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Unsteady forces on spheres during free-surface water entry

Published online by Cambridge University Press:  02 July 2012

Tadd T. Truscott ,
Brenden P. Epps  and
Alexandra H. Techet
Tadd T. Truscott*
Affiliation:
Brigham Young University, Provo, UT 84604, USA
Brenden P. Epps
Affiliation:
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Alexandra H. Techet
Affiliation:
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
*
†Email address for correspondence: [email protected]
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Abstract

We present a study of the forces during free-surface water entry of spheres of varying masses, diameters, and surface treatments. Previous studies have shown that the formation of a subsurface air cavity by a falling sphere is conditional upon impact speed and surface treatment. This study focuses on the forces experienced by the sphere in both cavity-forming and non-cavity-forming cases. Unsteady force estimates require accurate determination of the deceleration for both high and low mass ratios, especially as inertial and hydrodynamic effects approach equality. Using high-speed imaging, high-speed particle image velocimetry, and numerical simulation, we examine the nature of the forces in each case. The effect of mass ratio is shown, where a lighter sphere undergoes larger decelerations and more dramatic trajectory changes. In the non-cavity-forming cases, the forces are modulated by the growth and shedding of a strong, ring-like vortex structure. In the cavity-forming cases, little vorticity is shed by the sphere, and the forces are modulated by the unsteady pressure required for the opening and closing of the air cavity. A data-driven boundary-element-type method is developed to accurately describe the unsteady forces using cavity shape data from experiments.

JFM classification

Mathematical Foundations: Computational methods Vortex Flows: Vortex shedding
Type
Papers
Information
Journal of Fluid Mechanics , Volume 704 , 10 August 2012 , pp. 173 - 210
Copyright
Copyright © Cambridge University Press 2012

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