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The aerodynamic characteristics have a great influence on the steering stability and the fuel economics of a high speed vehicle. The diffuser located at the aft part of a car underbody is one of the most important aerodynamic add-on devices. The parameters of the diffuser, including the diffuser angle, the number and the relative position of longitudinal separator (LS), the shape of the end plate and etc, will affect the underbody flow and the wake. Here, diffuser with longitudinal separator of different number and relative position was investigated. Numerical simulation was used to study the aerodynamic characteristics of a simplified sedan with different diffuser of longitudinal separator. The study found aerodynamic coefficient of the car changes little when we change the relative position of diffuser's longitudinal separator. Besides, we also found that increasing the number of the diffuser's longitudinal separator will increase the vehicle's aerodynamic drag coefficient and reduce the vehicle's lift coefficient.

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In this paper, the external flow simulation has been carried out on a simplified passenger car model with two schemes adopted: a smoothed complex underbody and a flat underbody. The aerodynamic characteristics and the flow field structure of the passenger car model were obtained with CFD method, and the drag coefficients of the smoothed complex underbody and the flat underbody are respectively 0.275 and 0.251. According to the simulation, flow separation and energy loss of fluid was caused by protuberances on the complex underbody. As there are many differences between the aerodynamic characteristics of the smoothed underbody and the flat one, its suggested to use a smoothed complex underbody not a flat underbody in the studying of aerodynamic drag reduction of a passenger car.