2011 Chevrolet Cruze in the GM wind tunnel – Click above for high-res image gallery

Thirty years ago this week, General Motors opened its first full-scale wind-tunnel at its Warren, MI, technical center. Today, the facility is more important than ever. As automakers strive to hit new fuel economy mandates as well as develop electric vehicles, aerodynamics engineers at GM are working around the clock in the tunnel to best manage the air-flow around and through vehicle. We had the opportunity to take a tour of the facility this week and talk with some of the engineers.

GM engineers did a wind tunnel evaluation on the Firebird I concept vehicle back in 1953. That fighter jet-shaped single-seater was put into the aircraft tunnel at Cal Tech University. Over the next 27 years, GM used a variety of tunnels, including the NASA Ames research center, to test its vehicles. In the late 1970s as fuel economy became more important, GM decided it needed its own tunnel. Construction began in the southwest corner of the tech center campus in 1978 with operations beginning in August 1980. Senior project engineer Frank Meinert took us on a tour of what is officially known as the aerodynamics lab and you can learn all about it after the jump.

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The reinforced concrete structure consists of three main areas: the closed-loop tunnel, the test section and the offices. There are actually three separate foundations for the facility, one to support the tunnel, one for the offices and a third for the main balance. The balance is a mechanism that sits below the test section and actually makes the most important data measurements for evaluating vehicles.

The foundation for the main balance sits on pilings set 70 feet into the ground. The intention is isolate the balance from vibrations caused by trucks rolling by on nearby 12 Mile road so that the readings from the load sensors can be measured accurately. The entire balance system, along with the platform that vehicles sit on, can be rotated so that crosswind behavior can be evaluated. The system can monitor vertical, longitudinal and lateral loads at each of the four corners of the vehicle. The loads at each corner can be translated into lift/down-force and drag in the various directions. There are in fact two balance mechanisms on the same structure. The primary balance takes measurements from full-scale vehicles. A smaller balance is used to take measurements from one-third scale models.

The test section of the tunnel and the adjacent control room have seen the biggest changes over the last three decades. In those early days, the focus was entirely on aerodynamic drag. However, as drag has been reduced and vehicle sealing improved, noise has become more of an issue. In its original configuration the test section, like the rest of the tunnel, was lined with concrete. If you've ever stood in a large empty concrete structure, you will have noticed that the material is very acoustically active and echos abound.

In 2001, the test section and some other aspects of the tunnel were modified and lined with acoustical foam to slash the background noise levels. These days, about 50 percent of the work in the tunnel is focused on noise – increasingly important with hybrid and electric vehicles where power-train noise has been all but eliminated. The rest of the tunnel testing is split between drag and air-flow testing.

The addition of the acoustical lining meant that the operators in the control room could not see through the windows directly into the test section. The control room has been revamped with modern computerized measuring equipment in place of the old panels of dials and LCD monitors allow the operators to see what the cameras in the tunnel see.

The test section itself measures 18 feet high, 34 feet wide and 71 feet long. That large cross section means that interference from the walls is kept to a minimum and even GM's largest trucks only occupy about five percent of the sectional area. At each end along the floor there is a small step with a slot in it connected to a suction system. As air (or any fluid) flows through a passage, the friction of the surface causes a slower boundary layer to develop. The suction system literally slices off the boundary layer along the floor helping to ensure that the air speed around the vehicle is more uniform.

Many of the tunnels built in the last twenty years, especially for racing purposes, have tried to accomplish this by using what is known as a rolling road. In a rolling road tunnel, the vehicle actually rides on belts with the "road" moving along below the vehicle. While this has some theoretical advantages, it also poses its own problems because some mechanism must be used to hold the vehicle in place, inherently disrupting the air flow. One of the purported advantages of a rolling road is to be able to measure the drag around a rotating wheel. However, according to Meinert, these tunnels typically show a reduction in drag from the rolling wheels while the reality of testing actual cars shows the opposite to be true. GM does occasionally do some testing at other tunnels to validate its own results from the Warren tunnel and has found that its results correlate well with real-world results.

Aside from the test section, the rest of the Warren tunnel has a 988 foot closed-loop air-flow path. Turning vanes at each of the four corners get the air moving in the right direction. The larger vanes in corners one and three help reduce the noise levels while the smaller vanes in corners two and four are more aerodynamically efficient. Screens ahead of the vanes prevent any parts that might come off a vehicle from getting to the fan and damaging it. In between turns three and four is a heat exchanger that covers the entire tunnel section. This is used to maintain the air at about room temperature.

The fan itself is opposite the test section and measures 43 feet in diameter and is powered by a 4,500 horsepower DC electric motor. The six fan blades are made of sitka spruce which was chosen for its light weight and resistance to fatigue. At the time the tunnel was built, the only other real option was aluminum which has a tendency to work harden and crack over time. Even after 30 years, the fan is still using its original blades which have only needed occasional refinishing. Even with this relatively light wood construction, each of the 12 foot blades weighs in at one ton.

Because a rotating fan will causes the air behind it to swirl as well can cause turbulence, the nacelle supports behind the fan have an airfoil shape designed to straighten out the airflow. The tunnel contains 2 million cubic feet of air weighing 75 tons. It takes about 20-30 seconds to get the fan up to speed and then about another 10 seconds for the air-speed to stabilize in the test section. The maximum air speed in the test section is 138 miles per hour.

In the years since the tunnel was opened, simulation has become an increasingly important aspect of aerodynamic development. However, computational fluid dynamics (CFD) is still extremely processor intensive and at this point only about half of the aero work is done with CFD. The nature of aerodynamic development actually means that simulation is used in the opposite way that is done for areas like engine development.

CFD is used for overall shape development while fine tuning is done by hand in the tunnel because modelers and designers can quickly reshape details right on the car in the tunnel with quick iterations. Typically, the engineers can get through 20-30 iterations in an eight-hour shift. Machining new engine parts is far more costly and time consuming. Evidence of this kind of detail work is clear on the rear corners of the Volt where careful shaping helps to manage the air coming off the back the car.

Another, less visible example of the benefits of aerodynamic development is on the new Chevrolet Cruze Eco. The aperture behind the grille has been reduced in area to only what is required to meet the cooling requirements at highway speeds where the air flows faster. The lower grille area below the bumper is only needed at lower speeds. Here, a set of thermostatically controlled shutters is closed at higher speeds to force air around the car instead of allowing it through the engine compartment. The 0.016 reduction in the coefficient of drag produced by these shutters is responsible for a 1 mile per gallon improvement in highway fuel economy.

GM uses the tunnel almost full time these days, but it is occasionally available for outside use. Since GM of Canada is a sponsor of the Canadian downhill ski team, GM has done some testing with the skiers in the tunnel over the past eight years. When it's not being used to measure Volts or snow racers, the tunnel is available for about $2,000 an hour including tech support. This outside work is likely to shrink in volume as new CAFE standards approach.

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