Three-Dimensionality in the flow of an elastically mounted circular cylinder with two-degree-of-freedom vortex-induced-vibrations

TL;DR

This study numerically explores the transition from two-dimensional to three-dimensional flow in vortex-induced vibrations of an elastically mounted circular cylinder with two degrees of freedom, revealing mode-C transition characteristics and critical Reynolds numbers.

Contribution

It provides new insights into the 3D transition mechanisms in 2-DOF VIV of cylinders, including a transition map based on Reynolds number and oscillation parameters.

Findings

Mode-C transition from 2D to 3D flow observed in 2-DOF VIV.

Critical Reynolds number for transition around 250 at reduced velocity 6.

Transition characteristics depend on oscillation frequency ratio and Reynolds number.

Abstract

The study numerically investigates the three-dimensionality in the flow and two-degree-of-freedom (2 DOF) vortex-induced-vibrations (VIV) characteristics of an elastically mounted circular cylinder. The cylinder is allowed to vibrate in both streamwise and transverse directions. A low value of mass-ratio with the zero damping coefficient is taken for the simulations. The primary aim is to understand the vortex shedding behind the cylinder and the transition characteristics of the wake-flow from two-dimensional (2D) to three-dimensional (3D). The Reynolds number (Re) is varied from 150 (fully 2D flow) to 1000 (fully 3D flow), which lies inside the laminar range. The reduced velocity is varied which covers all three major VIV branches (Initial Branch (IB), Upper Branch (UB), and the Lower Branch (LB)). The oscillating cylinder sweeps the figure-eight trajectory. Two branches (IB, LB) and three branches (IB, UB, LB) amplitude responses are obtained for the low and high Re values, respectively. The wake behind the cylinder with 2-DOF VIV undergoes the mode-C transition of 2D to 3D flow as opposed to the direct mode-B transition observed for transverse only VIV in the literature. The critical Re range of the 2D to 3D transition for the 2-DOF VIV cylinder at a reduced velocity of 6 is around 250, less than the 1-DOF VIV. Also, this range varies with the variation in and the streamwise to transverse oscillation frequency ratio. A map is proposed for the 2-DOF VIV, highlighting the different modes of transition obtained for combinations of reduced frequency and Re.