The Brown researchers set up a belly-flop-like water experiment using a blunt cylinder and gave it a significant vibrational twist, ultimately leading to a counterintuitive discovery. Credit: John Antlik and Daniel Harris.
We’ve all had the misfortune of jumping into a pool and ending up with a painful stomach flop. Or maybe you’ve purposefully jumped in to show off, only to quickly regret your decision. Hitting water with such a body position will feel like hitting concrete and can lead to bruising and (if falling from a higher height) internal damage. Although the basic physics are well understood, scientists are constantly seeking deeper insights into phenomena in the hope of finding new ways to improve their impact.
Scientists at Brown University have surprisingly found that adding a little extra spring to a body that hits water can actually increase, rather than reduce, the impact force under certain conditions. discovered. new paper Published in Journal of Fluid Mechanics. The impact goes beyond just protecting divers. A deeper understanding of fluid mechanics will improve the design of warships, seaplanes, projectiles, and even underwater autonomous vehicles.
From a physics perspective, we are talking about an elastic body colliding with the water surface. The stress of moving from the medium of air to the denser medium of water exerts a large force on an object as it moves. The cohesive forces between water molecules are stronger at the surface, making it harder to break through. (This is why competitive diving often uses aerators to create bubbles in the water to break up the surface tension to protect the diver.) To match the speed of the object it hits, A large amount of liquid must be accelerated (moved) in a short amount of time. The greater the surface area of an object that hits the water, the greater the resistance. The belly flop has a much larger surface area than a simple swan dive, resulting in that signature slam.
Scientists have been studying this phenomenon for a very long time, including recent studies on diving birds such as gannets and kingfishers. Gannets and kingfishers are able to accomplish this feat over and over again, despite the stress placed on their bodies, without injury, especially concussions. It is able to enter the water without creating compression waves below the surface, thanks to its tapered, conical beak. Gannets fold their wings back when diving, giving them a streamlined shape.
This latest study investigates the effects of blunter objects hitting the water surface. In this case, the greatest impact occurs during the so-called “slamming” stage. “Most of the research done in this area has focused on rigid bodies that crash into water, and their overall shape doesn’t actually change or move in response to the impact.” Co-author Daniel Harris said: of Brown University. “The question we started thinking about was, ‘What if the object we’re impacting is flexible and can change shape or deform once it feels a force?’ How do we change it and, more importantly, how do we change the forces on these structures?”
JT Antolik et al., 2023
For their experiments, Harris et al. constructed a slender, stiff impactor with a blunt nose joined by an elastic spring element laser-cut from plastic. Springs act like a car’s suspension and must cushion the impact by distributing the load over a long period of time. This body contained an accelerometer and a ferromagnetic ball to measure the deceleration of the impact, and it was possible to drop the impacting object from various heights via electromagnets. The water collision was illuminated with a diffuse white backlight and filmed at high speed to capture the movement of the air-water interface.
This result surprised scientists. This is because adding a flexible spring to the impactor did not necessarily make the impact softer compared to a completely stiff impactor. In some cases, the maximum force exerted on the body actually increased. Subsequent experiments have shown that in such cases, the vibrations generated when the impactor hits the water can cause oscillations (harmonic oscillations) in the spring, thereby increasing (rather than decreasing) the impact force. It turns out that. Therefore, the spring must be soft enough to effectively absorb shock without vibrating too much.
“The structure is vibrating back and forth due to severe shocks, so we were taking measurements from both the shocks hitting the fluid and the vibrations from the structure itself shaking.” harris said. “If you don’t deal with it in a timely manner, it can basically make the situation worse.”
Journal of Fluid Mechanics, 2023. DOI: 10.1017/jfm.2023.820 (About DOI).
Listing images by John Antlik and Daniel Harris