Anyone who has ever belly flopped in a swimming pool knows that it ends with a dull-sounding splat, a big splash and a red sting. Why most people do not know what.
Daniel Harris does. The assistant professor at Brown University’s School of Engineering says the physics behind the phenomenon isn’t too complicated. What happened -; And what makes it so painful, he explains -; The forces from the surface of the water suddenly make a formidable resistance to the body of water, which is often stagnant from the wind.
“Suddenly, the water has to accelerate to catch up with what’s falling through the air,” said Harris, who studies fluid mechanics. “When that happens, that large reaction force is sent back to whatever impacted it, causing that signature slam.”
How and why this happens in fluid mechanics isn’t important to creating prize-winning belly flops for competitions or figuring out pool-party trivia about why belly flops hurt so much -; The understanding is important for naval and marine engineering, which often has structures that must survive high-impact air-to-water slamming forces. For that reason, the phenomenon has been thoroughly studied over the past century. But a research team led by Harris and Brown graduate student John Antolik found novel insights in a new study in partnership with Newport and scholars at Brigham Young University’s Naval Undersea Warfare Center.
for Journal of Fluid Mechanics study, the researchers set up a water test similar to a belly flop using a blunt cylinder but added an important pulsating twist to it, which ultimately led to their counter-intuitive results.
Most of the work done in this space looks at rigid bodies that crash into water, whose overall shape doesn’t really change or move in response to the impact. The questions we begin to get are: ‘What if the impacted object is flexible so that once it experiences a force it can either change shape or deform? How does this change the physics and then, more importantly, the forces felt in these structures?
Daniel Harris, Brown University
To answer this, the researchers attached a soft “nose” to their cylinder body, referred to as an impactor, with a system of flexible springs.
The idea, Antolik explains, is that springs -; which in principle works like the suspension of a car -; Should help soften the impact by distributing the impact load over a longer period of time. This technique has been floated as a potential solution to reduce the sometimes disastrous slamming effects in the air-to-water transition, but few experiments have ever looked closely at the basic mechanics and physics involved.
For this test, the researchers repeatedly dropped the cylinder into still water and analyzed both visual results and data from sensors embedded inside the cylinder.
This is where the unexpected happened.
The results show that while the strategy can be effective, surprisingly, it doesn’t always soften the impact. In fact, contrary to conventional thinking, sometimes a more flexible system can increase the maximum impact force on the body compared to a completely rigid structure.
This forced researchers to dig deeper. Through extensive experiments and building a theoretical model, they found their answer. Depending on the height from which the impactor is dropped and how stiff the springs are, the body will not only feel the impact from the slam but will also feel the vibration of the structure as it enters the water, complicating the slamming force.
“The structure is vibrating back and forth because of the violent impact, so we’re getting readings from both the impact of the liquid impact and an oscillation because the structure itself is shaking,” Harris said. “If you don’t time it right, you can basically make the situation worse.”
The researchers found that the key was the springs: they had to be soft enough to absorb the impact gently without leading to faster vibrations that add overall strength.
Working at Brown’s Engineering Research Center, Antolik recorded experiments using high-speed cameras and an impact measurement device called an accelerometer. “The whole back corner gets a little wet when I’m testing,” he joked.
The researchers are now looking at the next steps in their line of research, taking inspiration from diving birds.
“Biological studies of these birds have shown that they perform certain strategies as they enter the water to improve conditions so that they do not experience such high energy,” Antolik said. “The direction we’re moving in is basically trying to design a robotic impactor that can perform some active maneuvers to do the same for blunt objects when water enters.”
The research was supported by the Office of Naval Research and the Naval Undersea Warfare Center.
Antolic, J.T., etc (2023) Slamming force during water penetration of a simple harmonic oscillator. Journal of Fluid Mechanics. doi.org/10.1017/jfm.2023.820.