Test The Limits

Caught up in the turbulence

By Ales Alajbegovic

November 06 2015

We’ve all experienced turbulence on an airplane, but back on terra firma those troublesome, swirling pockets of air can have just as great an impact on how a car moves along the road and how much fuel it spends. And for automakers, the ability to accurately simulate turbulence helps to create quieter, slipperier, and more fuel-efficient vehicles.


The airflow around a car is always turbulent, particularly in the critical boundary layer – the few millimeters closest to the vehicle’s surface – and in the wake the vehicle leaves behind it. Not only that, but a turbulent flow is transient, as opposed to steady-state: it continuously changes with time. Smoke wands show this in a wind tunnel, and the phenomenon is reflected in the measurements that aerodynamicists record during tunnel testing.


Logically, any digital simulation of airflow ought to take account of its transient nature if we want to accurately replicate real-world behavior. If you ignore the transient nature of the airflow, it is impossible to predict what the aerodynamic force on the car will be.


At Exa, we believe that the turbulence model at the heart of our PowerFLOW software is one of the best models used in the industry. We can calculate how the turbulent vortices interact with the car and evolve around it, then use that information to very accurately predict the air pressure distribution around the car. That’s what distinguishes our software from the steady-state codes used by others.


Accurately predicting the aerodynamic forces is crucial to delivering the low drag coefficients (CD figures) with which automakers are helping to reduce their vehicles’ fuel consumption. By modeling how turbulence changes with time, PowerFLOW can provide unmatched simulation accuracy of ±1 count (equivalent to a CD of 0.001), as opposed to an estimated ±30 counts for the steady-state codes, which equates to about a 5% error in the predicted fuel economy!


The technology is built on the strong foundation of mathematical equations that underpin the software. Collectively known as the Lattice Boltzmann Method (LBM), this is the most efficient numerical scheme for simulating transient flows, making PowerFLOW faster and more robust at simulating transient flows than rival codes, which are all based on Reynolds-Averaged Navier-Stokes (RANS) equations. RANS cannot deliver the level of accuracy – the magical ±1 count – that automotive customers are now demanding.


The design of cars such as the Jaguar XE (CD 0.26), Chrysler 200 (0.26), or Tesla Model S (0.24), which have the lowest drag coefficient in their sectors, has been made possible only by the use of PowerFLOW. PowerFLOW gives our customers the confidence to reduce the number of costly and time-consuming physical prototypes and to limit their wind tunnel testing to validation. Transient simulations are critical for the real-world fuel economy predictions that will be mandated by regulations and demanded by the public.