Gas flow dynamics over a plunging breaking wave prior to impact on a vertical wall

J Westerweel,M Van Meerkerk,B Hofland,C Poelma

doi:10.1016/j.euromechflu.2021.09.008

J Westerweel, M Van Meerkerk + Show 2 more

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https://doi.org/10.1016/j.euromechflu.2021.09.008

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Abstract

We present an experimental study on the gas flow field development over a plunging breaking wave prior to impact on a vertical wall. The variability of wave impact pressure over repeated measurements is well known (Bagnold, 1939). The formation of instabilities on the wave crest are postulated to be the main source of impact pressure variability (Dias and Ghidaglia, 2018). However, the mechanism that results in wave impact pressure variability and the influence of the gas phase in particular are relatively unknown. The velocity field of the gas phase is measured with particle image velocimetry, while simultaneously the local free surface is determined with a stereo-planar laser-induced fluorescence technique. The bulk velocity between the wave crest tip and the impact wall deviates from the mass conservation estimate based on the velocity profile between the wave crest and the impact wall. This is caused by a significant increase of the local gas velocity near the wave crest tip. The non-uniformities in the seeding concentration accumulate near the wave crest tip and reduce the accuracy of the velocity measurements. However, the bulk velocity estimate is significantly improved with a fit of the velocity profile that is based on a potential flow over a bluff body. Additionally, the development of vortex is observed and quantified for two typical measurements with either a disturbance on the wave crest or a smooth wave crest. The circulation development is comparable to the formation and separation of a vortex ring, which results in a saturated vortex that separates from the wave crest (Gharib et al., 1998). Furthermore, the impact location of the wave tip is altered by the formation of secondary vortices. The secondary vortex enhances the lift locally and alters the trajectory of the wave crest tip, which may result in additional variability of the wave impact pressure.

Highlights

Stricter emission regulations result in new use cases of liquefied natural gas (LNG), which can serve as a ‘‘transition fuel’’ until greener options become economically viable
The gas velocity between the wave crest and the impact wall is determined, and the velocity profile is estimated from the valid velocity vectors
Several repeatable focused waves are generated that impact on a vertical wall

Summary

Introduction

Stricter emission regulations result in new use cases of liquefied natural gas (LNG), which can serve as a ‘‘transition fuel’’ until greener options become economically viable. This growth of the LNG market brings about an increased demand for floating liquefaction facilities, storage facilities, and shipping solutions [1]. Additional challenges arise with the widespread use of LNG, such as the requirement for lower filling levels and increased containment capacity [2]. The filling levels in LNG containment systems are currently restricted to limit the occurrence of sloshing. A fundamental understanding of wave impacts on walls is required before studying increasingly complex phenomena, such as LNG sloshing in containment systems [6]

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