串列双圆柱绕流的模态特征及流场重构

为探讨不同间距下,串列双圆柱绕流场特征的内在变化规律,在低雷诺数Re=100下,针对间距比为P/D=1.1~5的串列双圆柱,通过动力学模态分解（dynamic mode decomposition,DMD）方法对其绕流场进行模态分解,并基于DMD的主导模态建立降阶模型,重构了串列双圆柱的涡量场。结果表明:串列双圆柱的3种流态,即单一钝体（P/D=1.1~2）、剪切层再附（P/D=3）和双涡脱流态（P/D=4~5）,呈现明显不同的子模态特征；随着间距比的增大,与圆柱涡脱频率对应的模态结构会逐渐由下游圆柱尾流转移至上游圆柱尾流,此主模态表征了3种流态随间距比演变的内在机制；与单一钝体及剪切层再附流态相比,双涡脱流态的下游圆柱近尾流区的高阶模态结构更为复杂,流场重构时需要更多模态才能达到与其他两种流态相似的精度,在尾流旋涡影响区的重构误差最大。

Mode characteristics and reconstruction of flow field around two tandem circular cylinders

The flow mechanism around two tandem circular cylinders with different pitch ratios (P/D) was investigated. Numerical simulations were conducted on two tandem circular cylinders (P/D=1.1-5) at a low Reynolds number (Re=100). Dynamic mode decomposition (DMD) was adopted to decompose the flow around two cylinders. Reduced-order model was established based on the dominant DMD modes to reconstruct the vorticity field of two tandem circular cylinders. Results show that the typical wake flow regimes of two tandem circular cylinders were single bluff body (P/D=1.1-2), shear layer reattachment (P/D=3), and couple vortex shedding (P/D=4-5), which presented distinct wake flow features. With the increase in pitch ratio, the mode structure corresponding to the vortex shedding frequency moved from the wake of the downstream cylinder to the wake of the upstream cylinder. It indicates that the dominant mode characterizes the inherent flow mechanism of the transition of the three wake flow regimes for two tandem cylinders with different pitch ratios. The wake flow pattern of couple vortex shedding presented more complicated higher-order mode characteristics than that of single bluff body and shear layer reattachment. In terms of flow reconstruction, more sub-modes were needed for couple vortex shedding to achieve similar precisions with the other two regimes. The reconstruction error was mainly concentrated near the vortex formation region.