基于粒子图像测速技术的明渠圆柱上游行近流特征研究

Study on the features of approaching flow upstream of a circular cylinder in open channel flows based on PIV measurement

现有实验测量和数值模拟方法难以准确捕获近水面流动特征,为全面掌握墩柱上游行近来流的内部结构和水面特征,通过明渠圆柱绕流试验,采用粒子图像测速技术对柱体上游的对称垂面进行了测量,获得了行近来流的瞬时二维流场和水面线。在此基础上,对圆柱上游的时均流动和壅水特征进行了研究,得出上游行近来流从水面至床面依次由上升流、近水平流和下降流构成,形成典型的三层结构,其中,上升流厚度随柱体雷诺数增加而变厚,下降流无量纲厚度基本不随柱体雷诺数改变。在靠近柱体的床面附近,边界层与床面分离,分离区内为向上游流动的逆流以及一个马蹄涡和一个次生涡;在靠近柱体的水面附近存在表层逆流以及两个逆向涡。研究结果揭示出边界层分离、表层逆流及柱体本身依次促使行近来流减速,使得沿程水深雍高,壅水末端至圆柱迎水面的水面线符合一元四次函数分布,无量纲壅水长度随着柱体雷诺数的增加而延长。研究成果可作为基于流体体积法等具有自由水面计算能力的明渠墩柱绕流数学模型的验证依据。

Existing experimental measurements and numerical simulations encounter difficulty in resolving the near-surface flow in open channels. To research the flow pattern and free-surface characteristics upstream of bridge piers, the approaching flow of a vertical circular cylinder in an open channel was measured by particle image velocimetry （PIV）. The measurements were made in the plane of symmetry upstream of the cylinder, based on which the averaged velocity fields and water surface profiles were obtained. The results show that the approaching flow is composed of an upper up-flow layer, a middle hori- zontal-flow layer, and a lower down-flow layer. The up-flow is deflected upstream near the water surface and interacts with the main flow, inducing retrograde vortices in the shear layer. Similarly, the down-flow is deflected upstream at the bed and interacts with the separated boundary layer, generating horseshoe vortices at the base of the cylinder. The separated boundary layer, the surface backflow, and the cylinder retard the approaching flow successively, resulting in a quartic-polynomial backwater profile which increases in length with the cylinder Reynolds number. The results obtained in this work can be used to validate the numerical models for the flow around a cylinder in open channels with free-surface tracking methods.