Large-eddy simulations are used to investigate the origin of the wake asymmetry and symmetry behind notchback Ahmed bodies. Two different effective backlight angles, ${\beta _1} = 17.8\mathrm{^\circ }$ and ${\beta _2} = 21.0\mathrm{^\circ }$ , are simulated resulting in wake asymmetry and symmetry in flows without external perturbations, in agreement with previous experimental observations. In particular, the asymmetric case presents a bi-stable nature showing, in a random fashion, two stable mirrored states characterized by a left or right asymmetry for long periods. A random switch and several attempts to switch between the bi-stability are observed. The asymmetry of the flow is ascribed to the asymmetric separations and reattachments in the wake. The deflection of the near-wall flow structures behind the slant counteracting the asymmetry drives the wake to be temporarily symmetric, triggering the switching process of the bi-stable wake. The consequence of deflection that forces the flow structure to form on the opposite side of the slant is the decisive factor for a successful switch. Modal analysis applying proper orthogonal decomposition is used for the exploration of the wake dynamics of the bi-stable nature observed.

中文翻译：

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凹背型艾哈迈德体后尾流双稳定性的数值研究

大涡模拟用于研究凹口艾哈迈德体背后的尾流不对称和对称性的起源。两种不同的有效背光角度， ${\beta _1} = 17.8\mathrm{^\circ }$ 和 ${\beta _2} = 21.0\mathrm{^\circ }$ , 被模拟导致尾流不对称和对称，而没有外部扰动，这与之前的实验观察结果一致。特别是，不对称情况呈现出双稳态性质，以随机方式显示两个稳定的镜像状态，其特征是长时间左右不对称。观察到随机切换和多次尝试在双稳态之间切换。流动的不对称归因于尾流中的不对称分离和重新附着。斜面后近壁流结构的偏转抵消了不对称性，使尾流暂时对称，触发双稳态尾流的切换过程。偏转迫使流动结构在斜面的另一侧形成的结果是成功转换的决定性因素。应用适当正交分解的模态分析用于探索观察到的双稳态尾流动力学。