In this paper, the differences between a production car of the 2018 A-class and an early stage vehicle model with a mostly similar outer skin are examined experimentally and numerically. The aerodynamic development of vehicles at Mercedes-Benz is divided into several phases. When comparing force coefficients differences can be observed between these distinct hardware stages as well as when comparing steady state simulations to wind tunnel measurements. In early phases when prototype vehicles are not yet available, so-called aero foam models are used. These are well-defined full-sized vehicle models, as the outer skin is milled from Polyurethane. Important aerodynamic characteristics such as a motor compartment with a cooling module, deflecting axles with rotatable wheels and underbody covers are represented. In order to close the gap between the aerodynamic reference body and the production vehicle we conducted a convergence study with a geometric alignment of the two vehicle phases to make them comparable. The vehicles were completely sealed afterwards which led to an only surrounding flow and a good convergence of the integral force values. Nevertheless, differences in the flow topology were found by means of total pressure measurements in the wake of the vehicles, mostly resulting from different geometric representations of underbody parts as shown by CFD. The influence of the different surface roughness on the general flow is negligible as proven by boundary layer measurements. By gradually increasing the flow through the vehicles again, measurement and simulation values show a different behavior of the two vehicles. Among the shown differences are the flow out of the motor compartments through various exits and the flow through gaps at underbody parts.
