To estimate the unknown damping which is important in the study of dynamic response of the offshore structures to the wave forces, the damping of a circular cylinder oscillating in various depths of still water has been investigated using a rigid cylindrical pendulum. The damping factor is constant and independents of the amplitude of the cylinder oscillation in the range of low amplitude, and its value is explained theoretically by using the Stokes" solution introduced to explain the viscous effect of cylinder at low Keulegan-Carpenter number. At large amplitude, the damping factor becomes amplitude dependent and the drag coefficient associated with damping factor corresponds well the drag coefficients obtained for the circular cylinder in planar oscillatory flow at Keu1egan-Carpenter number above 2.
Damping is defined as the energy dissipated due to the vibration of a structure. The dynamic response of the structure to the force acting on it is restricted by the damping especially near the resonance. Therefore the damping must be considered significantly in the study of vortex-excited vibration of a structure in a fluid. However, the evaluation of the fluid damping are very difficult because the separation of the fluid damping force and the fluid force forcing the vibration of the structure(=forcing term in the vibration equation) is very difficult. The fluid damping may be considered to be composed of l)the viscous drag which is produced by shearing between the free stream and the surface of the structure, 2)the pressure drag which is produced by flow separating from the structure and forming a wake, and 3) the radiation force due to the creation of gravi ty wave induced by the oscillation of cylinder, radiation damping(gravity wave damping), (Blevins, 1977, Brounwers and Meijssen, 1985).