A few years ago, when Jaguar engineers commenced work on the recently introduced XE small sedan, they took what then seemed like a risky break from normal practice. Instead of building full-size clay models to develop the XE’s exterior details in a wind tunnel, they took a chance on a technology called Computational Fluid Dynamics (CFD). This enabled faster and less expensive development using digital simulation models instead of traditional physical prototypes. And a better chance of beating the class-leading BMW 3-series aerodynamic performance.
Wind tunnels were invented in the 19th century to test aerodynamic properties. The Wright brothers used one to develop their Flyer in 1901 and Gustav Eiffel conducted thousands of tests in a tunnel built near his Parisian tower. Today, wind tunnels are a key means of reducing aerodynamic drag to boost fuel economy as every maker strives to meet daunting federal mpg requirements on the books for 2025. But the success both Jaguar and Tesla have achieved developing cars with CFD suggests there’s a new kid on the testing block which might knock the wind out of wind tunnels.
CFD has existed for half a century but recent strides in computing power are finally making this branch of fluid mechanics useful to car developers. In the most modern CFD simulations, air molecules flow on streamlines that travel over, under, or though a hypothetical car. Every change in a molecule’s momentum as it moves around the car results in a small force applied to the vehicle. CFD sums up those momentum changes and their directions to quantify the total force in the horizontal plane—aerodynamic drag—and the changes in wheel loadings—aerodynamic lift—plus other useful information.
Jaguar and Tesla both used PowerFLOW (PF) CFD software developed by Exa, a Boston-based enterprise founded in 1991 to provide advanced simulation tools to transportation product developers around the globe. “Inside” a supercomputer, PF blows a digital blast of air over the detailed math model of a vehicle to measure things. In addition to quantifying aerodynamic drag and lift, PF can be coupled with other Exa software to assess the interior and exterior noise related to airflow, the cooling efficiency of powertrain and chassis components, and how well the air conditioning cools the cabin.
Jaguar began the XE’s design by creating detailed mathematics models for use throughout the car’s development. These computer-aided design (CAD) and computer-aided engineering (CAE) models quantify unibody stress and strain, predict crash performance, and reveal how much the finished car will weigh. They’re also useful for manufacturing the tools needed to cast the engine block, stamp the body panels, and mold the bumper covers. Math models are standard operating procedure these days because they shorten the car-design and development processes by years.
With the XE’s math model in hand, Jaguar engineers used Exa’s PF simulation software to fine-tune the car’s shape. An early concern was providing the front brakes with sufficient cooling airflow. Ducts guiding air through the fascia to the calipers and rotors solved that problem.
After 1200 CFD simulations requiring 8 million computational hours (equivalent to 8000 hours of wind-tunnel testing), Jaguar engineers had whittled the XE’s drag coefficient down to 0.26, the best it had ever achieved for a production model. Just to be sure, it built a full-size hard-foam validation model for testing. Chief program engineer Nick Miller was happy with the results. “We did a little bit of fine-tuning in the tunnel but the simulation and the validation models were very close.”
Tesla engineers developing the Model S electric sedan found Exa’s PF simulation software beneficial for studying and improving the complex airflow in the front-fender and wheel-well areas. The goal was guiding the air so it hit the front wheels head-on instead of at an angle. Tesla’s lead aerodynamicist Rob Palin noted, “The side of the tire can act like a bucket catching the air, producing significant drag. From the initial concept to the final design, we made huge improvements in this area of the car.”
Palin’s engineers also used the PF tool to minimize the drag and the noise erupting inside the cabin with the sunroof open and an air deflector raised. The simulation approach was quicker than wind-tunnel testing and provided more information. “The Model S’s early design concept had an 0.32 drag coefficient,” noted Palin. “Major shape changes reduced the drag to 0.27 and smaller changes provided further improvement to 0.24.” Tesla used standard wind-tunnel procedures to confirm these results, and then ventured beyond normal tests with CFD to diminish drag experienced during real road driving. Every bit of aerodynamic drag reduction counts when your maximum range is less than 300 miles.
Exa vice president for ground transportation applications Dr. Ales Alajbegovic explains, “Wind tunnels tell what happens with the vehicle, such as quantifying the aerodynamic drag force. Simulations go beyond that by explaining why. On the road, cars experience crosswinds and turbulence that tunnels can’t accurately simulate. This means that some air dams and deflectors developed in a wind tunnel may not work on the road when turbulence disrupts the wake behind the front tires.”
CFD tools such as PF offer the opportunity to factor additional road conditions into testing and development. The resulting efficiency gains not only will help carmakers meet tough fuel-economy standards, they should also benefit car owners. Testing that’s better aligned with road conditions might even narrow the gap between EPA estimates and real-world gas mileage.
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