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. Turbulent flow is also transient, as opposed to steady-state: it continuously changes with time - smoke wands will show this in a wind tunnel, and the phenomenon is reflected in the measurements that aerodynamicists record during tunnel testing. In addition, the air on the road is never steady. There are traffic turbulence and wind affecting the real world aerodynamic performance of the vehicle.
Logically, any digital simulation of airflow should take account its transient nature to accurately replicate the real-world behavior. If you ignore the transient nature of the airflow, it is impossible to predict the exact aerodynamic force on the car.