Characteristics of Unsteady Fluid Force Acting on a Rotating Cylinder Partially Submerged in Water.

This paper deals with an experimental investigation of unsteady fluid force acting on a rotating cylinder partially submerged in water. The rotating cylinder partially submerged in water is excited in one direction at a constant amplitude, and we measure the strain of the shaft installed in the rotating cylinder, displacement, and rotational angle for calculating the fluid force acting upon the cylinder. Experiments are carried out with variation in excitation frequency (f from 4 Hz to 14 Hz)and rotational speed (S from 3 s-l to 30 s-1). For evaluating the fluid force, we transform the strain signals measured on the rotating shaft to the absolute coordinate system consisting of two perpendicular axes, one for the excitation direction and the other for the direction perpendicular to the former, and subtract the inertial force of the cylinder from them. We calculated the energy input to this system with respect to two major experimental parameters, f and S. Plotting the energy calculated in the f-S diagram, we find the domains where the energy takes a positive value, which are confirmed to correspond to the unstable vibration of a rotating cylinder partially submerged in water.