Effect of aspect ratio on the near-wake flow structure of an Ahmed body

Through numerical simulations, the interaction between the longitudinal c-pillar vortices and the flow over the rear slant surface is established for an Ahmed-body geometry as its aspect ratio is varied. In turn this affects the flow structure and topology of the near wake, and has a significant effect on the drag. In particular, aspect ratio was found to influence the critical angle at which flow fully separates on the rear slant surface due to the interaction of the c-pillar vortices with the rear slant flow. This appears to be a consequence of the influence that downwash generated by the c-pillar vortices has on promoting rear slant flow reattachment. By isolating the pressure drag associated with individual surfaces, it is shown that the drag discontinuity is almost entirely due to a sudden change in the pressure field on the rear slant surface. On increasing the aspect ratio, once the flow becomes fully separated, c-pillar vortex strength and position change considerably, inferring a mutual relationship between flow reattachment and c-pillar vortex generation mechanisms for the Ahmed-body geometry.