旋转圆柱绕流的PIV实验研究

Experimental study on the flow past a rotating cylinder with PIV

投弃式海流剖面仪(Expendable Current Profiler,XCP)周围流场是典型的旋转圆柱绕流。探头周围流场对探头的运动状态起决定性作用,这直接关系到探头的测量性能,因此有必要对旋转圆柱周围流场进行实验研究。实验在循环水槽中进行,通过PIV对雷诺数保持不变(Re=1000)、不同圆柱旋转速度比(α=0、0.5、1.0、1.5、2.0、2.5、3.0、3.5、4.0、4.5和5.0)的圆柱下游尾流场进行研究。通过选取不同旋转速度比的任一时刻的瞬态流场,来分析旋转对圆柱尾流结构的影响。为了获得流场的频率信息,对所获得流场信息进行能谱分析来获取涡旋的脱落频率,并进一步使用正交模态分解对流场进行分析,给出了流场主要拟序结构及其能量与转速比的变化趋势。发现圆柱旋转改变圆柱尾流结构,使尾迹尺度变小。在旋转速度比0≤α≤2.0时,存在明显的周期性涡旋脱落,并且涡旋脱落的频率有逐渐升高的趋势;而且当转速比2.0<α≤5.0时尾迹流场的周期性减弱,涡旋脱落变得不明显,流场表现出低频、剪切层的区域特征。随着转速变大,涡旋尺度变小。在较高旋转速度比时,流场中能量被重新分布。

Flow field behind the expendable current profiler（XCP） is a typical flow past rotating cylinder. Flow field around a cylinder is crucial to the probe motion state which would affect the performance of the probe singnificantly. So it is necessary to investigate the flow field around the rotating cylinder using experimental methods. For the flow past a rotating cylinder, there are two important factors affecting the flow pattern of the rotating cylinder, namely the Reynolds number and the speed ratio. With constant Reynolds number（Re= 1 000） and different speed ratios, defined as the ratio of cylindrical rotating speed to stream velocity, the experiment was con- ducted in a circulated water channel, and the wake flow fields under different speed ratios were acquired by PIV. To analyze the rotating cylinder wake structure variation caused by rotation, the transient flow fields of different speed ratios at any time were chosen. In order to obtain the frequency information of the flow field, energy spectrum analysis was carried out on the flow field to extract vortex shedding frequency. Proper otrhogonal decomposition （POD） was used to extract large scale structure and energy contained in large structure which varied with the increase of speed ratio. It was found that the rotating cylinder changed the wake structure and made the wake scale smaller. As the speed ratio increased, there were two different transformation trends of cylinder flow wake. When speed ratio varied between 0 and 2.0, periodic vortex shedding was observed clearly. The periodicity of the flow field was the main character according to the relationship of spectrum and frequency. With the increase of the speed ratio, vortex shedding frequency had a rising trend. However, when the speed ratio varied between 2.0 and 5.0, the periodicity of the flow field became weak; flow field adopted a low frequency and high shear pattern. Furthermore, as the speed ratio increased, the vortex scale became smaller. At a higher rotational speed ratio, the flow field energy was redistributed.