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The water entry of a sphere in a jet

Published online by Cambridge University Press:  29 January 2019

Nathan B. Speirs ,
Jesse Belden ,
Zhao Pan Open the ORCID record for Zhao Pan [Opens in a new window] ,
Sean Holekamp ,
George Badlissi ,
Matthew Jones  and
Tadd T. Truscott Open the ORCID record for Tadd T. Truscott [Opens in a new window]
Nathan B. Speirs
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
Zhao Pan
Affiliation:
Department of Mechanical and Aerospace Engineering, Utah State University, Logan, UT 84322, USA
Sean Holekamp
Affiliation:
Naval Undersea Warfare Center Division Newport, Newport, RI 02841, USA
George Badlissi
Affiliation:
Department of Ocean Engineering, University of Rhode Island, Narragansett, RI 02882, USA
Matthew Jones
Affiliation:
Department of Mechanical and Aerospace Engineering, Utah State University, Logan, UT 84322, USA
Tadd T. Truscott*
Affiliation:
Department of Mechanical and Aerospace Engineering, Utah State University, Logan, UT 84322, USA
*
†Email address for correspondence: [email protected]
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Abstract

The forces on an object impacting the water are extreme in the early moments of water entry and can cause structural damage to biological and man-made bodies alike. These early-time forces arise largely from added mass, peaking when the submergence is much less than one body length. We experimentally investigate a means of reducing impact forces on a rigid sphere by placing the sphere inside a jet of water so that the jet strikes the quiescent water surface prior to entry of the sphere into the pool. The water jet accelerates the pool liquid and forms a cavity into which a sphere falls. Through on-board accelerometer measurements and high-speed imaging, we quantify the force reduction compared to the case of a sphere entering a quiescent pool. Finally, we find the emergence of a critical jet volume required to maximize force reduction; the critical volume is rationalized using scaling arguments informed by near-surface particle image velocimetry (PIV) data.

JFM classification

Flow Control: Drag reduction Interfacial Flows (free surface): Interfacial Flows (free surface)
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 863 , 25 March 2019 , pp. 956 - 968
Copyright
© 2019 Cambridge University Press 

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Speirs et al. supplementary movie 1

Supplemental movie for Fig. 3a. A 50 mm diameter sphere impacts a quiescent pool surface with velocity U=4.39 m/s forming a subsurface air cavity, that experiences surface seal, deep seal and cavity shedding. Movie played back at 3% of real speed.

Download Speirs et al. supplementary movie 1(Video)
Video 2.3 MB

Speirs et al. supplementary movie 2

Supplemental movie for Fig. 3b A 50 mm diameter water jet impacts a pool surface forming a subsurface air cavity followed by a 50 mm diameter sphere that impacts the bottom of the jet cavity at velocity U=4.35 m/s without forming a cavity. Movie played back at 3% of real speed.

Download Speirs et al. supplementary movie 2(Video)
Video 3 MB

Speirs et al. supplementary movie 3

Supplemental movie for Fig. 5a A sphere impacts an initially quiescent pool at 4.23 m/s accelerating the liquid in front of it. The colouring of the images shows the vertical velocity of the fluid uy with positive defined in the upward direction as shown in the colour bar on the right. Movie played back at 0.04% of real speed.

Download Speirs et al. supplementary movie 3(Video)
Video 1 MB

Speirs et al. supplementary movie 4

Supplemental movie for Fig. 5b. A jet impacts a quiescent pool at 4.23 m/s, deforms and creates a large, local downward flow. The colouring of the images shows the vertical velocity of the fluid uy with positive defined in the upward direction as shown in the colour bar on the right. Movie played back at 0.4% of real speed.

Download Speirs et al. supplementary movie 4(Video)
Video 769.8 KB

Speirs et al. supplementary movie 5

Supplemental movie for Fig. 5c. A sphere at 4.45 m/s impacts the bottom of a cavity formed by a jet with the same impact conditions as in Movie 4. The colouring of the images shows the vertical velocity of the fluid uy with positive defined in the upward direction as shown in the colour bar on the right. Movie played back at 0.4% of real speed.

Download Speirs et al. supplementary movie 5(Video)
Video 504 KB