This paper investigated two-phase flow around a surface-piercing circular cylinder by means of computational fluid dynamics (CFD) method. An algebraic VOF model is used to capture the interface of the free-surface flow. The turbulence flow is modeled with the two equation SST k-ω model. Forces, free-surface elevation and cylinder surface pressure are presented and analyzed. To further understanding the turbulent structures, the normalized Liutex definition is used to identify and extract the vortical structures in the wake of cylinder.

INTRODUCTION

Flow past vertical free surface-piercing cylinder is of great interest in both industry and academia. From an industrial point of view, it is the simplified model for offshore structures such as spars, tension-leg platforms and semi-submersibles. These floating platforms generally include surface-piercing vertical columns. Therefore, they are subjected to vortex-induced motions (VIM) when exposed to currents. From an academic point of view, the detailed hydrodynamics of how vortices are generated around a surface-piercing truncated and interact with free surface are poorly understood and should be further studied.

There have been several studies on this topic both experimentally and numerically. Sheridan et al. (1997) conducted experiments in a water channel to study the wake of a horizontal cylinder submerged beneath free surface with different submerged depths. They found that flow past a cylinder beneath free surface could generate near-wake structures that are distinctly different from the wake of a fully submerged cylinder. Inoue et al. (1993) experimentally investigated a vertical surfacepiercing cylinder with diameter of 0.05m for two Froude numbers, Fr=0.8 and 1.0. They revealed that the periodic vortex shedding is suppressed in the regions near free-surface, while it is strong in deep regions away from free-surface. Kawamura et al. (2001) performed large-eddy simulation (LES) to investigate the interactions between surface waves and underlying viscous wake for a turbulence flow past a free surface piercing circular cylinder at Re=2.7×104. Three Froude numbers, Fr=0.2, 0.5 and 0.8 are considered in their simulations. They also observed vortex shedding attenuation near free surface at high Froude number. Yu et al. (2008) also studied flow past a free surface piercing cylinder numerically using LES. In their study, they focused on flow with higher Reynolds number up to Re=1×105 and Froude number Fr=3.0. The air-water interface was captured using a two-phase volume-of-fluid (VOF) method and the volume fraction transport function was solved by a flux-corrected transport (FCT) algorithm. They compared drag and lift forces, as well as vortical structures between various Reynolds and Froude numbers. Rosetti et al. (2013) numerically investigated flow past free-surface piercing, low aspectratio (L/D=2.0) circular cylinder by means of unsteady Reynoldsaveraged Navier-Stokes (URANS). They also performed small-scale experiments in recirculating water channel. In their numerical studies, they compared the results of different boundary conditions for top of the domain and discussed the free-surface effect.

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