USU’s Splash Lab scientists test skipping spheres

Image courtesy USU

LOGAN – Two Utah State University researchers, Tadd Truscott and Randy Hurd, recently published a paper that examines the initial impact a skipping sphere has with water. Understanding the science behind this, according to a USU press release, can have many different applications.

“One of the questions that’s been unanswered for quite a while is how do solid object that are elastic, how do they respond to water impact,” Truscott said. “That’s what our paper answers. It’s just in a really generic way with a sphere.”

The research started about five years ago, but the idea was conceived shortly before that, when Truscott was on a Rhode Island beach skipping stones with his son and nephew. The trio went to a toy store after leaving the beach and found a ball for sale that had the ability to bounce on water. The three returned to the beach and quickly became fascinated with their new water-skipping toy.

When Truscott returned to work at the USU Splash Lab, he brought the spheres with him. He filmed them skipping with a high-speed camera to later show to his son. He also showed it to his boss, who liked what he saw and encouraged Truscott to pursue research on it. Two years later, a trip to the toy store had turned into a research grant from the United States Office of Naval Research.

“The basic research effort the (Office of Naval Research) is probably interested in most is the idea that we need to understand better how objects interact with the water’s surface at high speeds,” Truscott said.

Truscott and Hurd said solid objects will bounce off the water if they enter at the right speed and angle, while a more solid, rigid object will require a high speed and low angle. The two researchers have found that an elastic object, such as the sphere from the toy store, will very rapidly deform itself into a flat shape upon connecting with the water, giving it a larger lifting force. Once it returns to its original shape, it lifts itself off the water.

“That’s part of the reason why they skip so well, in the split instant they deform into kind of this ideal skipping stone shape,” Hurd said. “The interesting thing about it is they can do it in any orientation, whereas a stone has to be thrown very specifically to impact correctly. It doesn’t matter if it’s a formable sphere, you can throw it any way. Upon impact it will form at just the right angle, not just the right, but pretty dang close. That has very similar dynamics to a skipping stone after that.”

But it isn’t as simple as just having a flexible, stretchy ball. Truscott said timing is also an important factor. If a ball can deform and return to its original shape faster than the cavity of water it creates can form around it, it will bounce.

“On the flip side,” Hurd explained. “If you made a ball out of memory foam, this is going to hit the water and deform significantly, but it’s not going to return to its original shape before the water comes back.”

What made the toy water-bouncing balls skip so effectively, according to Hurd, is that they deform and reform at the same speed as the water.

The most applicable research, the kind of stuff the Office of Naval Research is probably interested in, involves large objects – objects too large to be brought into the Splash Lab and shot into the water. For those issues, the team used a numerical simulation based on the observations of the smaller objects.

“We had guys at the Naval Undersea Warfare Center and at Brown University look at the dynamics of what is happening with the sphere,” Truscott said. “They modeled the sphere as a solid elastic object and they also modeled water.”

Hurd said the other scientists tested and changed the numerical model until they got results that correlate with what the USU scientists found.

“At that point we feel pretty good about their model because we are getting really good matching,” he said. “And then that’s when we extrapolate some things that we can’t do in the lab.”

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