Wind Tunnel Investigation of Passive Spoiler Influence on Drag Characteristics of a 25° Ahmed Body

Improving vehicle aerodynamic performance is a key strategy for reducing fuel consumption and carbon emissions in the transportation sector. Among various aerodynamic factors, pressure drag generated by flow separation behind bluff bodies plays a dominant role in total drag. This study experimentally investigates the effect of a small rear lip as a passive flow control device on the wake structure and drag coefficient (Cd) of a 0.2-scale 25° Ahmed body model under low to medium Reynolds number conditions. Wind tunnel experiments were conducted at free-stream velocities of 8 m/s, 10 m/s, and 12.5 m/s. Drag forces were directly measured using a load measurement system, while velocity distributions in the wake region were captured using hot-wire anemometry. The results show that increasing velocity intensifies the wake structure and strengthens longitudinal vortices without eliminating flow separation. The addition of the small rear lip modifies the shear layer development and redistributes momentum in the wake region. At higher velocities, the passive device produces a more stabilized downstream flow pattern and smoother velocity gradients, indicating potential improvements in base pressure recovery. However, wake size reduction is not always directly proportional to drag reduction. These findings demonstrate that simple geometric modifications can influence wake dynamics under laboratory-scale conditions and provide empirical insight into passive aerodynamic optimization for energy-efficient vehicle design.