Numerical investigations on wake transition of a 2-DOF elastically mounted circular cylinder

Present is a three-dimensional (3D) numerical study on vortex-induced vibration (VIV) of an elastically mounted rigid circular cylinder in cross-flow. The cylinder motion has two degrees-of-freedom (2-DOFs), i.e. it can oscillate in both streamwise as well as in transverse directions. The open-source software, ‘OpenFOAM-7′ (Open Field Operation and Manipulation), has been used to solve the governing equations of fluid and cylinder motion. Computations are carried out to investigate the behavior of wake transition from two-dimension to three-dimension (2D to 3D) and the associated flow characteristics over range of values of reduced velocity (U_r), Reynolds number (Re) and mass ratio (m*). In particular, the range of U_r (2 ≤ U_r ≤ 10) encompasses both lock-in and non-lock-in regimes, where lock-in refers to synchronous oscillations of the cylinder and the wake. Effect of U_r on unsteady wake has been examined using vorticity contours and iso-Q surfaces. The temporal behavior of both the cylinder and wake oscillations has been analyzed using Hilbert spectra. The critical value of Re ($R{e}_{cr}$) for transition from 2D to 3D through ‘mode C’ type of instability is ascertained for fixed values of m* and U_r. Thereafter, the critical values of m* and Re are obtained by considering their mutual variation for fixed U_r and the associated mode transition is identified. At small values of m*, ‘mode C’ type of instability appears which changes to ‘mode A’ type for larger m*.