Experimental study of a spheroid falling in water: From prolate to oblate

This experimental study investigates the falling characteristics of spheroidal particles with varying geometrical anisotropy, characterized by the aspect ratio (χ). The aspect ratio is systematically varied from prolate (χ=0.1) to oblate (χ=10), while the Archimedes number (Ar), representing the ratio of gravitational force to viscous force, spans from 491 to 1389. The density ratio between the particle material and the fluid is maintained at λ≈1.2, providing controlled experimental conditions with two parameters, χ and Ar, influencing the spheroid's fall. High-speed cameras capture the falling patterns and positions, leading to a phase diagram categorizing the trajectories into quasi-vertical, rectilinear, zigzagging oblique for prolate, and zigzagging rotation and spiral for oblate spheroids. The characteristics of the paths, velocities, and inclinations are analyzed, drawing comparisons with prior studies and examining similarities and differences. Subsequent analysis focuses on the kinematic characteristics, particularly drag coefficients and oscillatory properties. The drag coefficients of falling spheroids are found to be generally larger than those reported for fixed spheroids, with the discrepancy more pronounced for oblates. The dimensionless oscillatory frequency, represented by the Strouhal number variant St^∗, is discovered to be geometry-independent when an appropriate characteristic length is adopted, converging to a value of approximately 0.124. Other oscillatory variables also normalize to a consistent curve. Finally, the fluorescent visualization technique reveals distinct wake structures behind spheroids in different falling modes. The vortex shedding influences spheroids differently in the fluid mode versus the fluid–solid mode, showcasing a strong coupling between wake structures and falling trajectories. This comprehensive study provides the first experimental map of dynamically varying falling characteristics across a broad parameter range of Ar for spheroids, enhancing our understanding of the impact of geometrical anisotropy on their falling behavior.