Finding Aerodynamic Balance Through Wind-Tunnel Testing

Wind-tunnel testing used to be the domain of OEMs and the highest echelons of racing. You know, GM, NASCAR, IMSA and Formula 1, those kinds of outfits.

In today’s motorsports industry, that caliber of testing facility has become available to all companies who can afford to rent the A2 Wind Tunnel in Mooresville, N.C. It still isn’t cheap, and it requires planning and execution to achieve any measurable results.

Two NASA racers have taken advantage of that in their racing-related businesses and to develop and refine their products on full-scale cars in the A2 wind tunnel. AJ Hartman launched AJ Hartman Aero in 2015, a performance-focused composites shop that specializes in manufacturing wings, splitters, diffusers and vents and ducts for a variety of racing applications. Hartman won the 2019 American Iron National Championship in his EcoBoost V6-powered Mustang, which serves as a test mule for many of his company’s products.

Al Watson is the national director for American Iron, which includes Spec Iron and American Iron Extreme. In 2019, Watson launched Race Louvers, a company that specializes in manufacturing hood and fender air extractors for high-performance street and racing applications. The two became friends at the track because they both raced in NASA Northeast in American Iron.

Watson and Hartman have since partnered up a few times to help develop their companies’ products in the A2 Wind Tunnel and split the costs of renting the facility.

“It’s really why you’re there, for aero balance. The wings that they make these days are highly effective. And in fact, they’re so effective that they usually overpower the front of the car,” Watson said. “So I guess where I’m going at is if you put a wing on a car, and you don’t do anything to the front, you’re going to have loads of back traction and no front traction, which is not going to make your car any faster.

“Same thing on the opposite end. If you were to put a splitter on there, or hood vents or stuff like that, but no wing, you’re going to have tons of front grip and no back grip,” he continued. “So there’s an aero balance, which is very important to racecar drivers. You want to get that aero balance close to 50/50 within reason. So that’s a very useful thing for any motorsports guy, is to bring their car there, and that should be their primary goal, is to get the car balanced.”

AJ Hartman Aero and Race Louvers conducted back-to-back testing on three different hood designs to measure, drag, downforce and pressure differentials in front of and behind the radiator.

Of course, both of them had put in a great deal of work in design and prototyping for years before they ever entertained the idea of testing in a wind tunnel. Hartman had partnered with several consultancy firms staffed with Ph.D.’s to help develop his products in computational fluid dynamics, so he was confident going in.

“There are a few secrets we’ve got to keep to ourselves, but generally, since we had extensive CFD work done on our wings, and canards, and splitter tunnels, and side plates, and all that stuff, we had a pretty good grasp of what they would do,” Hartman said. “And everything relatively performed how we thought it would. And now to the degree, putting an actual downforce number on it, sometimes might be a little bit different, but there weren’t really any huge surprises that were just mind blowing. Because we had a good grasp of aerodynamics heading into the tunnel.”

Installing and testing different hood louvers revealed flow characteristics and radiator differential pressure increases, which enhance cooling.

Watson started race louvers by buying a spare hood for his daily driver, a 2005 Nissan Altima, which he used for testing prototype parts. Watson is educated as an electrical engineer and at one time worked at the Brookhaven National Laboratory on a heavy ion collider project.

He partnered with a mechanical engineer who drafts the prototypes in SolidWorks. Watson built his own equipment to test pressure differentials and other parameters, lined up a supply chain then hung out a sign on the Internet.

For their part, the louvers create downforce by relieving high pressure under the hood. It’s not actually making downforce, per se, but it creates the same effect, and they increase the flow of cooling air through the radiator. The bonus is that the louvers create the downforce while adding only a nominal amount of drag. Of importance to NASA Super Touring drivers, you can buy vents that don’t protrude more than three-eighths of an inch above the hood, so they’re legal.

This bar chart shows the percentage of change in the coefficient of pressure differential when an Audi A4 was fitted with different Race Louvers fitted to the hood.

Race Louvers also sells vents for fenders but only in one height now based on their work in the wind tunnel, Watson said. Testing showed that the short fender louvers worked as well as the medium height and created less drag than any of them, so they only needed to offer one.

“You’re wasting a lot of energy trying to suck it through there, I guess. In any case, we confirmed that,” Watson said. “That told us right away that it was too aggressive.”

A2 also has a full sheet metal shop adjacent to the wind tunnel, so you can pop in to pick up scraps to add wickers and gurneys and subtle additions to whatever your testing. Watson and Hartman also made use of yarn tufts to get a better idea of how air flows over the car, where it stays attached and where it doesn’t. Hartman used the wind tunnel to test different carbon fiber canards, wings and splitters.

“Yarn is nice because it shows you the direction of air flow. If you get any separation bubbles or anything, you’ll see the yarn is flapping around like crazy,” Hartman said. “Sometimes, even the yarn can be coming forward, depending where it falls in a separation bubble. So they’re nice while the wind tunnel is going, you don’t really need to stop and do a smoke run. You can just look at the yarn and see how it changes.”

A good example of a separation bubble is where air will circulate between the rear glass and the deck lid rather than flow over the roof, down the back glass and off the rear of the car.

Yarn can be filmed on video during a test, which takes place at speeds at and above 80 mph, which is a good representation of racing speeds. The smoke wand is hand held and placed manually, so wind tunnel operators must use a slower speed, around 30 mph. Like yarn, the smoke can be filmed and analyzed later, but it also gives the observer an immediate indication of how air flows over, through, under and around a car or piece of aerodynamic equipment. In the wind tunnel, which costs $500 per hour, time is precious, so Hartman and Watson map out their test plans well in advance.

Conducted at around 30 mph, smoke-wand tests show how air flows over, under, around and through a car and its aerodynamic devices. It is a physical manifestation of what is calculated digitally in computational fluid dynamics during the design and development process.

“We’ve only been down there three times, but we spend all day. And then we’re robots, and we get 30, 40 runs in in a day. So we’re not dilly-dallying,” Watson said. “My motto with AJ and whoever’s car we’re borrowing, whoever else that comes with us, my motto is ‘There’s no talking in the wind tunnel.’ We’re just banging out parts. At the end, you can have a five minute conversation. And then there’s the eight-hour ride home that you can talk all you want. You’re not getting billed for it.”

The smoke wand is instrumental because the operators can be right there looking at a specific part, and it provides more detail than yarn tufts. The smoke is a physical manifestation of what is represented digitally in CFD software.

This bar chart shows the differences in pounds of downforce, as measured on floor sensors, that different Race Louvers components make at 80, 100 and 120 mph with and without grill restrictions.

“Again, a real cool one is how it comes out of the hood louvers, or the vortices coming off a wing,” Hartman said. “I know that got caught in a couple videos I did. But seeing the stream hit something, and the smoke, what it does afterward, is cool, where you can’t really get that from wool tufts. So it’s just another tool, another way to look at what air is doing around something.”

In addition to the video evidence from yarn and smoke, the control room also measures the weight of the car — actual downforce — on the front and rear axles and at each wheel if necessary. All the data is compiled on a spreadsheet so Hartman and Watson can compare different tests.

“They give you a spreadsheet of coefficients of all the numbers and you can just type in what run you want to compare to in case you want to compare run one, to run 12 … and it just does all the math for you,” Hartman said. “So you can get total gains across all of the things you changed. It’s pretty cool the amount of data that they give you.”

Sensors in the floor are connected to the control room where operators can measure actual downforce and put a number to it.

These two NASA drivers were able to launch their businesses from their garages and from a passion for racing. The time they spend in the wind tunnel allows Hartman and Watson to develop products that deliver in the real world, on the racetrack where it counts.

“For a small business, like AJ and myself, it’s definitely worth the time and the money,” Watson said. “The way AJ and I do it normally is we’ll find a specific car to take that lends itself to whatever we want to test. And then we’ll decide what we’re going to test.

“And then what we do is we’ll sit down and we’ll look at all the runs that we want to do,” he continued. “And then we’ll rearrange them in such an order than we can go in there in a machine-like fashion and just swap parts. Put a part on, test, put a part on, test, and so on. We’re not thinking about it or trying to figure out what we’re going to be doing while we’re at the tunnel. We do it ahead of time. It gives us a lot more runs per hour.”

AJ Hartman Aero
AJ Hartman Aero YouTube Channel
Race Louvers
Race Louvers YouTube Channel
A2 Wind Tunnel

Image courtesy of Race Louvers and AJ Hartman Aero

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