Vortex-induced vibration (VIV) is a fundamental phenomenon commonly encountered in various practical engineering. Owing to the complexities associated with this phenomenon, modelling and prediction of VIV is a challenging task. In this research, a new predictive phenomenological model is developed for VIV of an elastically mounted rigid cylinder subjected to a fluid flow and free to vibrate in both cross-flow (CF) and in-line (IL) directions. The ensuing dynamical system is based on double Duffing-van der Pol (structural-wake) oscillators with the two structural equations containing both cubic and quadratic nonlinear terms. The cubic nonlinearities capture the geometrical coupling of CF/IL displacements excited by hydrodynamic lift/drag forces whereas the quadratic nonlinearities allow the fluid-structure interactions. The model predictions are extensively compared with published and in-house experimental results. Experiments are carried out at the Department's towing tank to calibrate and validate numerical prediction results. Comparisons illustrate the qualitative resemblance between experimental and prediction results, highlighting how the new model can capture several important VIV characteristics including a two-dimensional lock-in, jump and hysteresis phenomenon, and figure-of-eight trajectory tracing the periodically coupled CF/IL oscillations. Moreover, the parametric studies reveal the important effect of geometrical nonlinearities, mass ratio, damping ratio and natural frequency ratio.Insights into hydrodynamic properties such as VIV-induced mean drag, added mass and damping are drawn based on the newly proposed model via analytical-numerical approaches and comparisons with published literature. Consequently, the new prediction model is applied to the VIV analysis of flexible circular cylinders subjected to uniform and linearly sheared currents. To capture a three-dimensional aspect of the flexible cylinder experiencing VIV, nonlinear equations of CF, IL and axial structural oscillations are considered to be coupled with the distributed van der Pol wake-oscillators. Governing equations are numerically solved via a space-time finite difference scheme, and the obtained numerical results highlight several aspects of VIV of elastic cylinders along with the axial motion effects. Apart from the validation of the numerical model with published experimental results, this study reveals how the effect of axial motion and its nonlinear coupling with the two transverse CF/IL motions can be very important. These depend on the reduced velocity, the fluid-structure parameters, the single or multi-mode lock-in condition, and the standing-wave versus travelling-wave features.
|Date of Award||16 Apr 2015|
- University Of Strathclyde
|Supervisor||Narakorn Srinil (Supervisor) & Sandy Day (Supervisor)|