Effect of Underbody Diffuser on the Aerodynamic Drag of Vehicles in Convoy

Current study examines the effect on coefficient of drag (Cd) of convoy of two reference car bodies (Ahmed body) by employing underbody diffuser on lead body. CFD analysis of convoy is done using Shear-Stress-Transport model under moving ground conditions. The lead body's diffuser length is taken as 222m with diffuser angle of 0° (no diffuser), 3°, 5°, 7°, 9°, 15°, 20°, 25° and 30° each at inter-vehicular 0.25 and 0.75 body length. Each configuration resulting was analyzed with lead body backlite angle of 25° (pre-critical) and 35° (post-critical) with follow body backlite angle remaining 25°. To understand the flow features developed on Ahmed body due to an underbody diffuser a preliminary CFD analysis is done on an isolated body with 25° and 35° backlite angles by applying each diffuser angle in current study. CFD analyses are conducted after performing two validation analyses from previous studies. The drag on lead and follow vehicles was found to also depend on the axial vortices due to diffuser in addition to those from backlite surface of lead body. Average drag on cases with diffuser is found to be lesser than the no diffuser cases up to a certain diffuser angle. Thus applying diffuser has resulted in potential for reducing the overall drag on convoy by deciding optimum configuration.