Effect of cooling air intake and outlet positioning on the aerodynamics of BEVs

Licentiate thesis, 2025

The transition towards battery electric vehicles (BEVs) has increased focus on improving vehicle energy efficiency, particularly by optimising vehicle aerodynamics and thermal management. Adequate airflow through the heat exchanger is essential for maintaining component and cabin temperatures but comes at the cost of increased aerodynamic drag. This penalty, termed cooling drag, depends on several factors, including the location of the air intake and outlet, and the required cooling airflow rate. This work investigates how intake and outlet positioning affects both exterior aerodynamics and cooling efficiency, using high-fidelity CFD simulations on a modified open-source AeroSUV model representative of a BEV configuration.

Two approaches were initially employed to compare different intake and outlet configurations. The first method allowed for comparison of vehicle drag by imposing a constant mass flow rate at the cooling inlets and outlets. This approach was used to conduct a parametric study by varying the intake positions—laterally and vertically—while keeping the total intake area constant. Results showed that relocating intakes away from the stagnation region increased drag, while the flow field downstream remained largely unaffected. The second approach evaluated cooling efficiency by studying the relationship between cooling drag and mass flow rate, and was applied to analyse the influence of six outlet configurations across the hood, underbody,
and wheelhouse. The findings showed that wheelhouse exits delivered the highest airflow through the heat exchanger, while the hood outlet provided the best cooling efficiency.

Since cooling airflow requirements vary throughout the drive cycle, active grille shutters and a fan were incorporated to explore performance across a wider range of flow rates. The inclusion of fan power, necessary to sustain airflow in the absence of sufficient ram air, was found to significantly affect overall efficiency. Assumption of linear relationship between cooling drag and airflow rate and exclusion of fan power limited the applications of the second evaluation method. To overcome these limitations, a third approach was developed to evaluate overall cooling system performance by integrating aerodynamic drag and fan power consumption across a range of operating conditions. This led to the creation of an objective function, which served as the primary metric for comparing different configurations. The results demonstrated that the optimum configuration was dependent on the cooling airflow requirements, and that the lowest averaged drag could still be achieved even when fan assistance was required to meet cooling demands.