Large eddy simulation of forces and wake modes of an oscillating cylinder

Abstract

The aim of this research is to examine the transition process between two different wake states of an oscillating circular cylinder in the frame of three-dimensional large eddy simulation (LES). The relationship between the hydrodynamic force and near-wake structure of the fully-submerged cylinder is considered in detail, and results will be used to understand the mechanisms of vortex-induced vibration at high Reynolds flows. The simulations are performed on parallel processing in the University's high performance computing facility. The spanwise- and phase-averaging tools are developed in open source code OpenFOAM environment for qualitative comparisons with the experimental visualisation studies. Verification and validation of the simulations at subcritical Reynolds numbers of 5500-41300 are conducted. The grid resolution and subgrid model, which highly influence on LES solutions, are carefully and properly chosen at the considered Reynolds numbers, and the resulting hydrodynamic forces and pressure are successfully predicted compared to the available experimental or direct numerical simulation data. In these studies, the variations of the near-wake structure and shear-layer frequency are examined with increasing Reynolds number. The numerical results exhibit the characteristic broadband spectral peaks of shear-layer frequency due to small-scale shear-layer vortices inside the freely-separating thin-boundary layers behind the cylinder, and clearly show the dependence of Re0:67 for which the shear-layer instability is fully developing in subcritical flow regime.

Based on the careful validations, turbulent flow behind an oscillating cylinder is investigated with various excitation amplitudes and a series of excitation frequencies. In these studies, the well-known flow features of an oscillating cylinder are reproduced and compared to the experimental observations. Two types of transition processes near flow resonance, namely the self-excited and reverse self-excited transitions, are captured first in a set of numerical simulations. Whilst the self-excited transition was only observed in the recent experiment especially at a certain flow condition, high amplitude excitation and lower Reynolds number, the present research shows numerical evidence that the one-time transition is possible even at lower excitation amplitude and higher Reynolds number as well. The change of the ensemble averaged wake patterns during the transition process strongly supports this numerical observation.

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ORCID for Philip Wilson: orcid.org/0000-0002-6939-682X

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