The mechanism of the occurrence for the fluctuating force on a stationary two-dimensional circular cylinder

Abstract

It is well known that the fluctuating pressures induced by the shedding of the Karman vortex on a stationary circular cylinder yield the periodic lift force, especially in a sub-critical Reynolds number. The Karmen vortex is formed away from the cylinder, so that the Karman vortex dose not seem to have direct effects upon the fluctuating pressures on the side surfaces of the cylinder.In this study, we investigated the mechanism of the occurrence for the periodic force on a circular cylinder in a range of the sub-critical Reynolds number, by using the simultaneous multi-pressure measuring system and the flow visualization technique. It is found that not the shed Karman vortex itself but the movement of separated shear layers, which are formed nearby side surfaces of the cylinder and affected by the Karman vortex shedding, directly contribute to the fluctuating pressures on the cylinder. It is found that the fluctuations of the stagnation point and the separation point on the cylinder are synchronized with the lift fluctuation, both of which have the same frequency as the Strouhal number. We also clarified the relationship between the shedding of the Karman vortex, the movement of the separated shear layer and the occurrence of the fluctuating pressures on the circular cylinder.