Skipping under water: Buoyant sphere hydrodynamics at the air–water interface

Abstract

We present an experimental study of the hydrodynamics of a buoyant sphere accelerated horizontally along an air–water interface. At low speeds, the sphere floats at the surface, while at higher speeds, the sphere starts oscillating, moving below and toward the free surface akin to underwater skipping. The sphere often breaches and forms an air cavity during its subsequent dive. These underwater air cavities become horizontal and are attached to the sphere surface near the laminar flow separation point (∼π/2). High-speed imaging is used to investigate the effects of changing the pulling angle and counterweight-induced velocity on the hydrodynamics. We examine the transition from underwater skipping oscillations to water exit, particularly above the critical Froude number of 1.2, where buoyant spheres experience complex fluid–solid interactions revealing the influence of the air cavity on drag and lift coefficients and overall sphere hydrodynamics. Finally, we analyze the novel phenomenon of the steady motion of the horizontally pulled sphere with an attached inverted-wing-shaped air cavity.