EXPERIMENTAL INVESTIGATION ON THE EVOLUTION MECHANISM OF UNIFORM MOMENTUM ZONES IN TURBULENT BOUNDARY LAYER
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摘要: 等动量区是瞬时流场中流体动量接近的局部区域,其生成和分布与相干结构密切相关. 对等动量区的研究有助于更深入认识湍流边界层相干结构,但目前对其演化过程还缺乏实验支持和机理分析. 设计并使用移动式高时间分辨率粒子图像测速技术(TRPIV)系统对光滑平板湍流边界层进行了跟踪测量,用滤波方式对数据进行降噪,结合对直接数值模拟数据的插值结果,获得脉动速度信号. 使用改进方法去掉非湍流的影响,检测边界层内的等动量区,得到其数量的时间序列,结合流向速度概率密度函数分布的变化,分析得出了等动量区的数量在大的时间尺度下从一个稳态到另一个稳态的阶梯状变化特点. 分解不同尺度的脉动速度,对大尺度和小尺度脉动信号进行条件平均,发现大尺度脉动对等动量区数量变化起主要作用,表现为不同速度流体通过发生不同猝发事件改变流向速度概率密度函数分布. 分析流向大尺度脉动空间分布的变化,发现等动量区内常含有多个大尺度脉动区域,不同区域的扩张、收缩、分裂、合并影响流向速度的集中程度,进而导致等动量区数量的变化.Abstract: Uniform momentum zone is one type of local regions where the instantaneous momentum of fluid approaches, and its generation and distribution are closely related to the coherent structure. The study of uniform momentum zone contributes to further understanding of the coherent structure in turbulent boundary layers, but there is still a lack of experimental support and mechanism analysis for the evolution process of uniform momentum zone. The moving TRPIV system was designed and used to measure the velocity fields of the turbulent boundary layer on a smooth surface as it moved downstream. The data is denoised by filtering, and the fluctuating velocity signal is obtained by combining the interpolation results of the direct numerical simulation data. After the influence of non-turbulence removed by an improved method,this thesis detects the uniform momentum zone in turbulent boundary layer, and obtain the time series of its quantity. In combination with the change of the streamwise velocity probability density distribution, the stepwise variation in the number of uniform momentum regions from one steady state to another over large time scales is obtained. The fluctuating velocity of different scales was decomposed, and the conditional average of large-scale and small-scale fluctuation signals were carried out. The results show that the large-scale fluctuations play a major role in the quantity change of the uniform momentum zone, the action mode is that the probability density function distribution of streamwise velocity is changed by different burst events of fluid in different velocities. By analyzing the changes of different fluctuation regions in the streamwise large-scale fluctuation space, it is found that a uniform momentum zone often contains multiple large-scale fluctuation regions. The expansion, contraction, splitting and merging of different large-scale fluctuation regions affect the concentration degree of streamwise velocity, leading to significant changes of the number of uniform momentum zones.
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