EXPERIMENTAL STUDY ON THE KINEMATICS OF FREELY FALLING PROLATE SPHERE IN THE WATER

The motion of solid particles in the liquid is frequently encountered in everyday life and engineering applications. It attracts the devotement of many researchers due to the abundant fluid dynamic phenomena behind the motion of solid particles in the liquid. In the present paper, we experimentally study the falling characteristics of a single prolate sphere particle descending in water under the influence of buoyancy force. The motion tracking platform which consists of two orthogonal high-speed cameras and light sources accompanied by the fluorescence visualization technology is adopted to obtain the paths and vortex structures of the falling prolate sphere particle. The density ratio between the selected prolate sphere and surrounding fluid is 1.2, while the aspect ratio of the selected prolate sphere varies from 2 to 10, the corresponding Archimedes number changes from 400 to 1400, and finally, the terminal Reynolds number is limited in the range from 120 to 1350. During the experiments, we observe five typical types of paths during the falling process of the prolate spheres, which corresponds to small amplitude irregular motion, small-amplitude high-frequency oscillation motion, large-amplitude low-frequency oscillation motion, highly nonlinear motion and rectilinear motion, respectively. And the evolution of the oscillation of velocity and the inclination angle is obtained. Furthermore, we analyze the relationships between the drag coefficient of the freely falling prolate sphere and the Reynolds number during the falling process. Then, by using the fluorescence visualization technique, we identify the different vortex shedding modes, which are responsible for different falling trajectories, and study the influence of vortex shedding on the path of free falling prolate sphere. Finally, comparing with some previous results on the falling characteristics of the slender cylinder, we find the similarities and differences of the motion characteristics between the prolate sphere and the slender cylinder and the potential physical mechanism.