Effects of under body diffuser on the aerodynamics of a generic car

Abstract

Flow structure and other aerodynamic properties such as drag and lift coefficients of one of the extensively investigated generic cars so-called Ahmed body are reported in the present numerical study for the fixed ground and upswept rear. For this purpose, Ahmed body with the slant angle of 25 degrees is considered and its rear bottom edge is upswept with various angles from 5 degrees to 30 degrees by 5-degree increments without any side plates. It is observed that the sizes of the upper and lower recirculation bubbles seen in the profile section are decreased compared with the classical Ahmed body, however, the existence of some additional recirculation bubbles are observed when the upswept angle increases. At the highest upswept angle, the upper and bottom vortices called counter-rotating vortices (CVP) become nearly symmetric in the near-far region however they merge and only one CVP forms behind the body in the far-field. Effects of the upswept angles on the drag and lift coefficients reveal that drag coefficient reduces the upswept angle to 15 degrees and then rises to 20 degrees while the lift coefficient increases after 25 degrees.