Experimental studies on vortices merging based on MSFLE and Liutex
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摘要: 应用具有拉格朗日性质的运动单帧长曝光（MSFLE）图像测量方法，对矩形管内湍流边界层涡结构合并现象开展了实验研究。研究了雷诺数Reθ在97到194之间管道湍流边界层流向-法向平面内涡合并现象。在实验中，测量系统以与涡运动速度相近的速度匀速移动，采用连续长曝光记录示踪粒子运动轨迹，以捕捉涡随时空的演变过程，骨架提取图像处理算法获得示踪粒子速度，并应用Liutex物理量表征涡的旋转强度。研究表明：MSFLE方法测量装置简单，对实验条件要求低，可从拉格朗日视角直观测量湍流边界层涡结构及周围流场的时空演变过程，MSFLE图像测量方法与Liutex涡识别算法结合可以很好地应用于湍流边界层涡结构的可视化与量化。矩形管内湍流边界层涡合并的条件是两个涡相邻、强度和尺寸基本相同且为同向旋转涡，合并中两个涡的强度呈反向变化，合并生成的新涡的强度和尺寸基本为初始合并时两个涡之和，且旋转方向与两个涡同向。Abstract: The vortices merging in a turbulent boundary layer of a rectangular channel were experimental studied with a moving single-frame and long-exposure (MSFLE) imaging method which is a Lagrangian-type measurement technique. The process of vortices merging was studied in the streamwise-normal plane with Reθ=97~194. During the experiments, the measurement system moved at a constant speed that is similar to the moving velocity of measured vortex to record continually the moving of vortices with long exposure time. An image processing algorithm based on skeleton extraction was employed to process the images of the flow field for obtaining the velocity of vortices and the flow field. The Liutex theory was adopted to process the flow field for characterizing the intensity of vortex structure. The study shows that the MSFLE imaging method is a cost effective method. It can intuitively show the spatiotemporal evolution of vortex and surrounding flow field in a turbulent boundary layer with a Lagrangian-perspective. The MSFLE image method combined with Liutex vortex recognition algorithm can be applied to the visualization and quantification of vortex structure in the channel turbulent boundary layer. The vortices merging most possibly occur in a pair of adjacent co-rotating vortices with the basically equal intensity and size. During the process of merging, the intensity of these two vortices changes in the opposite direction. Moreover, a new generated vortex has the same rotation direction as the two merging vortices and its size and intensity are about the sum of the two merging vortices at the initial merging.
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Key words:
- turbulent boundary layer /
- vortex structure /
- measurement /
- MSFLE /
- Liutex
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图 5 x=400 mm与1300 mm处边界层法向平均速度分布
Figure 5. Dimensionless velocity profile of experimental results at x=400 mm and x=1300 mm
图 6 U*=0.9时涡合并过程图像及处理结果
Figure 6. Experimental images and processing results of vortex merging when U*=0.9
图 7 A涡和B涡合并过程中RInt随时间的变化
Figure 7. Temporal distributions of RInt during the merging process of A vortex and B vortex
图 8 U*=0.8时涡合并图像及处理结果
Figure 8. Experimental images and processing results of vortex merging when U*=0.8
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