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Aerodynamic drag change of simplified automobile models influenced by a passing vehicle

On-road turbulence caused by atmospheric winds and other automobiles influences the flow field of automobiles and causes variations in aerodynamic forces. Furthermore, the disturbance caused by passing vehicles has a significant impact on drag increase and vehicle stability. Therefore, designing flow control techniques considering this impact will effectively improve the robustness of automobiles under on-road conditions. In this study, we investigated flow phenomena responsible for drag changes under passing vehicle conditions to reduce drag. Two simplified vehicle models were adopted for the test vehicle. These models are one-fifth scale models, which reproduce the flow structure of production vehicles. The tests were conducted in a wind tunnel that simulated the passing environment where the truck model was placed in the adjacent lane. The drag change and flow characteristics of the truck model were measured by changing the relative positions of the vehicle model and the truck model. The Reynolds number was 1.1×106 at the transition to the turbulent boundary layer in front of the vehicle model. An analysis procedure was proposed to identify the components affecting the drag change using the flow characteristics of a passing vehicle. As a result, three key factors that change drag were identified; variations in the pressure field, crosswind generated by passing vehicles, and the remaining part. Crosswinds are especially important for reducing drag because the contribution of the pressure field to the drag is uniquely determined by the flow characteristics of the passing vehicles and almost zero when the overtaking/being overtaken process is considered. These results provide guidance for designing flow control techniques that are robust against disturbances under on-road conditions.