Reducing aerodynamic drag by 12 counts (reducing Cd by 0.0012) is equal to gaining about 1 mpg in vehicle fuel efficiency-“and our goal is to eliminate as many counts as we can,” said Scott Miller, General Motors' Director of Global CO2 Strategy, Energy, Mass, and Aerodynamics, at the recent opening of his company's new reduced-scale wind tunnel at the GM Technical Center in Warren, MI.
The new 32,000-ft2 (2973-m2) facility represents a $30 million investment for improving vehicle efficiency and reducing greenhouse gas emissions through greater aerodynamic design, testing, and validation. “Every vehicle we develop is measured in terms of dollars-per-gram of CO2 emissions per mile,” Miller noted.
Designed by Jacobs Engineering, the new wind tunnel uses a belt-type moving ground plane to simulate full driving conditions on 40%-scale clay models up to an indicated 155 mph (250 km/h). The aim is to accurately capture aerodynamic forces and moments including drag and side-force data, individual tire contact-patch downforce data, and the influence of rotating wheels.
Moving ground plane simulations better optimize the vehicle underbody and wheel design, the GM engineers said.
Using reduced scale models enables faster, less expensive surface changes per shift, with higher fidelity results and greater repeatability, said Miller. “One pound of clay on a 40% model is equal to 16 pounds of clay on a full-scale model,” he noted.
The model is placed on a turntable setup with capability for 30° maximum yaw in both directions. There is also a boundary- layer suction function and capability for ride height adjustment. Wind speeds generated by the fan powered by a 1100-hp (820-kW) AC electric motor can accelerate the model to a simulated 250 km/h within 30 s, noted GM Senior Aerodynamicist Nina Tortosa. The fan is fitted with wood-carbon fiber composite blades.
Ghafari, a global architectural and engineering firm, designed the wind-tunnel building. As the new facility goes on line, GM will take its full-scale tunnel off line for extensive upgrades in early 2016. They include addition of a moving ground plane and full acoustics capabilities. Completed in 1980, the “big” tunnel was the first full-scale aerodynamic wind tunnel in the U.S. designed specifically for testing full-size automotive vehicles. Jacobs Engineering is performing the upgrade work. While the tunnel is out of commission, GM will use the Lockheed Martin tunnel in Georgia for its full-scale aero work.
During a media tour of the new reduced-scale tunnel facility, GM aerodynamics engineers showed the first example of its new 40%-scale model vehicles-a Chevrolet Silverado pickup that was impressively detailed. The models, which cost approximately $450 each to build in house, are constructed using GM's stereolithography/additive manufacturing machines along with CNC machining. The model truck features highly accurate underbody detail, working suspension systems, and spinning wheels, causing equally high fascination and covetous stares among the assembled reporters.