The importance of the aerodynamic performance, specifically meaning reducing both drag coefficient (CD) and lift coefficient (CL), is a growing issue in the modern automotive industry. The former is to improve fuel efficiency whereas the latter is to improve driving stability. These characteristics are quite associated with the geometric details of the external car body such that many studies are putting a lot of efforts to understand the contribution of geometrical details to the aerodynamic performance. For the design point of view, the comparison among local shape factors and the following trade-off study are essential during the early stage of the exterior design. In this paper, the qualitative and quantitative contribution of local shapes to overall aerodynamic performance is explored with a simplified vehicle model, especially the Ahmed body, by performing a multi-objective design optimization in a high-speed cruising condition. To achieve the goal of this research, Computational Fluid Dynamics (CFD) analysis is incorporated with various state-of-the-art design methodologies such as Design of Experiments (DOE), surrogate modeling, sensitivity analysis, Pareto-Optimum decision making, etc. Six design variables around the rear shapes of the Ahmed body are parameterized and populated into design space via a hybrid DOE method combining Central Composite Design (CCD) and Latin Hypercube. For CD and CL, corresponding Artificial Neural Networks (ANN) are created for the surrogate model. Then, the individual and collaborative contributions of design variables are scrutinized. For further detailed analysis, a Monte Carlo Simulation (MCS) is performed so that the empirical joint probability distribution is calculated to explore the feasible and optimum design space. Based on the simulation and analysis results, Pareto frontier is identified and multi-objective optimization is conducted to seek the best appropriate vehicle shapes for different design goals between CD and CL, which weigh on fuel efficiency and driving stability, respectively.

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