FREELY VIBRATING TWO SIDE-BY-SIDE SQUARE COLUMNS WITH COMBINED TRANSLATIONAL MOTIONS

摘要

This paper presents a set of numerical simulations of flow-induced vibrations (FTV) and coupled wake flow behind two identical square columns in a side-by-side configuration. To observe the four regimes as a function of different gap ratios, the computational results of the configuration at low Reynolds number (Re =(ρ~f UD)/μ~f) in stationary condition are firstly compared with existing experimental data of moderate Reynolds number. We next investigate the configuration of elastically mounted square columns, which are free to oscillate in both streamwise and transverse directions. The simulations are performed by the Petrov-Galerkin finite-element method and Arbitrary Lagrangian-Eulerian technique to account for the fluid mesh motion. The four regimes of stationary side-by-side configuration follow the same trend of the experimental data conducted at moderate Reynolds number, while the ranges of each regime differ due to the turbulent wake properties. For the freely vibrating condition, all the simulations are computed at low Reynolds number (Re = 200), mass ratio equal to 10 (m~* = M/m_f) and reduced velocity in the range of U_r ∈ [1,50] where U_r = U/(f_ND) and in free-damping condition ζ = C/2(KM)~(1/2)= 0. The four regimes in vibrating condition are investigated as a function of gap ratios g~*= g/D, which is the ratio of spacing between the inner column surfaces to the diameter of the column. The effects of reduced velocity on the force variations, the vibration amplitudes and the vorticity contours are analyzed systematically to understand the underlying FIV physics of side-by-side columns in the four regimes. Finally, we present a FIV study of the full semi-submersible model at moderate Reynolds number Re = 20,000 which can be considered as the application of side-by-side configuration.