椭圆柱绕流尾迹的PIV测量及DMD分析

PIV Measurements and DMD Analysis of Elliptical Cylinder′s Wake

为了研究椭圆柱绕流尾迹的流动特性,利用粒子图像测速技术(PIV)对长短轴之比AR=0.42的椭圆柱在雷诺数Re=3493,4657,5822三种流动工况进行了实验研究,得到了相应的尾流区瞬态速度矢量场和涡量场,并通过频谱分析得到旋涡脱落频率。结果显示:随着雷诺数的增大,椭圆柱尾流区域在竖直方向上的影响范围减小,近尾流区内旋涡形成区域缩短,旋涡脱落频率增加。同时采用动力学模态分解(DMD)方法对实验获得的速度场进行模态分解,提取了椭圆柱尾迹流场的相干结构及各个模态的频率和增长率信息。结果表明:低阶模态包含了原始流场的主要流场结构,以及与旋涡周期性脱落运动相关的主要模态。随着模态阶数的增加,衰减速度逐渐增大,涡尺度逐渐变小,并向竖直方向进行扩散。

The wake flow structure of elliptical cylinders at Reynolds numbers of 3493,4657 and 5822 is measured by particle image velocimetry(PIV)in a water channel.The axis ratio of the elliptic cylinder is AR=0.42,and the major axis of the ellipse is aligned in the stream-wise direction.The corresponding instantaneous velocity vector field and vorticity field were captured,and the vortex-shedding frequencies were obtained by spectrum analysis.Results indicate that with the increase of Reynolds number,the influence area of the elliptical cylinder’s wake in the vertical direction decreases,the length of vortex forming region in the near wake shortens,and the vortex-shedding frequency increases gradually.The dynamic mode decomposition(DMD)is used to extract the coherent structure of the flow in the wake of the elliptical cylinder,as well as the frequency and growth rate of each mode.It is found that the low-order modes contain the most dominant flow structure of the original flow and illustrate flow structures associated with the periodic vortex shedding.As the number of modes increases,the decay rate increases,the scale of vortices in the wake becomes smaller,and vortices diffuse gradually in the vertical direction.