The Ahmed body is one of the most studied 3D automotive bluff bodies and the variation of its slant angle of the rear upper surface generates different flow behaviours, similar to a standard road vehicles. In this study we extend the geometrical variation to evaluate the influence of a rear underbody diffuser which are commonly applied in high performance and race cars to improve downforce. Parametric studies are performed on the rear diffuser angle of two baseline configurations of the Ahmed body: the first with a 0° upper slant angle and the second with a 25° slant angle. We employ a high-fidelity CFD simulation based on the spectral/hp element discretisation that combines classical mesh refinement with polynomial expansions in order to achieve both geometrical refinement and better accuracy. The diffuser length was fixed to the same length of 222 ​mm similar to the top slant angle that have previously been studies. The diffuser angle was changed from 0° to 50° in increments of 10° with an additional case considering the angle of 5°. The proposed methodology was validated on the classical Ahmed body considering 25° slant angle, found a difference for drag and lift coefficients of 13% and 1%, respectively. For the case of an 0° slant angle on the upper surface the peak values for drag and negative lift (downforce) coefficient were achieved with a 30° diffuser angle, where the flow is fully attached with two streamwise vortical structures, analogous to results obtained from [1] but with the body flipped upside down. For diffuser angles above 30°, flow is fully separated from the diffuser. The Ahmed body with 25° slant angle and a diffuser achieves a peak value for downforce at a 20° diffuser angle, where the flow on the diffuser has two streamwise vortices combined with some flow separation. The peak drag value for this case is at 30° diffuser angle, where the flow becomes fully separated.

艾哈迈德（Ahmed）车身是研究最多的3D汽车钝器车身之一，其后上表面倾斜角的变化产生了与标准公路车辆相似的不同流动行为。在这项研究中，我们扩展了几何变化，以评估后部底盘扩散器的影响，后者通常在高性能和赛车中用于改善下压力。参数研究是针对Ahmed主体的两个基线配置的后扩散器角度进行的：第一个倾斜角度为0°，第二个倾斜角度为25°。我们采用基于频谱/马力元素离散化的高保真CFD仿真，将经典的网格细化与多项式展开相结合，以实现几何细化和更高的精度。扩散器的长度固定为与先前研究的顶部倾斜角相似的222毫米的相同长度。漫射器角度以10°的增量从0°更改为50°，在其他情况下考虑角度为5°。在考虑了25°倾斜角的经典Ahmed车身上验证了所提出的方法，发现阻力系数和升力系数分别相差13％和1％。对于上表面的0°倾斜角，通过30°扩散角获得阻力和负升力（下压力）系数的峰值，其中流完全由两个沿流的涡流结构连接，类似于获得的结果从[1]开始，但身体翻转过来。对于大于30°的扩散器角度，流量会与扩散器完全分开。具有25°倾斜角和扩散器的Ahmed阀体在20°扩散器角度下达到下压力的峰值，其中扩散器上的流具有两个沿流向的涡流并结合了一些流分离。在这种情况下，最大阻力值是在扩散角为30°时，流体完全分离。