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Water entry of deformable spheres

Published online by Cambridge University Press:  14 July 2017

Randy C. Hurd
Affiliation:
Department of Mechanical and Aerospace Engineering, Utah State University, Logan, UT 84322, USA
Jesse Belden
Affiliation:
Naval Undersea Warfare Center Division Newport, Newport, RI 02841, USA
Michael A. Jandron
Affiliation:
Naval Undersea Warfare Center Division Newport, Newport, RI 02841, USA
D. Tate Fanning
Affiliation:
Department of Mechanical Engineering, Brigham Young University, Provo, UT 84602, USA
Allan F. Bower
Affiliation:
School of Engineering, Brown University, Providence, RI 02912, USA
Tadd T. Truscott
Affiliation:
Department of Mechanical and Aerospace Engineering, Utah State University, Logan, UT 84322, USA
Corresponding

Abstract

When a rigid body collides with a liquid surface with sufficient velocity, it creates a splash curtain above the surface and entrains air behind the sphere, creating a cavity below the surface. While cavity dynamics has been studied for over a century, this work focuses on the water entry characteristics of deformable elastomeric spheres, which has not been studied. Upon free surface impact, an elastomeric sphere deforms significantly, giving rise to large-scale material oscillations within the sphere resulting in unique nested cavities. We study these phenomena experimentally with high-speed imaging and image processing techniques. The water entry behaviour of deformable spheres differs from rigid spheres because of the pronounced deformation caused at impact as well as the subsequent material vibration. Our results show that this deformation and vibration can be predicted from material properties and impact conditions. Additionally, by accounting for the sphere deformation in an effective diameter term, we recover previously reported characteristics for time to cavity pinch off and hydrodynamic force coefficients for rigid spheres. Our results also show that velocity change over the first oscillation period scales with the dimensionless ratio of material shear modulus to impact hydrodynamic pressure. Therefore, we are able to describe the water entry characteristics of deformable spheres in terms of material properties and impact conditions.

Type
Papers
Copyright
© 2017 Cambridge University Press 

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Hurd et al. supplementary movie 1

A sphere deforms significantly as it enters the water (G∞ = 6.70 kPa, D = 51 mm, U0 = 5.3 m/s and ρs = 1070 kg/m3) creating a subsurface cavity. After the initial deformation, the sphere oscillates between oblate and prolate shapes, eventually creating a second cavity within the first. Pinch-off occurs within this second, smaller cavity. The entry event is shown from both a top view and side view.

Download Hurd et al. supplementary movie 1(Video)
Video 6 MB

Hurd et al. supplementary movie 2

A sphere deforms significantly as it impacts and rebounds from a flat horizontal surface (G∞= 6.70 kPa, D = 51 mm, U0≈ 5 m/s and ρs = 1070 kg/m3). After impact the sphere oscillates between oblate and prolate shapes.

Download Hurd et al. supplementary movie 2(Video)
Video 2 MB

Hurd et al. supplementary movie 3

A rigid sphere enters the water forming a subsurface cavity (G∞ = 5.66E5 kPa, D = 51 mm and U0 = 6.5 m/s and ρs= 1070 kg/m3). The cavity pinches-off with a deep seal event.

Download Hurd et al. supplementary movie 3(Video)
Video 4 MB

Hurd et al. supplementary movie 4

A deformable sphere enters the water forming a subsurface cavity (G∞ = 70.2 kPa, D = 51 mm, U0 = 6.5 m/s and ρs = 1070 kg/m3). The cavity pinches-off with a deep seal event, very similar to a rigid sphere.

Download Hurd et al. supplementary movie 4(Video)
Video 7 MB

Hurd et al. supplementary movie 5

A deformable sphere enters the water forming a subsurface cavity (G∞ = 6.70 kPa, D = 51 mm, U0= 6.5 m/s and ρs = 1070 kg/m3). Pinch-off occurs within the second cavity formed.

Download Hurd et al. supplementary movie 5(Video)
Video 5 MB

Hurd et al. supplementary movie 6

A deformable sphere enters the water forming a subsurface cavity (G∞ = 1.12 kPa, D = 51 mm, U0 = 6.5 m/s and ρs= 1070 kg/m3). Pinch-off occurs within the second cavity formed.

Download Hurd et al. supplementary movie 6(Video)
Video 5 MB

Hurd et al. supplementary movie 7

For the largest and most compliant spheres tested (G∞ = 1.12 kPa, D = 100 mm, U0 = 6.5 m/s and ρs = 1070 kg/m3) the sphere decelerates more rapidly, occupying the space where pinch-off would occur. The attached cavity recedes upward along the sphere and pinch-off occurs at the top.

Download Hurd et al. supplementary movie 7(Video)
Video 7 MB
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