A vehicle's drag coefficient is reduced to a single number, and that number is marketed. From an engineering standpoint it is useful only when you know where it comes from.
The dominant component: the rear wake
On road vehicles most of the drag is pressure drag, and most of that comes from the rear wake. The low-pressure separated region behind the vehicle pulls it backwards.
That is why the rear design — backlight angle, boot edge, diffuser, rear bumper — has more influence on drag than the front. Where separation begins is decisive. See the separation note.
The overlooked items
Wheels and wheel housings. The rotating wheel and the circulation inside the housing make up a surprisingly large share of the total drag. An analysis without modelling wheel rotation gets this component systematically wrong.
Cooling flow. Air passing through the radiator loses momentum and constitutes a drag item. That is why active grille shutters exist. If the engine bay flow is not in the model, both drag and cooling come out wrong.
The underbody. An irregular underside behaves like a rough surface. Covering panels reduce both drag and noise.
Lift and balance must also be measured
A change that reduces drag can generate lift at the rear axle and spoil high-speed stability. Front and rear axle lift should be reported separately; Cd alone is not a design criterion.
Modelling notes
The road and wheels must move. A stationary floor produces an unrealistic boundary layer.
Keep the blockage ratio low. If the domain is not large enough, the vehicle sees its own blockage. See the boundary condition note.
The rear wake is unsteady. A steady RANS gives an average; for the detailed wake structure and its oscillations a scale-resolving approach is required.
References
- Spalart, P. R., Jou, W.-H., Strelets, M. & Allmaras, S. R. (1997). Comments on the feasibility of LES for wings, and on a hybrid RANS/LES approach. In Advances in DNS/LES, Greyden Press, Columbus, OH.
- Menter, F. R. (1994). Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal, 32(8), 1598–1605. DOI: 10.2514/3.12149