Want the secret to a less painful belly roll? These researchers have the answer

Want the secret to a less painful belly roll?  These researchers have the answer

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Anyone who has ever belly-jumped into a pool knows that it ends with a sharp hit, a big splash, and a burning red sting. What most people don’t know is why.

Danielle Harris does. The physics behind this phenomenon isn’t very complicated, says the assistant professor at Brown University’s College of Engineering. He explains that what happens – and what makes it so painful – is that the forces from the surface of the water create fierce resistance to the body that suddenly goes from air to water, which is often at rest.

“Suddenly, the water has to accelerate to catch up with what’s falling in the air,” said Harris, who studies fluid mechanics. “When this happens, this large reaction force is sent back into whatever is doing the impact, resulting in that characteristic knockout.”

How and why this happens in fluid mechanics is not just important for developing award-winning belly flops for competitions, or handing out trivia at a pool party about why belly flops are so painful – understanding is crucial to marine and marine engineering, which often has structures that need To survive high impact forces from air to water.

For this reason, this phenomenon has been comprehensively studied throughout the past century. But a research team led by John Antolic, a graduate student at Harris and Brown, has found new insights in a new study conducted in partnership with scientists at the Naval Warfare Center in Newport and Brigham Young University.

For the study published in Journal of Fluid Mechanics the researchers conducted a water experiment similar to belly floping using a blunt cylinder but adding a significant vibrating touch to it, which ultimately produced counterintuitive results.


Brown University researchers conducted a water experiment similar to belly floping using a blunt cylinder but adding a significant vibrating touch to it, which ultimately yielded counterintuitive results. Credit: John Antolek and Danielle Harris.

“Most of the work done in this space looks at solid objects that impact water, which do not change their overall shape or move in response to the impact,” Harris said. “The questions we begin to answer are: What if the object that collides is so elastic that once it feels a force it can either change its shape or deform it? How does that change the physics and therefore, more importantly, the forces acting? These structures are felt by us?”

To answer this, the researchers attached a soft “nose” to the cylinder body, referred to as an impactor, with a system of elastic springs.

The idea, Antolic explains, is that the springs — which work similarly in principle to a car’s suspension — should help soften the impact by spreading the impact load over a longer period. This strategy has been floated as a potential solution to reduce the sometimes catastrophic knocking effects in air-to-water transitions, but few experiments have looked closely at the underlying mechanics and physics involved.

In this experiment, the researchers repeatedly dropped the cylinder into still water and analyzed both visual results and data from sensors embedded within the cylinder.

Here the unexpected happened.

The results show that although the strategy can be effective, surprisingly it does not always mitigate the effect. 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 by developing a theoretical model, they found their answer. Depending on the height from which the shock device is dropped and how stiff the springs are, the body will feel not only the impact of the strike, but also the vibrations of the hull as it enters the water, doubling the impact force.

“The structure is shaking back and forth due to the violent impact, so we were getting readings from the impact of the fluid impact and oscillation as the structure is shaking itself,” Harris said. “If you don’t do it in time, you can make the situation worse.”

The key, the researchers found, are the springs: They must be soft enough to gently absorb impact without triggering more rapid vibrations that add to the overall strength.

While working at the Brown Engineering Research Center, Antolic recorded experiments using high-speed cameras and used an impact measurement instrument called an accelerometer. “The whole back corner gets a little wet when I’m experimenting,” he joked.

The researchers are now looking at the next steps in their line of research, inspired by diving birds.

“Biological studies of these birds have shown that they perform certain maneuvers when entering the water to improve conditions so as not to encounter such high forces,” Antolic said. “What we’re moving toward is trying to design what is essentially a robotic impact device that can perform some active maneuvers while entering the water to do the same for blunt objects.”

more information:
Impact forces during water entry for a simple harmonic oscillator, Journal of Fluid Mechanics (2023). doi: 10.1017/jfm.2023.820

Magazine information:
Journal of Fluid Mechanics

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