Verification of wake branch response regions observed in 2-DOF forced motion of a circular cylinder

Vortex induced vibrations (VIV) are an essential consideration in the engineering design of long, slender underwater structures subject to currents, such as moorings, line arrays, and pipelines. Some semi-empirical methods used to predict this fluid-structure interaction may utilize forced motion experiments conducted physically or through simulation to estimate forces on a simplified cross-section (such as a rigid circular cylinder) to utilize slender body assumptions for the prediction of the behavior of long, flexible structures. When the cylinder is allowed to vibrate in both the in-line and cross-flow directions, for a given set of kinematic conditions, multiple branches of response have been observed along with unusually excitation forces. Additionally, limited experimental data sets evaluating these forced motions show significant discrepancies in measured forces. In the present study numerical simulations are performed to investigate discrepancies between published 2-DOF forced motion datasets. It is found that results from both datasets can be observed numerically through variation of the initial conditions, indicating additional multiple wake branch response regions that may exist in such datasets, but may not have been observed experimentally. The cause of these differences is a combination of differences in experimental setups and testing environments, coupled with the highly nonlinear nature of VIV in 2 DoF. This observation is significant as it illustrates the sensitivity of such datasets to unobserved nonlinearities that would affect predictions based on forces derived from these datasets.