Statistical Properties Of Structures In The Cylinder Placed Near A Wall Based On Clustering Method

This article takes many simplified physical models of near-wall cylinder flow in practical engineering as the research object.Through numerical simulation and statistical analysis,the complex and rich multi-scale turbulent structures,especially the geometric properties and evolution characteristics of vortex structures in the flow field,are studied,which has important theoretical significance.First,the large eddy simulation method was used to obtain the flow field data of near wall cylinder at Reynolds number Re=U∞D/υ =1500,gap ratios G/D=0.5 and 1.0;Secondly,the clustering methods was modified and expanded to extract fluctuation velocity structures and Reynolds shear stress structures,and then combine it with vortex recognition methods to extract vortex structures;Finally,the geometric properties of vortex structure and flow fluctuation velocity structure in the near wake region,as well as the evolution characteristics of vortex structure and Reynolds shear stress structure,were studied.The main research content and conclusions of this article are as follows:1.The statistical characteristics of vortex structures in the near wake region under a gap ratio G/D=1.0 was studied,with particular attention paid to the geometric characteristics of secondary vortex structures.Using the modified clustering method to identify and extract vortex structures,it was found that the structure was divided into two spatial statistical regions by the wall normal boundary(y/D≈0.8)in the flow field.However,the statistical results show that the large-scale secondary vortex structure is geometrically self-similarity.During its formation,the average lateral geometry is approximately circular,and the average lateral geometry of the lower tail vortex is approximately elliptical,and its scale is larger than that of the secondary vortex structure.In addition,the Lead-lag phenomenon of the lower wake vortex and secondary vortex was quantitatively characterized from the perspective of the distribution of the center position of the vortex structure.2.The statistical characteristics of flow fluctuation velocity structure in the near wake region under gap ratio G/D=1.0 was obtained through research.Using the modified clustering method to extract high-speed and low-speed structures,the velocity structure in the flow field is also divided into two spatial statistical regions by the wall normal boundary.The large-scale high-speed and low-speed structures in y/D(?)0.8 have self-similarity in geometry.The deflection angle of the low-speed structure center is the same as that of the secondary vortex center,and the high-speed structure promotes the generation of the secondary vortex structure.The geometric properties of low-speed structures are similar to those of secondary vortex structures.In addition,conditional averaging is used to establish distribution models of secondary vortex structures and high-speed and low-speed structures.3.The evolution characteristics of Reynolds shear stress structure and vortex structure in the near wall cylindrical flow field with gap ratios G/D=0.5 and 1.0 were studied.At G/D=0.5,when the upper wake vortex on the cylinder merges with the secondary vortex,the Reynolds shear stress structure around it also merges.The merged Reynolds shear stress intensity increases,and the secondary vortex that deviates also deviates upwards from the surrounding Q2(ejections)structure and Q4(sweeps)structure.At G/D=1.0,the scale of the Reynolds shear stress structure is larger,the inclination angle of the structure is relatively small,and the evolution of the Q2(ejections)structure is closer to the evolution of the secondary vortex structure.