With the widely application of cylinders in offshore structures, flow around cylinder has been investigated extensively to explore the mechanism behind it. Nevertheless, most experiments and numerical simulations focused on the single-phase flow and the two-phase flow around the surface-piercing cylinder which considers the existence of free surface has not received much attention. In the present work, free surface effect on the flow around a surface-piercing cylinder at the Re=27000 and Fr=0.8 is investigated. All the numerical simulations are conducted by the in-house CFD solver naoe-FOAM-SJTU, which is developed based on the open-source toolbox OpenFOAM. RANS equations with turbulence model are adopted to simulate the flow and VOF method is used to capture the free surface. The comparison with experiment and LES results by others shows that the current method is reliable and can capture the time-averaged characteristics. Through the analysis, it is found that the existence of free surface has a significant influence on the hydrodynamic forces and the wake flow characteristics. A reduction of drag coefficient can be observed. The delayed transition of the shear layers and the attenuation of regular vortex shedding can be found near the free surface.
With the rapid development of offshore platforms such as spars and semi-submersibles, it is necessary to study the flow characteristics of them. So as the basic hydrodynamic problem, the flow around cylinder has received much attention because it can reveal complex flow phenomenon like separation, reattachment and vortex shedding. And with the help of computational fluid dynamics (CFD), we can get more detailed information including streamlines, velocity profiles and vortical structures to explore the mechanism behind it.
However, most experiments and numerical simulations (Williamson, 2003; Yeon et al., 2016; Pereira et al., 2019; Ye et al., 2017; Zhao et al., 2016) are about single-phase flow around the cylinder which ignores the effect of free surface. Actually, the existence of free surface makes the flow more complicated due to the interaction between air and water. Bow wave at the front, depression behind the cylinder and Kelvin wave in the far field can be observed. So, it is of great importance to study the effect of free surface for future work.