Research On The Flow Around Two Tandem Cylinders With Different Diameters Under Flow Interference

The flow around bluff body structures is widely existed in daily life and engineering applications,such as high-rise building structures,bridge structures,ocean structures,etc.When there are multiple bluff bodies,the flow interference between adjacent structures will occur,making the flow characteristics significantly different from that of a single bluff body.The flow around a cylinder has always been a typical subject for the study of the flow around a bluff body,and has attracted extensive attention from researchers.As a generalization of the flow around a single bluff body,two equal diameter cylinders naturally become an ideal model for studying the flow around bluff body groups.Although the two cylinders are arranged in a variety of ways,the flow interference of the tandem cylinders is the strongest and the flow is the most complex,so it has always been the focus of researchers.However,in real life and practical engineering,a group of closely spaced bluff body structures do not always have the same size,so it is of great practical and engineering significance to consider the flow characteristics of tandem cylinders with different diameters,as well as important scientific research value.The purpose of this work is to study the flow characteristics of tandem cylinders with unequal diameters under wake interference through experimental and numerical methods,so as to understand the variation of flow topology,fluid force,frequency properties and stability characteristics with the diameter ratio(d/D),spacing(L/D)and Reynolds number(Re)of the two cylinders.To explore the physical mechanism of the flow and improve the understanding of the flow around two tandem cylinders,provide a theoretical basis for engineering applications.The specific work are as follows:(1)Firstly,the flow around two tandem cylinders with equal and unequal diameters is studied experimentally.The dependence of Re on mean drag,fluctuating drag,fluctuating lift and vortex shedding frequency is studied respectively,and the effects of Reynolds number,spacing ratio and diameter ratio on the mechanical characteristics of two cylinders are obtained.The analysis of the mechanical characteristics shows that Reynolds number has less influence on the variation of the fluid force on the cylinder than changing the spacing and diameter ratio.(2)The topological characteristics of the flow around two tandem cylinders with same diameters are studied.Based on the discontinuous Galerkin spectrum/hp element method,a set of simulation program is compiled by Matlab,which can be used to simulate the flow problem.Through calculation,the transient flow topology and the transient mechanical characteristics of the cylinder in the process of reattachment and transition to the co-shedding regime are obtained respectively.The results show that the feature of the transition process of flow regime is the change of gap flow,that is,the flow in the gap between two cylinders changes from unidirectional flow to‘horseshoe-shaped’ bidirectional flow.(3)The dynamic characteristics of tandem cylinders with different diameters are studied systematically.In view of the "double frequency" phenomenon observed in the experimental study of tandem cylinders,the generation mechanism of the harmonic frequency in the lift signal of downstream cylinder is described in detail through numerical methods,and the relationship between the harmonic frequency and the fundamental frequency is studied through wavelet analysis.The results show that the generation of harmonic frequency is due to the shedding of secondary vortex.(4)The first instability characteristics of the flow around two tandem cylinders with unequal diameters are studied.Through linear and nonlinear stability analysis,the instability characteristics in the transition process from steady flow to periodic flow are reseached,and a relatively compehensive description of the characteristics of primary instability is obtained.The results indicate that the flow around tandem cylinders has convective instability,and the flow has three different linear instability modes according to the difference of the distance between two cylinders and the diameter ratio.The change of instability mode may lead to the change of the core region of the instability mechanism,and show the nonlinear characteristics of supercritical bifurcation,subcritical bifurcation and secondary bifurcation.This paper has 87 pictures,6 tables,and 166 references.