Flow fields around a circular cylinder moving along circular orbits for Keulegan-Carpenter numbers (Kc) less than 3.5 have been· experimentally investigated when the motions are impulsively started from rest. The results of flow visualizations demonstrated that a separated vortex rotating with the opposite sense to the orbital motion initially sheds from the cylinder at a half period after the start of motion, and the shedding vortex induced a circulating flow similar to a starting vortex described by a wing theory. The circulation strength, which is obtained from the photo-image, drastically increases soon after the start of motion, and reaches a stable value from t = T/2 through t = T. The value of the circulation strength at steady state increases with increasing Kc number for Kc less than 3.0.


Flow fields around a vertical cylinder in regular waves or a horizontal cylinder in shallow water regular waves may be idealized as plane oscillatory flow. However, in the case of a horizontal cylinder in deep water regular waves, the water particle orbits are almost circular, and an instantaneous incident flow vector rotates with the same sense as the orbital motion during a wave period. At very low Keulegan-Carpenter numbers, hydrodynamic forces on a circular cylinder in plane oscillatory flow may safely be predicted by potential flow theory because there is no large flow separation. However, Chaplin (1984a, 1984b) reported that the inertia force on a horizontal cylinder, with its axis parallel to wave crests, decreases to a value as low as one-half of the inertia force calculated by potential theory at very low Kelilegan-Carpenter numbers (Kc of about 2). He pointed out that the cause of the inertia force reduction is the effect of the lift force, which is always in the opposite direction to the inertia force vector, due to the combination of incident flow and steady circulation generated by the oscillatory boundary layer.

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