用拉格朗日粒子跟踪识别二维剪切流中的拉格朗日相干结构

2019/07/16   下载量： 0 次

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用德国LaVision公司的DaVis8.1.3软件平台中的PTV分析功能，对二维剪切流中的拉格朗日相干结构用拉格朗日粒子跟踪方法进行了识别分析研究。

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The quantification of Lagrangian Coherent Structures (LCS) has been investigated using a novel algorithm based on the tesselation of unstructured data points. The applicability of such an algorithm in resolving an LCS was first tested using a synthetically-generated double-gyre flow with a known LCS solution. Subsequently, the algorithm was tested on experimental data acquired using two-dimensional particle tracking velocimetry (2D-PTV) of a nominally
two-dimensional free shear flow. To quantify factors linked to accurate LCS identification, dimensionless parameters are developed to characterize their relationship to LCS resolution. Specifically, the study considers turnover number (TN), which is defined here as the fraction of the particle track length to the vortical time scale, and cell ratio (CR), which can be thought of as the relative density of particles with long track lengths per characteristic vortex length scale. When examining the double-gyre finite-time Lyapunov exponent (FTLE) field, as expected, the resolution of the well-defined ridge improved with increasing TN. Upon use of tracks lengths >30% of the overall vortex time scale, only modest improvement in ridge resolution was observed. The LCS of the experimental data set was well-resolved using track lengths on the order of 20-30% of the vortex time scale. However, the use of greater TN values presented LCSs with an increasingly disordered coherence suggesting an inadequate particle count to accurately define the structure. Additionally, an exponential relationship was observed between CR and TN. This suggests that a compromise must be made between
track length and the density of tracked particles to adequately resolve an LCS where an insufficient number of particles have long track lengths.

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北京欧兰科技发展有限公司
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