Experiments by a Brown-led research team investigated belly flop mechanics and found surprising insights about air-to-water impacts that could be useful for marine engineering applications.
PROVIDENCE, R.I. [Brown University] - Anyone who's ever done a belly flop into a swimming pool knows it ends with a blunt-sounding splat, a big splash and a searing red sting. What most people don't know is why.
Daniel Harris does. The assistant professor in Brown University's School of Engineering says the physics behind the phenomenon aren't too complex. What happens - and what makes it so painful, he explains - is that the forces from the water surface put up a fierce resistance to the body suddenly going from air to water, which is often still.
"All of a sudden, the water has to accelerate to catch up to the speed of what's falling through the air," said Harris, who studies fluid mechanics. "When this happens, that large reaction force is sent back to whatever's doing the impacting, leading to that signature slam."
How and why this happens in fluid mechanics isn't just important for developing a prize-winning belly flop for competitions, or dolling out pool-party trivia on why belly flops hurt so much - the understanding is critical to naval and marine engineering, which often have structures that need to survive high-impact air-to-water slamming forces. For that reason, the phenomenon has been studied thoroughly for the past century. But a research team led by Harris and Brown graduate student John Antolik found novel insights in a new study done in partnership with scholars at the Naval Undersea Warfare Center in Newport and Brigham Young University.
For the Journal of Fluid Mechanics study, the researchers set up a belly flop-like water experiment using a blunt cylinder but adding an important vibrating twist to it, which ultimately led them to counterintuitive findings.
"Most of the work that's been done in this space looks at rigid bodies slamming into the water, whose overall shape doesn't really change or move in response to the impact," Harris said. "The questions that we start to get at are: 'What if the object that's impacting is flexible so that once it feels the force it can either change shape or deform? How does that change the physics and then, more importantly, the forces that are felt on these structures?"
To answer that, the researchers attached a soft "nose" to the body of their cylinder, referred to as an impactor, with a system of flexible springs.
The idea, Antolik explains, is that the springs - which act in principle similar to the suspension of a car - should help soften the impact by distributing the impact load over a longer period. This strategy has been floated as a potential solution for reducing sometimes catastrophic slamming impacts in air-to-water transitions, but few experiments have ever looked closely at the fundamental mechanics and physics involved.
For this experiment, the researchers dropped the cylinder repeatedly into still water and analyzed both the visual results and data from sensors embedded inside the cylinder.
This is where the unexpected happened.
