 The colors of the streamlines around the cars indicate the pressure in the surrounding air from Red (high pressure) to Blue (low pressure). High pressure on the nose and headlights of the Monte Carlo SS (left) shows that this area generates the majority of the car’s front downforce; yet this area is Blue on the Impala SS (right), indicating low pressure and lift in the same places. Dark Red on the horizontal surface of the Impala SS’s front splitter shows this aerodynamic device generates significant downforce. NASCAR allows the splitter to be adjusted from four to six inches ahead of the bumper recess to fine tune the aerodynamics—increase or reduce this downforce.
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Most importantly for the teams bringing the new Impala SS to the track is that the new COT racecar's aerodynamics significantly differ from the Monte Carlo SS it replaces. GM engineers used CFD to gain insights into the effects of the new body shape, front air splitter, and adjustable rear wing on aerodynamic performance differences.
"NASCAR specifications require the Impala SS to be wider and taller than the Monte Carlo SS it is replacing, so its frontal area is larger and its aerodynamic drag is approximately 10% greater," Bayless commented. "Initially the Impala SS will have about 15% less downforce than the Monte Carlo SS, which has been highly refined over the years. The change from a spoiler to an adjustable rear wing appears to reduce turbulence in the wake of the car, so when two cars are running nose-to-tail, the less turbulent air behind the first car should alleviate some of the aerodynamic-push experienced by the trailing car."
Although CFD is a powerful engineering tool, Bayless makes it clear it is not a replacement for conventional wind tunnels and track testing. The cars still have to have air blown over them in the real world. Cup teams are going to need test and track time to equal and exceed the aero-balance and baselines they had achieved with the Monte Carlo SS.
 The effects of a conventional rear spoiler on a NASCAR Monte Carlo SS (top) and the rear wing used on the Impala SS (bottom) are contrasted here. Blue in the wake of the car indicates turbulence. The flow of air between the deck lid and the wing (bottom) produces significantly less turbulence (Blue) behind the car than the spoiler (top). This less turbulent air allows a second car to run close behind without suffering as much aerodynamic instability (nose wash-out). NASCAR teams can adjust the wing’s angle of attack from 0--16 degrees to suit track conditions. |
"The computing time required to run CFD programs is a limitation even with a supercomputer," Bayless noted. "A team can run through more tests in a day in a wind tunnel than can be done with CFD. At this point in its development, CFD supplements the testing that's done in wind tunnels and on race tracks. We share the information that GM Racing obtains through CFD with the Chevy teams. Given the level of competition in NASCAR today, it's vital that every team takes advantage of every available resource."
The teams that can marshal all these resources the quickest will be the ones who get a winning grip on the COT.
