Strouhal number effect on synchronized vibration range of a circular cylinder in cross flow

Synchronized vibrations were measured for a circular cylinder in a water cross flow at subcritical Reynolds numbers to compare the synchronization range between the subcritical and supercritical regions and clarify the effect of the Strouhal number on the range. A small vibration in the lift direction was found in only the subcritical region when the Karman vortex shedding frequency was about 1/5 of the cylinder natural frequency. The ratio of the Karman vortex shedding frequency to the natural frequency where the self-excited vibration in the drag direction by the symmetrical vortex shedding began was about 1/4 in the subcritical region, and increased to 0.32 at the Strouhal number of 0.29 in the supercritical region. The frequency ratio at the beginning of the lock-in vibration in the drag direction by the Karman vortex shedding was about 1/2, and that in the lift direction decreased from 1 to 0.8 with decreasing Strouhal number.