Characteristics of fluid dynamic forces acting on a circular cylinder oscillated in the streamwise direction and its wake patterns

This paper describes the characteristics of fluid forces and wake patterns of a circular cylinder oscillating in the streamwise direction in a cross-flow, on the basis of precise measurements and flow-visualizations in forced-oscillation tests in a water tunnel at subcritical Reynolds numbers. The added mass and added damping coefficients of a circular cylinder in a streamwise oscillation were calculated. The results of the forced-oscillation tests confirmed that two reduced-velocity ranges with negative added damping exist. The two ranges agree well with the two streamwise excitation regions observed in the free-oscillation tests in several previous studies. The added mass coefficient for the streamwise vibration varies greatly, depending on the reduced velocity, and that can affect the dynamic behavior of the streamwise vibration under low mass-ratio conditions. Furthermore, the mean drag coefficient attains its maximum at the reduced velocity between the first and second excitation regions where the added damping coefficient reaches its local maximum. This suggests that the increase in the mean drag coefficient induced by the formation of alternate vortices magnifies the fluid damping effect and contributes to the damping region between the first and second excitation region.