In the present paper hydrodynamic forces acting on oscillating circular cylinders as well as flat plates perpendicular to the motion in regular and irregular modes due to vortex shedding are experimentally investigated in order to clarify the memory effect or history effect on the hydrodynamic forces. The experimental results for sinusoidal motion show that the drag and added mass coefficients in steady-state condition are significantly different from those of the first swing from the start because of the existence of old vortices generated in the previous cycles around the cylinder. It is found from the experiments in irregular modes that the drag and added mass coefficients significantly vary every swing. It is confirmed that these coefficients during a swing depend on the relative amplitude or the Keulegan-Carpenter number of the previous swings as well as that of the swing under consideration.

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