Experimental study on the impulsively started motion of a close-to-neutral buoyancy freely decelerating sphere

Abstract

An experimental study is presented that addresses the problem of a freely decelerating sphere in a still water tank. The diameter of the sphere was 0.04 m. Three different solid-to-fluid density ratios were considered: 0.78, 0.88, and 0.94. The submerged sphere was impulsively started upon being rammed by an actuator-mass system. Six initial velocities were considered: 0.91, 2.03, 2.54, 2.94, 3.29, and 3.78 m/s. The Reynolds number of the initial velocities based on the sphere diameter was 3.6 × 104, 8.1 × 104, 1.01 × 105, 1.17 × 105, 1.31 × 105, and 1.51 × 105 (subcritical). It was observed that both sphere dynamics and associated flow topology (identified via an optical system and a particle image velocimetry system, respectively) differed significantly from the case of an accelerating sphere. In the present case, a large vortex ring structure (both torus diameters of the order of the sphere's diameter) formed and attached to the sphere surface. This vortex ring followed the sphere motion all the way down the falling trajectory. From the data reduction standpoint, it was found that a suitably defined dimensionless acceleration parameter allowed for collapsing the kinematics variables of the sphere trajectory, namely, position, velocity, and acceleration, into a single ordinary differential equation.