The Society of Automotive Engineers (SAE) International - Getting the Aero Advantage

Championship car racing is challenging in itself, with about 16 teams involved in over 20 races each year. But when regulations change as often as they do, coupled with new race tracks and a longer season, the sport has become as much of an engineering competition as a racing challenge. To remain in the upper echelons of the sport, Team Rahal engineers have sought ways to implement and integrate new computer software simulation techniques into their design cycle. One of these solutions is Fluent's computational fluid dynamics (CFD) software.

One of the major benefits of CFD is that it enables engineers to visualize the airflow over a vehicle without having to conduct resource- and time-intensive smoke tunnel testing. According to Ray Leto, Technical Coordinator at Team Rahal, thermal, flow visualization, and pressure plots enable engineers to determine optimal positioning of wing fences and vortex generators. The positioning of such devices is important to obtain the maximum amount of downforce needed to keep the vehicle under control at such high speeds. "CFD has really allowed us to play with the tuning of where we think these vortex generators should be," he said.

CART rules limit much of what can be done with regard to the diffuser. "The best you can do is tune the front half of the car," said Leto. "This year we've found a lot of success in tuning these vortex generators (to improve the flow into the diffuser). The more mass flow we can get through here (diffuser), the lower the pressure we're going to end up with underneath the car, which is going to give us the downforce we're looking for."

CFD has allowed Team Rahal to explore many aerodynamic possibilities and solutions for its vehicles both during the race season and off-season. Once these options are narrowed down, they can be further explored in the wind tunnel.

"It's kind of a concurrent process," said Leto. "What we tried to do with our team is integrate simulation methods with wind tunnel testing before, during, and after the tests are conducted."

Wind tunnel testing enables engineers to characterize the car, giving basic aerodynamic information about the various components of the vehicle and how these components interact with each other. "I'll make basically a cookbook spreadsheet of various possible brake ducts, front wings, front wing angles - all the variables on the car - from which race engineers can pick setups," said Leto. "So if they run that setup in the wind tunnel, they would know the balance of the car and ride-height characteristics."

Along with CFD and wind tunnel testing, Team Rahal engineers also use a mechanical seven-post rig to gain additional insight into the ride and handling characteristics of the car's setup. "It's basically a four-pad hydraulic test rig that you set an actual car on," said Leto. "It has three hydraulic rams that fit to the front and rear of the vehicle to simulate aerodynamic forces. We can run various road profiles underneath the car and look at the chassis response. You are basically looking at transfer functions from the road to the hub to the chassis. That is another type of simulation where you run very limited types of maneuvers or road profiles and then try to capture some characteristics of what the car is sensitive to."

CFD has also given engineers the ability to measure the lift that occurs on the vehicle's tires. According to Leto, the team has been unable to devise any other method for measuring aerodynamic lift on the vehicle's tires in the wind tunnel.

The team uses Reynard's wind tunnel facility in Indianapolis, IN, to conduct many of its tests. Engineers use a six-component balance to measure forces occurring on the vehicle body while a separate three-component balance is employed to measure forces on the front nose and wing and the rear wing by itself. Drag arms are fitted with strain gauges to measure wheel drag. "But we have no way of looking at the lift on the wheels," added Leto. "CFD is really the only way that we can look at the lift changes on the rear wheels." Using CFD simulation, Rahal engineers discovered 220-265 N (50-60 lb) of lift per axle.

According to Rahal engineers, CFD and wind tunnel testing are used to determine and preserve aerodynamic balance to the car, which can be based on the driver's needs (feel of the car), the particular race track, and rules changes. Often, changing one component or variable will cause other changes to be made to retain the balance and performance of the car. "Maybe you do something with the front wing of the car that will make it a lot more pitch sensitive," said Leto. "With that you might need to change the ride height of the car or the stiffness of the springs. They are coupled together. It is difficult to make a complete determination, so you try to use the simulations to give you an idea."

The various racetracks and the regulations imposed at them are major determinants of the car's setup for a particular event. "The rules allow us to design the wing within a different envelope and track," said Leto. "So you end up with a road course setup with 17.8-22.2 kN (4000-5000 lb) of downforce and a super speedway setup around 7 kN (1600 lb) of downforce."

With a road course the car has to be able to turn both directions effectively and be capable of braking and accelerating hard (these cars brake at 3.5 g and accelerate at 1.5 g). "For this situation, we run different brakes with heavy brake cooling," Leto continued. "Mechanically, the focus on the car is running more of the symmetric suspensions because you will have to turn left and right. The settings on the rear of the car are mostly biased towards getting rid of wheel spin and putting the car's 670 kW (900 hp) to the ground. You also want a car that changes direction very quickly. So the car is going to be a lot less stable than on an oval situation where you want the car to have an immediate reaction and some pitch sensitivity. One of the biggest problems on road courses is understeer, in which the car will tend to go straight when cornering. You want the car to react quickly, but also not go into oversteer during power applications."

Engineers make the cars more stable for oval tracks. For superspeedways, teams go with a mechanical setup that provides the most grip as possible so as to reduce drag and improve speed down the straight. "To bias the setup for the corner, you have to make the car completely asymmetric in its settings," said Leto. "The cambers are all tipped into the corner negative cambers on the outside and positive cambers on the inside. The aerodynamics are biased to keep the balance in the middle of the corner, while allowing you to go down the straight as fast as possible. On an oval you don't want the car to be too quick turning into the corner, yet you don't want to have a lot of understeer. So there is a very fine balance between keeping the car stable, but also letting it carry a lot of speed through the corners."

"It is kind of a unique challenge for our series the variation of the types of tracks. We deal with several different rules packages, tracks, and completely different criteria. We do it around the same basic chassis, but the aerodynamic setups are completely different, and we manipulate suspension geometry the center height, motion ratios of the dampers and springs, and camber characteristics for different types of tracks."

Because of the differences between tracks and the setups needed for them, Team Rahal engineers prepare for each race by modeling the racetracks. "We have real 3-D maps of all the racetracks," said Leto. "We take those track profiles and find a driver line around those tracks. Then, on that driver line, we find the maximum performance that we can get out of the car. And that basically gives us the speed profile and a lap time around those tracks."

With those profiles and lap times, Leto would conduct several simulations of the track to find out the lap time sensitivity to lift and drag. For example, at a particular track the optimum lift-over-drag ratio for the vehicle is 2.3:1. This means that for every 445 N (100 lb) of drag added to the car, engineers must find 1023 N (230 lb) of downforce for the vehicle to travel at optimum speed.

"Even though you're getting new cars every year and things are changing, it's always a progression of building on the knowledge you had before," said Leto. Manufacturers typically will watch and learn about the various changes that team engineers make to the cars week to week. Some of these changes may find their way into the following year's design. "You'll see maybe a large shape change in the side pod or the underbody of the car for next year," said Leto. "On the suspension side, you'll see that people have found a few areas for improvement, and those will come with the model change for the following year. And there are always small things that manufacturers try to do on a yearly basis, such as eliminating friction from the suspension by going to flexures instead of joints, or looking at different bearing packages for the dampers and pushrods."

However, Leto says manufacturers still are careful about doing too much to the cars each year. "They're not going to change the entire suspension geometry or weight distribution of the car by some huge amount because this would completely disrupt their customers," he said. "You learn a certain amount of setups that fit your driver. If you have a completely different thing show up the next year, you might be tempted to pull out your old car. There is an evolution. You will see more discrete steps from season to season because the manufacturers will try to pack in a lot of improvement at that point."

Other changes that teams must be wary of are rules changes from year to year. One such change came this year, when CART placed a restriction on the diffuser size, causing Team Rahal to narrow the structure up to 254 mm (10 in) off the centerline on either side of the car. "That was a big restriction for us," said Leto. "It cost us probably 10-15% of the downforce we had on the car from last year to this. To get that downforce back there was a lot of development work that went on. The change also made the car more flat underneath, making it more sensitive to ride height."

Beginning with the 2001 season, Champ car teams will be faced with a new regulation that bans track testing during the race season. However, they will still have opportunities to test the cars during practice runs at the start of the race weekend. "I think it's going to put more emphasis on the process that you develop as a team for integrating design tools, rig testing, and other simulation tools to get that information back to the race engineers so that they can implement improvements right at the race track," Leto added. "Because you're not going to get the chance to try them until you have it on the car."

Frank Bokulich, Associate Editor

Fluent Inc. - Automotive Division, 220 E. Huron, Suite 470, Ann Arbor, MI 48104 . Tel: 734-213-6821; Fax: 734-213-0147.