The Reynolds number for the onset of flow unsteadiness is determined for several canonical geometries (triangles, rectangles, ellipses and lozenges) at different sectional breadth ($L$) to height ($d$) ratios (aspect ratio $\mathrm{AR}=L∕d$), for more than 70 shapes. The flow is modeled using a direct Navier–Stokes incompressible two-dimensional solver and the shape is defined by an Immersed Boundary Method. The employed procedure takes the fluctuation of the velocity in the wake as the criterion to define the unsteadiness and a binary search to find the transition. This procedure yields critical Reynolds number ${\mathrm{Re}}_c$ values in agreement with available data in the literature. When $\mathrm{AR}$ approaches zero, the five shapes lead to almost the same value of 31, which corresponds to ${\mathrm{Re}}_c$ for a flat plate normal to the flow. It is then found that ${\mathrm{Re}}_c$ grows exponentially with the aspect ratio, the influence of the cross section shape being accounted for by a single regression parameter. For all aspect ratios, the ellipse exhibits the highest ${\mathrm{Re}}_c$, and the front-pointing triangle the lowest, the three other geometries laying in between those two. The physics of the influence of cross-section shape on ${\mathrm{Re}}_c$ is analyzed, considering its link with recirculation length in particular. An exploitation of the results is outlined for the analysis of recent aeroacoustic shape optimizations at fixed $\mathrm{Re}=150$, through correlation between the lift fluctuation at this regime with the distance to the onset of unsteadiness it corresponds to.

流动不稳定开始的雷诺数由不同截面宽度的几种典型几何形状（三角形、矩形、椭圆和菱形）确定（$升$) 到高度 ($d$) 比率（纵横比 $\mathrm{增强现实}=升∕d$)，超过 70 种形状。流动使用直接 Navier-Stokes 不可压缩二维求解器建模，形状由浸入边界法定义。所采用的程序以尾流中速度的波动为标准来定义不稳定和二分搜索来找到过渡。此过程产生临界雷诺数${\mathrm{关于}}_C$值与文献中的可用数据一致。什么时候$\mathrm{增强现实}$ 接近于零，这五个形状导致几乎相同的值 31，这对应于 ${\mathrm{关于}}_C$对于垂直于流动的平板。然后发现${\mathrm{关于}}_C$随纵横比呈指数增长，横截面形状的影响由单个回归参数解释。对于所有纵横比，椭圆表现出最高的${\mathrm{关于}}_C$，前向三角形最低，其他三个几何图形位于这两者之间。横截面形状影响的物理学${\mathrm{关于}}_C$分析，特别是考虑到它与再循环长度的联系。概述了对结果的利用，以分析最近在固定条件下的气动声学形状优化。$\mathrm{关于}=150$，通过在该状态下的升力波动与其对应的不稳定开始的距离之间的相关性。