Optimizing Vehicle Aerodynamics: The Impact of an Elliptical Rear Design on Drag and Soiling

ABSTRACT

In automotive aerodynamics, the Ahmed body serves as a simplified model to study airflow patterns around vehicles. Our research introduces a modified version with an elliptical rear end, termed the Elliptical Ahmed Body (EAB), to investigate how this shape alteration influences airflow, potential drag reduction, and surface contamination, commonly referred to as soiling. Utilizing Particle Image Velocimetry (PIV), we captured detailed flow structures around the EAB at a 25° rear slant angle, focusing on a Reynolds number of 43,100 based on the model's height. Complementary Detached Eddy Simulations (DES) were conducted at Reynolds numbers of 14,700, 43,100, and 190,000 to provide a comprehensive analysis. Our findings reveal that the elliptical curvature significantly reorganizes the airflow. Notably, it eliminates certain flow features present in the standard Ahmed body, such as the slant separation bubble, longitudinal C-vortices, and the lower recirculation bubble. Additionally, the upper recirculation bubble shifts toward the slant surface. These changes suggest a transformation from high-drag to low-drag flow structures, resulting in a 21% reduction in aerodynamic drag. Importantly, the elimination of the lower recirculation bubble is associated with reduced accumulation of dirt and debris on the vehicle's rear surface, a phenomenon known as soiling. By mitigating soiling, the elliptical design can help maintain cleaner rear surfaces, which is particularly beneficial for preserving the functionality of rearview cameras and sensors. This study underscores the impact of rear-end design modifications on airflow characteristics, surface cleanliness, and drag reduction, offering insights for improving vehicle aerodynamics and reducing soiling.

Numerical Investigation of the Quasi-Vortex-Ring State of the Propulsive Wing in Vertical Descent

ABSTRACT

A multi-axis propulsive wing aircraft is a new type of vertical takeoff and landing aircraft. When the aircraft descends vertically, it enters a quasi-vortex-ring state (QVRS) similar to the vortex-ring state of the rotor. Based on the computational fluid dynamics method, the sliding grid technique was used to simulate and analyze the QVRS of a single propulsive wing in vertical descent. First, a computational method for the propulsive wing flow field was established, and the variation laws of the aerodynamic forces of the propulsive wing were analyzed. Then, the change in the flow field structure and wake of the propulsive wing was further studied, and the evolution mechanism of the QVRS of the propulsive wing was explained. Finally, recovery strategies for the QVRS and windmill–brake state (WBS) of the propulsive wing were proposed. The simulation results show that when the descent speed is less than the critical speed, the propulsive wing enters the QVRS. The interaction between the propulsive wing wake and reverse airflow provides an aerodynamic amplification of up to 17.3%; thus, the propulsive wing has satisfactory safety in the QVRS. When the descent speed is greater than the critical speed, the propulsive wing enters the WBS and its aerodynamic force is almost zero. The aerodynamic force of the propulsive wing can be increased by increasing the rotational speed of the cross-flow fan. Then, the descent speed of the propulsive wing slows down and the propulsive wing recovers from the dangerous state.