Ahmed body; trailing vortices; aspect ratio; time-resolved PIV; spectral POD & The dynamic structures in the wake of the Ahmed body with a 25° slant angle were investigated. It is well established that the wake is dominated by two, counter rotating, streamwise vortex structures (C-pillar vortices). The influence these vortices have on the flow over the slant back was altered by modifying the width of the body between 60% to 120% of the standard width. The wake was measured using time-resolved particle-image velocimetry in several streamwise and spanwise planes for a height-based Reynolds number of $2.6\times {10}^4$. The wake measurements were decomposed with a spectral proper orthogonal decomposition technique, revealing two oscillatory modes. Firstly, the C-pillar vortical structures alternately contract and expand with a Strouhal number of 0.24, mildly dependent on the width. Secondly, the boundary layers on the sides of the body separate at the end of the body, forming shear layers with a dominant frequency of St2.30. Reducing the width of the body to 60% rotates the symmetry axis of the shedding mode. Furthermore, changing the spacing between the C-pillar vortices, by changing the width of the body, alters the nature of the spanwise vortical structures that form behind the base of the model. Intermediate width bodies produce two stable spanwise vortices. For wide bodies, the flow remains separated over the entire slant, and the top spanwise vortex does not form. Rather, the flow over the slant resembles a free shear layer with a Kelvin–Helmholtz like instability leading to periodic shedding of small-scale vortex structures. Conversely, for narrow bodies, this phenomenon is observed at the bottom of the body and the lower spanwise vortex no longer forms while the flow over the slant remains attached.

艾哈迈德身体；尾涡；纵横比；时间分辨 PIV；光谱 POD & 研究了具有 25° 倾斜角的 Ahmed 体尾迹的动态结构。众所周知，尾流由两个反向旋转的流向涡流结构（C 柱涡流）主导。通过将主体的宽度修改为标准宽度的 60% 到 120%，这些涡流对斜背上的流动的影响发生了变化。尾流是使用时间分辨粒子图像测速法在几个流向和展向平面上测量的，以获得基于高度的雷诺数$2.6\times {10}^4$. 尾流测量值用频谱适当的正交分解技术分解，揭示了两种振荡模式。首先，C 柱涡旋结构交替收缩和膨胀，斯特劳哈尔数为 0.24，略微依赖于宽度。其次，主体两侧的边界层在主体末端分离，形成主频率为St2.30的剪切层。将主体的宽度减少到 60% 会旋转脱落模式的对称轴。此外，通过改变车身的宽度来改变 C 柱涡流之间的间距，会改变模型底部后面形成的展向涡流结构的性质。中等宽度的物体产生两个稳定的展向涡流。对于宽体，流动在整个斜面上保持分离，并且没有形成顶部展向涡。相反，倾斜上方的流动类似于具有开尔文-亥姆霍兹式不稳定性的自由剪切层，导致小尺度涡旋结构的周期性脱落。相反，对于窄体，这种现象在体的底部观察到，并且在斜面上的流动保持附着时不再形成较低的展向涡旋。