We’ve come a long way since the early days of analyzing airflow. Before the advent of wind tunnels and computerized simulations, automotive designers would tape small tufts of yarn, called telltales, on a car body in order to observe turbulence. While driving alongside, if they saw those snippets of woolen fringe wiggling too much in the wind, it was back to the drawing board. Crude, and only somewhat effective.
 In addition to wind-tunnel testing and on-track experience, Computational Flow Dynami
Much more recently, development of the new Corvette Stingray’s wind-cheating shape involved sophisticated CFD (Computational Flow Dynamics), leading up to lengthy hours of wind-tunnel testing. Computer-generated meshed models and “ribbons” have replaced those bits of yarn on a fullsize vehicle. These digital ribbons visually display the path of a single particle as it travels through and around the body. And this powerful analytic tool provides data on dozens of parameters—such as heat, velocity, and pressure coefficients—among many others, allowing for an unprecedented level of precision.
For instance, Corvette designers found through CFD that adjusting the shape of the side mirrors by as little as a single millimeter impacted the smoothness and direction of the airflow over the ’14 Stingray’s body, as well as through the rear ducts and spoiler. In addition, CFD analysis facilitated the design process, as it minimized the amount of time required for wind-tunnel testing. Even so, many hours went into evaluating airflow at speeds as high as 120 mph in the tunnel, and then 180 mph in a rolling road tunnel as a final check.
But the design of the new Corvette was not determined solely by digital readouts generated in a virtual setting, or in the controlled environment of a wind tunnel. As noted in last month’s feature on the design of the ’14 Stingray, the initial R&D on aerodynamics was derived from real-world experience on the track with the C6.R competition Corvette. One area in particular focused specifically on tilting the angle of the radiator forward, along with releasing air from the center of the hood. In this follow-up article in our series on the new Stingray, we’ll be digging into more detail on the interplay of drag and lift, along with other aspects of the arcane world of aerodynamics.
In addition to speaking with Kirk Bennion, the Corvette’s Exterior Design Manager, we gathered input from John Bednarchik, Corvette Lead Aerodynamics Engineer. A 15-year veteran who’s worked on a variety of Chevrolet vehicles, Bednarchik has degrees in mechanical engineering from both Lawrence Technological Institute and the University of Michigan, with special studies on fluid dynamics. While he notes that the primary reason for refining aerodynamics on Chevrolet vehicles in general is to improve fuel efficiency, the Corvette presented added challenges.
 Every surface of the Stingray was analyzed for airflow, including the vents in the si
As Bednarchik worked early on with Bennion to develop surfaces for the Stingray, “One of the big goals for the C7 was to enhance the lift performance,” he says. On the other hand, “We wanted the Cd [coefficient of drag] to be as low as possible.” Those are competing concerns, because as you reduce drag, lift increases. And conversely, as lift decreases and the car hunkers down, drag goes up. Balancing these two key elements more than doubled the amount of aerodynamic work, he notes.
Indeed, a critical question for each of the final five thematic, third-scale models of the seventh-generation Corvette was whether they hit the aero targets. If they didn’t, the team couldn’t move forward with them. Bednarchik didn’t specify what sort of design elements impede airflow, but he did allow that certain “more aggressive” styles either weren’t as slippery or created more lift.
“The new exterior is not just about styling,” Bennion confirms. In effort to create uniqueness, much of the shape was “aero driven, a sign of the times.” Partly because of the increase in amateur racing in recent years, the new Stingray had to be more track capable right off the showroom floor. So the airflow aspect had to not only reduce fuel consumption and optimize lift/drag characteristics, but also enhance several other areas such as brake and differential cooling.
Starting at the front, the leading edge of the front fascia needed to set up the airflow using a parabolic shape (as seen from the top down). “You really have to nail that front corner,” Bednarchik points out. In addition, the frontal area has to address multiple demands: lift/drag, noise levels, manufacturability, and even European pedestrian-safety requirements. Balancing this mix of competing concerns actually drives the entire process. Put another way, Bednarchik says his task as aerodynamic engineer for the C7 was, “To make it look good and still function—we gave Kirk what he needed.”
Doing so required not only developing an aero-optimized grille and air dam, but also a removable center section for the Z51 performance option, to provide better stability at high rates of speed. Bednarchik points out that while shapes for improving fuel efficiency typically begin to have an effect at highway speeds, lift and drag components become critical from 150 mph to maximum velocity.
In addition, as mentioned previously, the previous configuration of “bottom breather” airflow to the radiator didn’t work all that well at speeds that Corvettes are now seeing on racetracks. The change to a forward-angled radiator would be the first of many refinements forged in the heat of competition. In this configuration, air is force-fed into the engine bay and then ducted out over the top of the hood, an arrangement that provides more inherent downforce. About half the air goes over the top, with less than half spilling out through the side vents.
 Every surface of the Stingray was analyzed for airflow, including the vents in the si
Even the shaping of the air duct in the hood makes a difference. Rather than being merely an added-on aesthetic element, the extractor’s vanes are angled to manage airflow, minimizing the separation of the air from the body.
The CFD and wind-tunnel analyses led to other subtle features that streamlined the shape, “to make sure the airflow sticks to the body,” Bednarchik says. If it doesn’t, turbulence and wind resistance can develop, increasing the coefficient of drag. For an aerodynamic engineer, that’s the enemy.
Yet minimizing this aspect required tempering by certain legacy styling cues. For example, the design team determined early on that retro-inspired side coves would be an important element of the new body. Since the shape of the early coves created some turbulence, Bennion and Bednarchik took pains to ensure that the C7’s modernized treatment did, in fact, relieve underhood pressure and thus lower the car’s Cd.
But it doesn’t stop there. When we offhandedly inquired about what went into the shaping of the side mirrors, Bednarchik enthusiastically responded, “I’m really glad you asked about them!” Clearly we had touched a nerve for him on a subject that casual observers wouldn’t normally give a second thought.
It turns out that nearly 20 different mirror configurations were considered and tested in the wind tunnel, leading to the discovery of that millimeter of difference mentioned at the outset. “You can’t just put mirrors on from another car and expect the airflow to work the same,” Bednarchik explains. “Their wake affected airflow to the [auxiliary] coolers and Z51 spoiler. You have to look at it as a system. You can’t develop a mirror on one car and transfer it to another.”
 While the ducts in the rear quarters might look inconsequential, an extraordinary amo
Why so much time spent on such seemingly simple elements? Bednarchik admits that aerodynamics at times is a “black art, trial and error at times.” So, surprisingly enough, yarn tufts are still found in the designer’s toolbox (to evaluate the smoothness of large body panels), along with another telltale: ink drops. These were applied numerous times on the Stingray’s mirrors, so much so that the side of the test car was completely stained after 20 sessions in wind tunnel, visibly displaying the air paths and possible disruptions in flow.
Another area required even more experimentation. “We spent a lot of time with the CFD in designing the rear aux coolers,” Bednarchik notes. Again emulating the work done by the race team, designers configured the bodywork so that air enters the tops of the quarter panels, flows to the coolers housed in the lower outboard corners (trans on driver side, differential on the passenger’s), and exits at the rear fascia.
“We learned a lot from them, in aligning the vanes and the grille’s spider-weave mesh. Certain configurations would either choke or improve airflow.” All told, these rear cooling ducts alone required 100 different iterations evaluated over a year’s time.
Why was this element so critical? Because the ducting had to be “…not just functional but optimal,” Bennion notes. “In feeding the right amount of air—eight cubic meters per minute—each vane has its own particular location and shape. We don’t have a car unless we get all the airflow into it.”
Stepping back and considering the airflow dynamics as a whole, “The big takeaway is that the Stingray is great looking, but every aspect is aerodynamic, with low drag and enhanced lift,” Bednarchik concludes. (We’ve reviewed some preliminary figures, but final numbers were not available as we went to press.)
Suffice it to say that every smidgen of slipperiness is like free horsepower, giving the car an edge over its competition. And less lift means superior road-holding at high speed. All the CFD work in the wind tunnel paid off, as the lift numbers on a box-stock C7 are lower than on the aero-optimized C6 ZR1.
And while a wind-cheating shape is just one weapon in the C7’s formidable performance arsenal, it works in concert with the brawnier powertrain, stiffer structure, and other tangible enhancements to yield what is sure to prove the best-driving Corvette to date. Says Bednarchik, “The feedback from the ride-and- handling guys is that the ZR1 is faster…but the Z51 Stingray is better.”