In recent years, the dielectric barrier discharge plasma actuator (DBD-PA), which is a fluid control device, has been investigated for achieving both high aerodynamic performance and pleasing styling of transportation equipment. In this study, the authors installed a DBD-PA system on a simplified three-dimensional bluff automobile body to reduce the aerodynamic drag. In particular, the authors focused on the sides of the rear end of the body, where the local shape has high sensitivity regarding both styling and aerodynamic drag. At the rear sides of the automobile-like bluff body, a sharp edge rather than a smooth rounded corner often reduces the aerodynamic drag by promoting airflow separation. Therefore, the authors aimed to reduce the aerodynamic drag by using a DBD-PA system to promote flow separation at the rear end while retaining its rounded shape. Aerodynamic measurements using a one-fifth scale simplified automobile model were conducted in a wind tunnel. Preliminary investigation of the aerodynamic effect at the rear clarified how the longitudinal vortices from the rear pillar and the side edge of the trunk deck cause the drag increase at the rear-end corners. Two parallel DBD-PAs were installed on the rear surface to shift these vortices away from the corners by promoting flow separation. The drag reduction rate reached 3% at the highest applied voltage using the DBD-PA system on a rounded shape, and it achieved approximately half the effect of the sharp-edged shape. The longitudinal vortices were successfully kept away from the rear-end corners by the DBD-PAs. The surface pressure increased with the displacement of the vortices, which led to the drag reduction observed.

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To minimize fuel consumption, it is important to reduce the aerodynamic drag on a vehicle. Previously, significantly low zero-yaw aerodynamic drag coefficient (C D) values of a production vehicle have been achieved. However, low-C D vehicles tend to be more sensitive to wind disturbance (Windsor, 2014). Therefore, robustness against wind disturbance for on-road conditions should be investigated for the advancement of C D reduction technologies. In recent years, many researchers conducted real-world experimental studies on wind properties (Cooper

In this study, skin friction around a ½-scale Ahmed body was measured experimentally at a Reynolds number of Re = 2×105. The slant angle of the Ahmed body was 25° and the yaw angles ranged from 0° to 8°. This study focused on the flow structure on the slant surface under different cross-wind conditions. A force balance system was applied to measure the aerodynamic drag of the model. The global skin-friction topology was measured by applying a luminescent oil layer with a sub-grid data processing algorithm. The method used to measure the skin friction was conducted for the first time on the Ahmed body. The results indicated that the technique is highly capable of extracting the skin-friction topology. For a yaw angle below 3°, the flow on the slant surface was not significantly affected by the cross-wind condition and the drag of the model was nearly constant. However, at yaw angles above 3°, the flow on the slant surface was highly affected by the roof longitudinal vortexes on the windward side, leading to a dramatic increase in the drag of the model. High consistency in the drag and skin-friction fields was observed. The detailed skin-friction structure at different yaw angles will be discussed in this study.

We studied the mechanism of drag reduction due to textured hydrophobic surfaces in Newtonian laminar flow through a rectangular channel. The test wall surfaces were fabricated with different fine groove patterns and groove area ratios, and were then coated with PTFE to produce hydrophobic surfaces. Drag reduction was estimated by pressure loss measurements in a 12 × 12 mm channel. Visualization of the gas-liquid interface was carried out using a 0.5 × 5 mm microchannel to investigate the mechanism of drag reduction. A series of experiments showed that the gas-liquid contact area ratio and the air layer thickness influence the drag reduction, the maximum drag reduction ratio is 3.7 %.

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