# Smile! Dimples boost your mileage

Trucks covered with tiny indentations, like those on golf balls, experience less friction

Part of the energy used to move a car, truck or plane is spent overcoming the friction posed by air. The force caused by that friction is known as aerodynamic drag — and it can really hold a vehicle back. Over the years, researchers have developed ways to cut that drag. Now, an 18-year-old engineer has come up with a new approach. It’s specifically designed to boost the fuel efficiency of large trucks. And her solution: Just add lots of tiny dimples.

When air slams into an object, it has nowhere to go except around it. That flow around those edges starts out smooth. But at some point, it turns chaotic and turbulent. That turbulence sends the air swirling, almost like tiny, slow tornadoes. Scientists refer to the whirling air as vortices. (If there is just one, it is called a vortex.)The more quickly the flow turns turbulent, the greater air’s drag will be, explains Daniela Jimenez. She’s a junior at Preparatoria del Tecnologico de Monterrey in Zapopan, Mexico.

Key to reducing drag is keeping the flow of air around the vehicle smooth for as long as possible. On small objects like golf balls, tiny indentations — or dimples — keep the air moving smoothly around the ball. Jimenez wondered whether adding big dimples to big objects would offer a similar benefit. To test this hypothesis, she decided to place small models of trucks in a wind tunnel. (Engineers use wind tunnels to measure airflow around scale models of vehicles such as cars, planes and rockets. Then, they scale up those measurements to estimate what airflow around a full-size vehicle would be.)

Jimenez studied the flow of air as it curved around 30 different models of a tractor-trailer. These are the trucks that typically deliver goods long distances. Some model trucks had golf-ball-like dimples on their fenders (wheel coverings). These are tiny indentations shaped as if half of a ball had been pushed into the metal. On a full-size truck, each scoop-shaped indent would range between 0.5 to 5 centimeters (0.2 to 2 inches) across, Jimenez says. For her small models, she made the dimples proportionately tinier. In all cases, the dimples on her models were arranged so they simulated features that would be spaced 1.25 centimeters apart on a full-size truck.

The wind-tunnel tests simulated air flowing past full-size trucks zooming down the road at 88 kilometers per hour (55 miles per hour). Some dimple sizes improved air resistance better than others, of course. Ones that measured about 4 centimeters (1.6 inches) across on a full-size truck would cut drag the most — by about 28 percent, the teen found. This should boost a truck’s fuel efficiency by almost that much, Jimenez calculates.

She presented the results May 13 in Phoenix, Ariz., at the Intel International Science & Engineering Fair. The Society for Science & the Public, which created the fair in 1950, still runs the competition. (SSP also publishes Science News for Kids.)

A large machine bends and squeezes the metal used in a truck’s body to create its curves. Creating a dimpled surface should cost no more than making a smooth one, the teen says. So introducing the fuel-saving dimpling that she proposes shouldn’t add much, if anything, to a new truck’s costs, she says.

And that should bring a big smile (and possibly dimples) to even a grizzled trucker’s face.

Power Words

aerodynamic drag The force on an object caused by the friction of air flowing over its surface.

hypothesis A proposed explanation for a phenomenon. In science, a hypothesis is an idea that hasn’t yet been rigorously tested. Once a hypothesis has been extensively tested and is generally accepted to be the accurate explanation for an observation, it becomes a scientific theory.

turbulence The chaotic, swirling flow of air. Airplanes that run into turbulence high above ground can give passengers a bumpy ride.

vortex A region of fluid that has a spinning flow. Vortices (the plural of “vortex”) come in all shapes and sizes. Tornadoes are vortices, and so are the tornado-like swirls inside a glass of tea that’s been stirred with a spoon. Smoke rings are donut-shaped vortices.

wind tunnel A facility used to study the effects of air moving past solid objects, which often are scale models of real-size items such as airplanes and rockets. The objects typically are covered with sensors that measure aerodynamic forces like lift and drag. Also, sometimes engineers inject tiny streams of smoke into the wind tunnel so that airflow past the object is made visible.