Computational investigations of base drag for squareback automotive geometries

Drag reduction continues to be a high priority for automotive manufactures, to reduce CO2 emissions targets and improve the range of electric vehicles. For squareback geometries, a popular vehicle shape, base pressure drag contributes a large proportion of the overall drag and therefore a robust computational method was developed to characterise wake structures, for chosen simplified models, and their influence on base pressure drag. Within the method development both spatial and temporal resolution were considered and the importance of the replication of flow near the floor of the computational domain identified. Further to this the sensitivity of a highly simplified model, with lateral and vertical symmetry, to experimental misalignment was indicated through analysis of the time-dependent wake. Analysis of the flow fields demonstrated a relationship between the main recirculating structures and the near base flow velocities. A method of manipulating this structure was identified, in the form of rectangular cross-section, horizontal base slats. The slats were designed to be geometrically small to ensure minimal aesthetic impact. A statistically significant drag reduction was achieved due to flow impingement on the upper slat surface and a reduction in near base velocity of the lower recirculation, these mechanisms were also observed on a fully detailed geometry. Further configurations considered an asymmetric slat design and a combination of slats, with the optimum configuration producing a drag reduction of 6 counts (ΔCD = 0.006), worth approximately 0.75gCO2/km or a 3km increase in electric vehicle range. This is especially significant given how small the geometry change is, and highlights how many localised flow modifications can produce large cumulative effects; indicating existing vehicle development strategies should be supplemented by localised feature optimisation.