The hydrodynamic forces acting on a circular cylinder forced to oscillate transversely in a uniform flow were measured, and the effect of synchronization of vortex shedding on the forces were examined. The vortex shedding frequency fv becomes 1/3 of the frequency of cylinder oscillation fa, when the cylinder oscillates at the frequency fa ∼ 2f. where f. is the natural vortex shedding frequency so called Strouhal frequency. This phenomena is called "frequency demultiplication". The measured velocity field by means of the image measurement technique showed the vortex shedding process in the case of frequency demultiplication, and the measured results show that two vortices which have same vorticity direction shed in one and half cycle of the cylinder oscillation. The measured hydrodynamic forces, drag and lift, were analyzed by the FFT technique. The lift force contains a low frequency component fvo, the component lower than fa under frequency demultiplication. However the ratio of fvo/fa is not 1/3 neither 2/3, but continuously changes between 1/3 and 2/3. The drag force has two components beside the fa. It is found that the drag force has the component of fvo + fa and fvo - fa from the analysis of measured hydrodynamic forces.
The Karman's vortex street occurs behind a stationary circular cylinder spanning a uniform stream. Strouhal number St = f.D/U is an important parameter for the vortex shedding, where U represents uniform flow velocity, D the diameter of the circular cylinder, and fs. Oscillations of the cylinder at a frequency fa which is close to f. may cause vortex shedding at the frequency fa. This phenomena is generally called the "lock-in" (Bearman, 1984, Sarpkaya, 1979) which occurs over a range of flow speeds depending on the amplitude of cylinder oscillation a and the cylinder frequency fa.