Large Eddy Simulation Of The Compressible Flow Past A Circular Cylinder And Its Flow Control

The compressible flow past a bluff body is of importance in fundamentals and applications.The relevant work has been studied using large eddy simulation in this dissertation.In the present study,the three-dimensional Favre-filtered compressible Navier-Stokes equations are solved by means of a finite-volume method.Three typical cases,including transonic flow past a circular cylinder with different free-stream Mach numbers,the flow past a wavy cylinder,and the flow past a circular cylinder with tabs,are investigated in detail.The results and conclusions are described briefly as follows:(1) Effect of the free-stream Mach number M_∞on transonic flow past a circular cylinder has been investigated.Results show that there exist two flow states dependent on a critical Mach number M_cr.One is an unsteady flow state for M_∞＜M_cr,and the other is a quasi-steady flow state for M_∞＞M_cr.Based on carefully examining the flow evolution for both the flow states,local supersonic zones are identified in the near wake and generated by two typical processes, i.e.,vortex/vortex interaction and shock-wave/vortex interaction.In the unsteady flow regime,the convective Mach number M_c of turbulent shear layers is identified nearly as M_c＜1,and the instability process is mainly related to bi-dimensional one with the spanwise rolling eddies originating from the Kelvin-Helmholtz instability.In the quasi-steady flow regime,we find that M_c≥1 and oblique modes dominate the shear layer instability process.(2) The transonic flow past a wavy cylinder has been investigated to analyze the flow control behaviors.Comparing with the forces exerted on the wavy cylinder and circular cylinder,it is found that a drag reduction is obtained and lift oscillation on the wavy cylinder is greatly suppressed.Based on analysis of the flow structures,turbulence characteristics,vortex dynamics,and instability of shear layers in the flow field,the control mechanisms are discussed.Results show that the three-dimensional separation of the wavy cylinder may suppress the formation of shock wave and improve the stability of shear layers,leading to weakening the shock wave induced drag and surface pressure fluctuation,as well as the increase of base pressure.Therefore,the wavy cylinder can reduce drag and suppress the lift oscillation effectively.(3) The transonic flow past a circular cylinder with tabs(CCT) has been investigated for flow control.The control effectiveness of the CCT is analyzed by comparing with the forces exerted on the CCT and circular cylinder.Based on analysis of the self-sustained feedback mechanism in the shear layers in the front of tabs,the behaviors associated with the shear layer instability,turbulence characteristics,vortex dynamics in the flow field are discussed.It is found that the drag and lift-oscillation of the CCT decrease compared to the circular cylinder.Moreover,two control effects of the tabs have been identified.One is suppressing the shear layers growth upstream of tabs and raising the basepressure of cylinder behind tabs,and the other promoting more rapid vortex roll-up in the shear layer downstream of tabs,decreasing the base-pressure of cylinder before tabs and increasing the friction.