10 th International Congress on Advances in Civil Engineering, 17-19 October 2012 Middle East Technical University, Ankara, Turkey 1 A comparison between 3D LES of vortex shedding of uniform and tapered cylinders in subcritical Reynolds number M. Zeinoddini 1 , V. Tamimi 1 , S. Shahvaghar- Asl 2 1 Department of Civil Engineering, K.N.Toosi University of Technology and TWI Central Asia, Tehran, Iran, vahidtamimi@msina.kntu.ac.ir 2 Department of Civil Engineering, K.N.Toosi University of Technology, Tehran, Iran Abstract The physics of flow regime around a tapered cylinder becomes strongly three-dimensional (3D) and complex. These pose numerical difficulties in capturing the vortex shedding phenomena in the wake of cylinder. In the present study, self excited transverse response of a uniform and tapered circular cylinder with mean diameter of 0.028 m and aspect ratio of about 14 is numerically simulated. The circular cylinders have medium mass ratio (5.93, 6.1) and low mass- damping parameters (0.0275, 0.0279). A fully coupled two way fluid structure interaction (FSI) analysis is used to simulate the phenomena of vortex induced vibration in vicinity of lock- in range. A 3D computational fluid dynamic model is employed to solve incompressible transient Navier- Stokes equations. The LES turbulence model with Smagorinsky subgrid scale model is considered to include high turbulence effects at the present subcritical Reynolds number. Structural displacements are calculated through transient structural analysis in mechanical application (Computational Structural Dynamics- CSD). Interactions between CSD and CFD analyses take place at the fluid- structure interfaces. The transverse vibrations of uniform and tapered cylinders are compared against available experimental data. The comparison reveals that the model is capable to reasonably predict the initial and upper branch of the responses but falls short to properly predict the lower branch. Time histories of reduced amplitudes of transverse vibrations of the elastic uniform and tapered cylinders are also provided. The simulation results of the fixed and elastic mounted tapered cylinders are compared and discussed afterwards. The results indicate that the flow field in the case of the elastic mounted tapered cylinder is completely different from fixed tapered cylinder. For the case of the elastic tapered cylinder no vortex cell is forming in the lock-in region and a single frequency response dominates the entire spanwise length. Keywords: Fluid structure interaction, uniform and tapered circular cylinders, vortex cell, lock-in region 1 Introduction Vortex induced vibration is a complex multi-physics problem which concerns with both solid and fluids dynamics. There are three main different approaches toward the VIV research. i) Experimental studies j) semi empirical models k) numerical simulations. Experimental investigation of VIV, especially when we are looking for vortex patterns behind the cylinders (vortex visualization), is costly and time consuming. This is especially the case when using the methods to provide both qualitative and quantitative data like Digital Particle Image Velocimetry or DPIV. Instead, if correctly simulated, the numerical methods will make vortex pattern studies at the wake of the cylinders much easier. Numerical simulation of the flow field at the wake of fixed or forced vibrating cylinders are relatively more straightforward compared to resonance vortex induced vibration simulation of an elastic cylinder from rest. The difficulties lies in accurate simulation of the fluid field and correct coupling of two structural and fluid fields to let the vortices to become synchronizes with the cylinder oscillation across the 'lock-on' range. This is why most of the previous numerical researches have been concentrated on vortex shedding studies and force measurements on fixed or forced vibrating cylinders (Gabbai and Benaroya, 2005).