单方柱绕流的直接数值模拟研究

为了揭示柱体绕流的湍流流动机理，采用直接数值模拟方法对雷诺数为1200的单方柱绕流工况进行研究。首先通过与文献中斯特劳哈尔数、平均流速和表面压强系数等统计量进行对比，验证了数值方法的可靠性。其次采用本征正交分解方法系统地提取流场中的相干结构，结果表明：对于速度场的模态分解，第一阶模态代表平均速度场的特征，第二、三阶模态提取的是流场中的低频大尺度旋涡特征，第四、第五阶模态提取的是流场中的高频小尺度旋涡特征。最后基于联合概率密度函数分析速度梯度张量第二、第三不变量，发现方柱下游大致可分为两个流动阶段：发展阶段，流场以涡流层结构和耗散作用为主，涡流管结构逐渐生成；成熟阶段，流场中湍流结构伴随着高涡量拟能和高能量耗散率。

Direct Numerical Simulation of Flow Around a Square Cylinder

Direct numerical simulation was used to study the flow around a square cylinder with an inlet Reynolds number of 1200. Firstly, the accuracy of the numerical method adopted was verified by comparing the statistical data e.g. the Strauhal number, the mean velocity along the centreline and the pressure coefficient over the surfaces with the available experimental data. The Snap-Proper Orthogonal Decomposition method was used to systematically extract the coherent structure in the flow field. The returned results show that the first mode represents the characteristics of the mean velocity field, and the second and third modes extract the large-scale vortex shedding characteristics whereas the fourth and fifth modes extract the high-frequency small-scale vortex features. Based on the joint probability density function of the second and third invariants of the velocity gradient tensor, it is found that the downstream of a square cylinder can be roughly divided into two flow regions. The first one is the developing region where the flow field is mainly dominated by vortex sheets structure and dissipation. In this region, the vortex tubes structure are gradually formed. The second region is the mature region where the high enstrophy and the high energy dissipation rate turbulent structures coexist.