Research On Flow Characteristics And Vortex Dynamics Of A Span-changing Cylinder

The problem of flow around the cylinder structure has always been an important issue in the study of fluid mechanics.In recent years,the application of the cylinder structure in the engineering field has become more and more extensive.The vortex-induced vibration generated in the flow around it will bring safety hazards to the structure,causing fatigue damage and even structure damage brought by resonance.Therefore,it is of great engineering significance and research value to study the flow characteristics of the cylinder and then to suppress the vibration caused by the vortex shedding.In this paper,we used a numerical simulation method based on the computational fluid dynamics(CFD),employing CFD open source computing software OpenFOAM.Aiming at two kinds of cylinder structures proposed in the existing research which can improve aerodynamic characteristics and suppress vortex shedding: Wavy cylinder and Twisted cylinder,we studied its flow characteristics at lower Reynolds number,the multi-vortex system coexistence and interference phenomenon caused by its span change,revealing the vortex dynamic mechanism behind it.In addition,we focused on the three-dimensional effect of the change of the spanwise direction on the physical quantities generated in the flow.Firstly,the aerodynamic coefficients of the two cylinders are calculated,and the aerodynamic characteristics of the two special cylinders are compared with the usual smooth cylinder to compare their aerodynamic improvement effects.By comparison,we can see that the aerodynamic optimization effect of the Wavy cylinder is not obvious under the low Reynolds number and the corresponding cylinder size set in this paper,and the aerodynamic improvement effect of the twisted cylinder is very significant.Then,from the variation law of Strouhal number among different Reynolds numbers,the vortex shedding mechanism under low Reynolds number is classified.In addition,the new changes of various mechanisms in the flow around a smooth cylinder after introducing the complex span wise structures are analyzed.Through analysis,we can conclude that the change of the two cylinders in spanwise has a great influence on the vortex-shedding mode and its instability path.A new vortex mode also appears in the twisted cylinder,which is unlike conventional smooth cylinders.From this,we can use it to promote turbulence in advance and to control turbulence.Finally,for the obtained vortex shedding mechanisms,we selected representative Reynolds numbers to compare the vortex structure,vortex development and shedding,wake velocity profile,etc.,focusing on the flow characteristics and vortex dynamics mechanism under different mechanisms.Through the analysis of these physical quantities,we can clearly see the threedimensional effect of the two types of cylinders,for example,the average velocity and instantaneous vorticity at each position of the spanwise direction are different.At the same time,each physical quantity shows a typical periodicity along the spanwise direction under certain specific working conditions,indicating that the periodic variations of the structure's spanwise direction extend well in the flow.