Sinusoidal motion of a cylinder in viscous flow has been extensively studied in the past decades. Distinction of flow patterns exists between cylinders in cross-flow freedom restricted and freely vibrating conditions when experiencing oscillatory flow. In this paper, a series of numerical simulations are carried out by the in-house CFD code naoe- FOAM-SJTU, which is developed basing on the open source code OpenFOAM with overset grid capability. The diameter of the cylinder is 0.02m and the KC numbers varies from 3 to 12 corresponding to the attached vortices regime and the transverse street regime. Results of vortex evolution, flow regimes and hydrodynamic force coefficients are compared.


In actual production, offshore floating structures subject to waves, currents or winds will cause the platform to move periodically in the water. Then relatively oscillatory flow is generated between the riser and the water. In recent decades, researches of the sinusoidal motion of a cylinder in viscous fluid have been extensively studied by Bearman (1984, 1985), Sarpkaya (1986,1995) and Williamson (1985).

Williamson (1985) conducted a series of experiments to investigate development of vortices around a single cylinder in relative oscillatory flow. And several vortex regimes were identified within particular ranges of Keulegan-Carpenter (KC) Numbers: the attached vortices regime (0#x003C;KC<7), where no major vortices shed during a cycle; the single pair regime (7#x003C;KC<15); the double pairs regime (15#x003C;KC<24); the three pairs regime (24#x003C;KC<32) and the four pairs regime (32#x003C;KC<40). For further KC regimes, the number of vortices pairs shed in each oscillating period would be increased by one each time the KC regime changed to a higher one.

Kozakiewicz et al., (1996) conducted experiments of a cylinder exposed to oscillatory flow for two Keulegan-Carpenter numbers, KC=10 and 20. Then numerical simulations of a cylinder freely vibrating in the cross-flow direction were carried out at the same KC numbers. Comparisons showed that the number of vortices generated over one oscillating cycle increased when the cylinder was freely vibrating in the cross-flow direction. The vortex shedding direction changed to the opposite side of the cylinder in the transverse street regime when KC=10.

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