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Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

Controlled and impulsive compression of an entrapped air bubble during impact
Utkarsh Jain and Devaraj van der Meer
Physics of Fluids Group and Max Planck Center Twente for Complex Fluid Dynamics, MESA+ Institute and J. M. Burgers Centre for Fluid Dynamics,
University of Twente
Enschede, The Netherlands(a)ABSTRACT
Wave slamming onto a structure is often accompanied by the entrapmentof an air pocket. A large scale impact typically has a rapidly evolving and disturbed liquid-gas interface, such that several bubbles are entrapped upon impact. While it is largely understood how the peak pressure is created by liquid coming into contact with the solid structure, it ismore challenging to ascertain how an isolated air pocket is pressurised by
an impulsive impact, and how the maximum impact pressure inside thisbubble evolves. We study such a Bagnold-type impulsive compressionof an air bubble by performing well-controlled experiments, where weuse an inverted, hollow cone as an impactor. The cone is kept immersed
throughout in a water bath, such that it encloses an air bubble of known
and controlled volume. A high-sensitivity sensor measures pressures atthe vertex of the cone. Using high-speed imaging we show how incoming liquid deforms the air bubble enclosed in such a geometry, and how
an impact peak is registered inside the bubble, which can be traced backto the impact of a liquid jet onto the pressure sensor. We compare themeasured pressures to a Bagnold model, and discuss the dominant resonances in the bubble. From visualisations of the deforming bubble, wealso discuss the air-pocket’s deformations, resulting from the presenceof surrounding rigid geometry (such as corrugations in an LNG containment membrane).

(b)Highspeed
cameraWater(c)
45°dDFig. 1 Experimental setup: (a) The inverted cone is placed in awater bath and moved downwards by means of a linear
motor (not shown). A high speed camera can be used toview the bubble’s behaviour through a transparent cone.
(b) Example of an air bubble trapped under the cone, thesurface of which can be seen from the specular reflection atthe water-air interface. (c) The pressure sensor is mounted
in a stainless steel disc to which the 3D-printed cone is attached. Note that D and d represent the outer and innerdiameter of the base of the cone, respectively. The sensor
width is 5.55 mm.

KEY WORDS: gas pocketed impact; adiabatic bubble compression;
Bagnold impact

INTRODUCTION
In liquid-solid slamming events, air entrapment in between the formertwo phases is a common feature. It may occur due to air-cushioning,whereby the liquid surface deforms by a small amount due to highstagnation pressure, entrapping a thin air layer (Verhagen 1967, Peters,
Gordillo & van der Meer 2013, Jain et al. 2021). Alternatively it canoccur when either the fluid or the solid phase has a concave surface,
thereby fully entrapping a large pocket of air (Mathai et al. 2015).
Examples of this type of entrapment can occur when an overturned wave
slams onto a structure (Bogaert et al. 2010, Peregrine 2003, Kapsenberg
2011, van Meerkerk et al. 2020), or during the re/-entry of water-skiswith concave-shaped floats into water.
Air-pocket-mediated water slamming pressures have attracted attention
from the sloshing and wave-slamming community due to the critical role

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