Good news for clumsy divers: Physics holds the key to less-painful belly-flops

We have all had the misfortune of botching a dive into the pool and ending up in a painful belly-flop—or maybe we have executed it intentionally to indicate off and immediately regretted that call. Hitting the water in that physique place can really feel like hitting concrete and result in bruising or (if one is falling from a larger peak) inside accidents. Whereas the fundamental physics is well-understood, scientists are all the time on the lookout for larger perception into the phenomenon in hopes of discovering novel methods to ameliorate the impression.

Scientists at Brown College have discovered that, surprisingly, including a bit of additional spring to a physique hitting the water can really enhance the impression pressure as a substitute of lower it beneath sure situations, based on a brand new paper revealed within the Journal of Fluid Mechanics. The implications transcend defending divers; a greater understanding of the hydrodynamics will enhance designs of naval ships, seaplanes, or projectiles, in addition to underwater autonomous automobiles.

From a physics standpoint, we’re speaking about an elastic physique hitting the floor of water. The stress of shifting from the medium of air to the a lot denser medium of water exerts an enormous pressure as that physique displaces it. The cohesive forces between water molecules are stronger on the floor, making it tougher to interrupt via. (It is why diving competitions usually use aerators to create bubbles within the water, breaking the floor rigidity to guard the divers.) A big quantity of fluid should be accelerated (displaced) in a brief timeframe to match the pace of the impinging physique. The bigger the floor space of the thing hitting the water, the extra resistance there will probably be—and with belly-flops there will probably be a a lot bigger floor space than with a easy swan dive, leading to that signature slam.

Scientists have been finding out this phenomenon for a really very long time, together with current work on diving birds like gannets and kingfishers. Regardless of the stress on their our bodies, gannets and kingfishers handle the feat repeatedly with out harm, particularly concussions, thanks partly to their tapered cone-shaped beaks, enabling them to enter the water with out making a compression wave beneath the floor. Gannets additionally fold their wings again as they dive for a streamlined form.

This newest research investigates the impression of extra blunt our bodies hitting the water floor, by which the best impression happens through the so-called “slamming” part. “A lot of the work that is been executed on this area seems to be at inflexible our bodies slamming into the water, whose general form does not actually change or transfer in response to the impression,” mentioned co-author Daniel Harris of Brown College. “The questions that we begin to get at are: ‘What if the thing that is impacting is versatile so that when it feels the pressure it may both change form or deform? How does that change the physics after which, extra importantly, the forces which are felt on these buildings?”

For his or her experiments, Harris et al. constructed a slender inflexible impactor with a blunt nostril coupled collectively by elastic spring parts laser-cut from plastic. The springs ought to act like a automobile’s suspension, softening the impression by distributing the load over an extended interval. The physique housed an onboard accelerometer to measure impression deceleration in addition to a ferromagnetic ball in order that the impactor may very well be dropped from various heights by way of an electromagnet. The impacts with the water have been illuminated with diffuse white backlight and filmed at excessive speeds to seize the air-water interface motion.

The outcomes shocked the scientists, as a result of including the versatile springs to the impactor didn’t all the time soften the impression in comparison with a totally inflexible impactor. Typically, it really elevated the utmost pressure on the physique. Subsequent experiments revealed that in these instances, the vibrations created because the impactor hit the water typically additionally precipitated the springs to vibrate (harmonic oscillation), thereby rising (somewhat than diminishing) the pressure of impression. So, the springs need to be delicate sufficient to successfully soak up the impression with out vibrating an excessive amount of.

“The construction is vibrating backwards and forwards because of the violent impression, so we have been getting readings from each the impression of hitting the fluid and an oscillation as a result of the construction is shaking itself,” Harris mentioned. “If you happen to do not time these proper, you may principally make the state of affairs worse.”

Journal of Fluid Mechanics, 2023. DOI: 10.1017/jfm.2023.820  (About DOIs).

Itemizing picture by John Antolik and Daniel Harris