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There’s more to designing a car’s exterior than making it look cool. You have to make it aerodynamic as well. The science of smoothing airflow is of increasing importance as automakers strive to make their cars more fuel-efficient and as batteries play a greater role in propulsion systems. The earliest experiments in aero design can be traced to the ’20s and ’30s, but it wasn’t until the ’70s that automakers took it seriously. Now most of them develop their cars in wind tunnels. General Motors has the biggest in the industry, and it runs 24/7.Left: This composite shows what the Chevrolet Volt looks like in the wind tunnel. That isn’t smoke flowing over the car, it’s a stream of propylene glycol. Engineers use it to study airflow over and around vehicles in the wind tunnel.
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Abandon hope all ye poor designs that enter here! GM’s tunnel went online in 1980, and these days just about everything the General designs passes this through these doors to be tested. The all-electric EV1 was – and remains – the most aerodynamic production car in history when it emerged from these doors in the early ’90s.
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Mission control. Engineers can monitor and record airflow velocity and pressure and their effect on a vehicle’s drag, lift, pitch and yaw. "We’re trying to replicate how a vehicle moves through the air," says Nina Tortosa, the engineer who guided the Chevrolet Volt through wind tunnel development. "Drag gets the most emphasis because of its impact on fuel efficiency. But 40 percent of our work is on wind noise – making the interior quiet."
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Whatever you do, don’t push the red button! Given the size of the wind tunnel – it’s a closed loop 988 feet long with a volume of two million cubic feet – and its importance in vehicle design, the control panel is remarkably straightforward.
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The data display tells engineers everything they need to know about the 43-foot fan at the heart of the wind tunnel. It’s capable of generating wind speeds of 138 mph. Engineers can make as many as 16 runs during a shift.
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This platform in the test chamber is where mock-ups – usually clay models – are placed for testing. The turntables allow engineers to move the models to measure their behavior in crosswinds. Testing starts on 1:3 scale models, and changes are made one at a time to see how they affect the rest of the car. "It’s a dynamic system," Tortosa says. "Make one small change at the back and it completely alters things at the front." Once the design is perfected in 1:3 scale, it’s transferred to a full-size mockup and the tests begin anew.
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The acoustically treated walls of the wind tunnel are a minimum of 18 inches thick. Some 20,000 cubic yards of reinforced concrete were used to build the wind tunnel, which stands on pilings that extend 80 feet into the ground.
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Turning vanes in each of the four corners of the wind tunnel manage airflow through the tunnel, which has a maximum height of 48 feet. The vanes are made of acoustically absorptive material to reduce the noise generated by the massive fan as it spins up to 270 rpm.
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The flow-conditioning screen at the mouth of the tunnel helps straighten out the air before it flows into the semi-anechoic test chamber. Just prior to the screen, a heat exchanger with 175 aluminum vanes keeps the air at 72 degrees Fahrenheit.
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The heart of the beast is a 4,500-horsepower, variable-speed, DC electric motor that turns a six-blade fan at up to 270 rpm.
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The fan has six blades, each weighing one ton. They’re 12 feet tall and made of laminated Sitka spruce, which was selected for its high strength-to-weight ratio. With the motor spinning at maximum velocity, the tip speed of the blades is 415 mph.
credit Photo: Joe Brown/Wired.com
GM’s Nina Tortosa, with her favorite colleague: the fan.
