BACKGROUND: Outdoor and indoor wave pools and water theme parks are hugely popular in the US. These are sanitized, manmade versions of nature's wild surfs. In wave pools, the water is chlorinated, the beach is concrete, and the waves arrive like clockwork once every few minutes. The densest collection of wave pools can be found in the Wisconsin Dells, home to 18 water parks. Such parks are good clean family fun, but they also illustrate some fascinating basic science about waves.
MAKING WAVES: Waves are the result of wind traveling over water. Imagine a breeze blowing gently across the surface of lake, creating small waves. The waves arise from the surface tension of water. The molecules on the water's surface hold together and form a sort of 'skin', which makes the surface stretchy, and therefore 'sticky.' As more air passes over that sticky surface, it grabs some molecules and pushes them into molecules ahead, which push on other molecules, and so on, so that the wave travels to the opposite end of the shore. The water mostly stays in place; it's the disturbance caused by the wind that is moving across the water. In strong wind, the waves become choppy. The stronger the wind, the larger the waves, because as the waves move, they run into each other and merge adding their energy together to become bigger and move faster.
HOW WAVE POOLS WORK: There are a number of ways to recreate wave action with just a basin of water and a means of creating a periodic disturbance: a strong blast of air, perhaps, or a rotating paddle wheel. In one such approach, there is a pump room below the pool, which causes a high-speed fan to blow air into a wide metal pipe, leading to an exhaust port. In the middle of the pipe is a butterfly valve, a wide disc with a swiveling metal axis rod. When the rod swivels so the disc rests horizontally in the pipe, it blocks the air flow, while swiveling the rod the other way moves the disc to a vertical position, allowing air to flow. A hydraulic piston swivels the rod back and forth a regular intervals, causing short burst of pressurized air to flow up the exhaust port and blow on the surface of the water ı an artificial wind. This creates small waves across the water poolıs surface.
BIGGER IS BETTER: A large wave pool doesn't push on the water with air or a paddle; instead, the wave machine dumps a large volume of water into the deep end of the pool. The surge in water travels all the way to the artificial 'breach', so that the water level in the pool once again balances out. Dumping more water into the pool increases the size and strength of the wave. There are five basic components to such a system: a water pumping system, a water collection reservoir, a series of release valves at the bottom of the reservoir, a giant slanted swimming pool, and a return canal, leading from the breach area back to the pumping system. In this scenario, the water is constantly circulating, moving from the deep end of the pool, out to the canal, around to the pumping system, and back into the deep end of the pool.
The American Association of Physics Teachers contributed to the information contained in the TV portion of this report.
