and unnoticeable waves on the water. These waves are the result of both surface tension and gravity. Typically, it’s the wavelength of the gravity waves that limit a swimmer or boat’s speed. When the wavelength of the gravity waves a swimmer creates meets the size of the swimmer, the waves generated ahead of the swimmer start to reinforce the waves forming at the back of the swimmer. This traps the swimmer (or boat) in a trough between its bow and stern waves and limits the max speed of the swimmer since overcoming this critical hull speed requires excessive amounts of power.

    The tiny whirligig beetle overcomes this natural speed limit cleverly. It is smaller than the shortest possible gravity wave in water. Thus, it can never be trapped between its bow and stern waves! This allows the tiny swimmer to zip across the water’s surface at speeds above 0.5 m/s. That’s over 30 beetle body lengths per second! (Image credit: H. L. Drake, source; research credit: V. Tucker; submitted by Marc A.)

  • Swimming with Corkscrews

    Swimming with Corkscrews

    E. coli, like many bacteria, swim using corkscrew-like appendages called flagella. Because the bacteria are extremely tiny – their flagella may be less than ten microns long – their swimming is overwhelmingly dependent on viscosity. (Inertial effects are 100 to 10,000 times smaller than viscous effects for swimming E. coli.) Rotating their helical flagella generates viscous drag along the surface of the corkscrew. Because the flagella is asymmetric when you add all of those drag components together, the net force is thrust that moves the bacterium forward. Watch carefully in the animation above and you’ll see that E. coli have multiple flagella and will swing one out to the side during maneuvers. (Image credit: L. Turner et al., source; reproduced in a review by E. Lauga, pdf)

  • Starfish Vortices

    Starfish Vortices

    Starfish larvae, like other microorganisms, use tiny hair-like cilia to move the fluid around them. By beating these cilia in opposite directions on different parts of their bodies, the larvae create vortices, as seen in the flow visualization above. The starfish larvae don’t use these vortices for swimming – to swim, you’d want to push all the fluid in the same direction. Instead the vortices help the larvae feed. The more vortices they create, the more it stirs the fluid around them and draws in algae from far away. The larvae actually switch gears regularly, using few vortices when they want to swim and more when they want to eat. Check out the full video below to see the full explanation and more beautiful footage.  (Image/video credit: W. Gilpin et al.)

  • Plesiosaur Swimming

    Plesiosaur Swimming

    Plesiosaurs are marine reptiles that thrived during the Jurassic period and went extinct some 66 million years ago. Since the first discoveries of plesiosaur fossils centuries ago, scientists have debated how the four-limbed creature would have swam. One approach to answering this question is to examine the efficiency of different strokes. Researchers have done this computationally by building a digital plesiosaur with biologically realistic joint motions. They then couple the model plesiosaur’s body motions with the movement of fluid around the body. With this computational model, they then simulate many different methods for moving the plesiosaur’s limbs and search for the most efficient one.

    What they found is that the plesiosaur’s propulsion is dominated by its forelimbs, which likely moved with a flight stroke similar to that of a penguin or sea turtle. Despite their size, the hindlimbs were able to produce very little thrust, suggesting that they were primarily used for stability and maneuverability. (Image credits: S. Liu et al., GIF source)

  • 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: 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!

  • Swimming at Microscale

    Swimming at Microscale

    Tiny organisms live in a world dominated by viscosity. There’s no coasting or gliding. If a microorganism stops swimming, friction will bring it to a halt in less than the space of a hydrogen atom! To make matters worse, simply flapping an appendage forward and backward will get them nowhere. As we’ve seen before, these highly viscous laminar flows are reversible, meaning that a backward power stroke is simply undone by a mirrored forward recovery stroke. Instead, microorganisms like the paramecium swimming above are covered in tiny hairlike cilia which beat asymmetrically. They extend to their full length during the power stroke, but they stay bent during the forward recovery stroke. That asymmetry guarantees that they move more fluid backward than forward, thereby letting the paramecium make progress. (Image credit: C. Baroud, source)

  • Fluids Round-up

    Fluids Round-up

    Last week was supposed to have a fluids round-up, but we were having too much fun walking on water instead. So here it is now!

    – NASA has asked Congress for funding for new X-plane programs to explore solutions for greener airliners and quieter sonic booms to enable next-generation air travel. Popular Science, Gizmodo, and Ars Technica take a closer look at the proposed projects. I won’t lie – as an aerospace engineer I am hugely in favor of this. The first ‘A’ in NASA has been neglected for quite a while and projects like these are needed if we want to advance the state-of-the-art in aeronautics.

    – The New York Times’ ScienceTake video series took a look back at their most popular videos, and 3 of the top 5 videos are fluid dynamics-related. Because we are just that awesome. (via Rebecca M)

    – I made a guest appearance on last week’s Improbable Research podcast, where we talked about bizarre experiments trying to unravel swimming.

    – Physics Girl shows us 5 weird ways to blow out a candle. There’s some neat and potentially non-intuitive fluid dynamics involved!

    – SciShow offers an explanation of why we sneeze. Spoiler alert: it’s more than just to get rid of irritants.

    – Fluid dynamics made the short list for NPR’s Golden Mole awards with the discovery of dancing droplets. Here’s Skunkbear’s take on it.

    – Ernst Mach, of Mach number fame, was also a bit of an artist and philosopher. (via @JenLucPiquant)

    – It’s not quite fluid dynamics, but this Slow Mo Guys video of spinning burning steel wool might be their most beautiful video yet. Check it out!

    (Image credit: NASA)

  • Featured Video Play Icon

    Fluids Round-up

    Here’s to another fluids round-up, our look at some of the interesting fluids-related stories around the web:

    – Above is a music video by Roman Hill that relies on mixing and merging different fluids and perturbing ferrofluids for its visuals as it re-imagines the genesis of life.

    – GoPro takes viewers inside a Category 5 typhoon with 112 mph (180 kph; 50 m/s) winds.

    – Astronaut Scott Kelly demonstrates playing ping pong with a ball of water in space. (via Gizmodo)

    – See fluid dynamics on a global scale with Glittering Blue. (via The Atlantic)

    – To make a taller siphon, you have to find a way to avoid cavitation.

    – Speaking of siphons, Randall Munroe tackles the question of siphoning water from Europa over at What If? (submitted by jshoer)

    – The Mythbusters make a giant tanker implode using air pressure.

    – Sixty Symbols explores how tiny things swim.

    – What happens when you bathe in 500 pounds of putty? Let’s just say that bathing in an extremely viscous non-Newtonian fluid is not recommended. (via Gizmodo)

    (Video credit and submission: R. Hill et al.)

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  • Fluids Round-Up

    Fluids Round-Up

    New year, new (or renewed) experiments. This is the fluids round-up, where I collect cool fluids-related links, articles, etc. that deserve a look. Without further ado:

    (Video credit and submission: Julia Set Collection/S. Bocci; image credit: IRPI LLC, source)