The Physics of . . . Waterslides

When waterslides reach a certain speed, no scientist can predict their behavior

By Elizabeth Svoboda|Sunday, July 24, 2005
RELATED TAGS: WATER

Standing 76 feet above Orlando, Florida, staring down the throat of the Blue Niagara slide at the Wet’nWild water park, some riders have been known to have second thoughts. Powerful pumps churn the cascading water at the top of the slide into a furious froth, muffling the sound of parkgoers seven stories below. “Just go for it!” the ride operator urges, and another human torpedo surrenders to the froth, accelerating to a velocity of about 40 feet per second while careering through the slide’s 300 feet of looping tubes.

“Yup, that’s a crazy one,” engineer Marvin Hlynka says, chuckling. Hlynka works for WhiteWater West Industries, a firm in Richmond, British Columbia, that designed the Blue Niagara and other extreme waterslides. It’s his job to use science to find new ways to scare riders out of their minds, but there’s a limit to how much his equations and computer programs can help him. In the alternate universe of waterslide design, Newtonian models don’t always work, and chaos lurks in every hairpin turn. “In terms of actually predicting where a particular drop of water or a particular body is going to be in the slide at any given time, you can’t do it,” Hlynka says. “It’s just not possible.”

The path that water takes through a channel is primarily a function of its volume and the channel’s shape, slope, and the roughness of its surface. As the water picks up speed, however, it starts to develop eddies—zigzagging, swirling currents—that make the flow less predictable. The velocity at which a smoothly flowing stream produces eddies depends on the water’s viscosity, the diameter of the channel, and other factors, but no one fully understands what triggers all the turbulence. As the physicist Werner Heisenberg put it, “When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first.”

Turbulent water, like thunderstorms and other chaotic systems, is extremely sensitive to minor disturbances. The tiniest contaminant—a speck of dust, a blade of grass—can disrupt a waterslide’s flow, touching off larger irregularities that in turn create swirling eddies, and so on. Newtonian principles predict that minute initial variations in a system lead to similarly minute final ones. Not so in chaotic systems. “There are tiny perturbations to start out with, and they get bigger and bigger in unpredictable ways,” says Parviz Moin, director of the Center for Turbulence Research at Stanford University. “You’ve heard how a butterfly flapping its wings in China can change the weather in California. In a sense, that kind of thing is what’s happening inside a waterslide.”

Because no equation can predict these changes in a straightforward way, waterslide designers can’t know for certain how the frothy water will behave when they switch on the jets. Yet they rely on water flow and gravity to safely convey riders to the splash pool. “If you’re building a roller coaster or a similar ‘hard’ amusement-type ride, you strap the riders in so they can’t move,” Hlynka says. “With a waterslide, there’s none of that control.”

Other variables further complicate the equation. Different bathing suits, for instance, generate differing amounts of friction as they rub against a slide, and that affects the character of the turbulence as well as the speed of the rider. Go down the Blue Niagara in a pair of cotton shorts and you might find the ride ho hum. Go down it in a pair of Spandex minitrunks and you might need some time to recuperate—especially if you happen to wear a bikini top, which fast slides have been known to tear off.

Hlynka’s only safeguard against such uncertainty is real-world testing. He creates detailed designs based on past experience, then uses an army of eager volunteers to act as guinea pigs to verify the slides’ safety. “Experience helps,” Hlynka adds. “After 20-odd years, you know that if a typical person with a typical bathing suit rides a fiberglass slide at a 9 to 10 percent slope, there aren’t going to be any problems.”

Just to be sure, Hlynka and his team tend to overengineer the safety features on their slides. Their nonenclosed flumes have inward-curving sides that are anywhere from 2 1/2 to 5 feet high. These ensure that even riders who encounter powerful turbulence won’t be catapulted off the slide. Since 1995, according to the Consumer Product Safety Commission, eight people in the United States have died on waterslides—half as many as have died on roller coasters over the same period.

Slide designers also have a few predictable physical principles in their favor. In WhiteWater West’s new High Speed 32 slide, for instance, riders veer through a succession of gentle curves at more than 30 feet per second. Each body’s speed and subtle rotation creates a force of about 3 g’s that pins the riders against the fiberglass chute as they slide. “The faster you go, the more the g-force holds you against the wall and locks you in position,” Hlynka says. “It’s almost like a restraint.”

Still, even the best-laid slide can sometimes throw a wrench in engineers’ plans. A few years ago, a WhiteWater slide in Minneapolis was such a hit that the park’s owners wanted one just like it at another location, this one in Ohio. The new slide was an exact copy of the original, its measurements identical within a quarter of an inch. Hlynka and his fellow engineers figured the design would be a no-brainer. They were wrong. “We had it all set up, and we saw water pouring out of one particular place. We said, ‘What the heck is that?’ ” Hlynka remembers. “The water went down a completely different path than it had in the Minneapolis slide—it zigged where it should have zagged.” In the end, the team had to design a new set of plastic splash guards to take care of the unexpected spillovers. Hlynka still isn’t sure what chaotic inconsistencies were at fault. “It could be something as prosaic as one city being dustier than another,” he says.

New computer models may help minimize such mishaps. Parviz Moin and his Stanford team are tackling turbulence by breaking it down into manageable chunks, using a computer program to determine a fluid’s velocity and pressure at closely spaced points. This approximates the overall flow, much as a pixelated image approximates an actual scene. Yet each “flow event” depends on the one before it, so simulating the entire sequence in detail might take even the speediest supercomputer hundreds of years.

In the meantime, Hlynka will continue to revel in the inexactitude of his science. “I like to think of waterslide designers as similar to cathedral builders,” he says. “In the Middle Ages, they created these beautiful buildings basically because they knew how. They couldn’t express their designs in calculations or formulas, but they got it right anyway. They were guided by visual proportion and a sense of the way the materials behaved.” It takes more than a slide rule, you might say, to build a waterslide.

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