In the past decades, sinusoidal motion of a circular cylinder in viscous flow had been extensively studied by researchers all around the world. While difference of flow patterns exists between fixed cylinder and freely vibrating cylinder. In this paper, numerical simulations are carried out by the in-house CFD code naoe-FOAM-SJTU with overset grid capability, which is developed based on the open source code OpenFOAM. The diameter of the cylinder is 0.02m and the KC numbers vary from 3 to 15. Results of vortex evolution, flow regimes and separation points of the cylinder with crossflow freedom restricted and freely vibrating are compared.
In actual production, the periodic oscillation of floating structures may generate relative oscillatory flow between the riser and the water. Experiments and numerical simulations of oscillatory flow around a circular cylinder have been studied by Williamson (1985), Sarpkaya (1986, 1995), Zhao et al., (2011), Graham (1980), Nehari et al., (2004), Tatsuno et al., (1990) and Obasaju et al., (1988).
Experiments of a periodically oscillated circular cylinder in still water were conducted by Williamson (1985) and Sarpkaya (1986). KC numbers ranged from 0 to 40 and flow regimes were identified within particular KC region, such as the attached vortices regime (0<KC<7), vortices attach to the cylinder and no shedding occurs; the single pair regime (7KC<15); the double pair regime (15<KC<24); the three pairs regime (24<KC<32) and the four pairs regime (32<KC<40). It was found (Sarpkaya, 1986) that no separation phenomenon happened when KC<1.1. The Honji vortices (Honji, 1981) and separation occurred when KC number between 1.1 and 1.6. A pair of symmetric vortices generated with KC numbers ranging from 1.6 to 4. While during the region of 4<KC<7, a pair of asymmetric vortices generated. For further KC regimes, the number of vortices pairs shed in an oscillating cycle increased by one each time the KC number changed to a higher one.