组合台阶柱绕流涡结构与水动力特性研究

Wake Structure and Hydrodynamic Characteristics ofFlow around Stepped Cylinders

该文利用开源软件OpenFOAM模拟研究了圆柱与方柱按不同形式串接而成的组合台阶柱的三维绕流流场与水动力特性。基于单位圆柱直径,来流雷诺数为500,方柱与圆柱的直径比为1.5,两者长度比为1:1。研究发现,台阶处存在“下洗”效应,且起始端相连的连接涡与边缘涡成对出现,造成台阶附近出现明显的尾涡干涉现象。单台阶柱的展向涡干涉区介于交接处上下±0.3D范围内,干涉区内流向时均速度为零的等值线断裂呈多根并存。受尾涡干涉与壁面涡的影响,单台阶柱的圆柱尾迹增宽约30%,背流面平均压力系数比单一方柱小25%,而双台阶柱的圆柱尾迹增宽约50%,背流面压力系数比单一方柱小33%。这表明尾流特征尺寸和表面的压力系数沿轴向均有明显波动,且双台阶更为显著。台阶柱尾流存在涡位错和涡分裂现象,双台阶柱后方的涡管分裂成更多小涡,涡脱频率带宽较大。表面平均压力系数与台阶个数有关,台阶柱的升力系数脉动值较单一形状柱体小,且双台阶柱的阻力系数脉动值最小。

The three-dimensional flow around stepped cylinders is numerically investigated using the open source software OpenFOAM.At the flow Reynolds number of 500 based on the unit circular cylinder diameter,the stepped cylinders are formed with the connection of circular and square cylinders along the axis,where the ratio of square diameter to circular cylinders'is 1.5 and the total length ratio of them is 1:1.The numerical results indicate the existence of downwash effect and the pairs of junction and edge vortices which are connected at the beginning.Consequently,the vortex interference is obviously observed around the steps.For a single stepped cylinder,the interference zone is in the range of-0.3D to 0.3D with respect to the connection location.The wake of a single stepped cylinder is widened by about 30%,and the average pressure coefficient on the back flow surface is 25%less than which of a single square cylinder.While the wake of a double stepped cylinder is widened by about 50%,and the pressure coefficient on the back flow surface is 33%less than which of a single square cylinder.Due to the influence of vortex interference and existence of junction and edge vortices,the wake characteristic size and pressure coefficient evolve spatially.Moreover,the double stepped cylinders present more predominant spatial evolution.The dislocation and splitting of vortices occur in the wake of stepped cylinders.The vortex tubes in the wake of the double stepped cylinders split into smaller vortices,resulting in broader band of vortex shedding frequency.The pressure coefficient is related to the number of steps.In comparison with the single cylinder,the root-mean-squared lift coefficient is reduced for stepped cylinders,and the double stepped cylinders possess the minimum root-mean-squared drag coefficient.