Large Eddy Simulation Of Flow Around Rectangular Cylinder At Low Subsonic And Transonic Speed

Flow around bluff bodies is common in nature and engineering, which is also oneof the important basic problems in fluid mechanics. It contains several kinds of typicalflow phenomena, such as flow separation and reattachment, instability and transition ofshear layer, periodic vortex shedding and development of wake in low subsonic regime,when the Mach number is raised to transonic range, the interaction between shock waveand vortex, shear layer, wake etc. occurs, and the formation and development of localsupersonic zones in wake can also be observed.In this paper, the flow characteristics around rectangular cylinders with differentaspect ratios at low subsonic and transonic speed are studied by large eddy simulation.The mixed finite volume and finite element method and unstructured grids areemployed to solve the compressible N-S equations. The dynamical Smagorinsky modelis used in large eddy simulation (LES). At low Mach number, the preconditioningmethod and MUSCL interpolation are adopted. At transonic speed, modified Roe’sRiemann solver and WBAP limiter are used.At Re=22000, large eddy simulations are performed to the flows aroundrectangular cylinder with the aspect ratio of1:1,2:1,3:1and4:1at Ma=0.1,0.75and0.95, respectively, to study the effect of aspect ratio and compressibility on the flowcharacteristics. The main results obtained in the present study are as the following:At Ma=0.1, the mean recovering pressure grows with the increase of the aspectratios. When the aspect ratio is equal to1:1or2:1, the flow does not exhibitreattachment on the upper and lower surfaces of the cylinder. Flow reattachment isobserved for cylinders with aspect ratios of3:1and4:1. The mean quantities such aspressure coefficient, drag coefficient, Strouhal number, turbulence intensities and vortexstrength decrease with the increase of the aspect ratios.At Ma=0.75, the drag coefficient, Strouhal number, the maximum velocity inrecirculation zone and the turbulence intensities decrease, while the recovering velocityat the outlet, the lowest pressure, the pressure at the centre of the back side increase withthe increase of the aspect ratios. There are two kinds of mechanism to form the localsupersonic zones in the near wake region, i.e., vortex and vortex interaction, shock-wave and vortex interaction. The lift coefficient exhibits obvious periodicvariations, indicating the existence of regular periodic vortex shedding. The threedimensional vortex street structure can be clearly observed. At Ma=0.95, the flowaround square cylinder is steady. The oblique shock waves appear which restrain thevortex shedding. The drag coefficient is larger than that at Ma=0.75.