Water entry and exit of 2D and axisymmetric bodies

TL;DR

This paper develops a numerical potential flow model to accurately simulate water entry and exit of 2D and axisymmetric bodies, improving predictions of hydrodynamic loads and wetted areas during impact.

Contribution

It introduces a fully non-linear boundary element model with enhanced stability for water impact analysis, including the often-overlooked effects of gravity.

Findings

Model accurately predicts free-surface shape and pressure distribution.

Better estimates of hydrodynamic loads during entry and exit phases.

Gravity effects are significant, especially during exit.

Abstract

The present paper is dedicated to the development of a numerical model for the water impact of two-dimensional (2D) and axisymmetric bodies with imposed motion. The work is a first step towards the implementation of a 2D+t procedure to be used for the analysis of aircraft ditching. The problem is investigated under the assumptions of an inviscid and incompressible fluid, which is modelled by a potential flow model with fully non-linear boundary conditions at the free-surface. The unsteady boundary value problem with a free-surface is numerically solved through a boundary element method, coupled to a simplified finite element method to describe the thinnest part of the jet. The study is aimed at describing the entry and exit phases. Specific numerical solutions are developed to tackle the exit phase and to improve the stability of the model. Results are presented in terms of free-surface shape, pressure distribution and hydrodynamic load acting on the impacting body. The model is used to study the water entry and exit of a 2D wedge and an axisymmetric cone, for which numerical or experimental results are available in the literature. The numerical investigation shows that the proposed model accurately simulate both the entry and exit phases. For the exit phase, it is shown that the proposed model, being fully non-linear, provides a much better prediction of the loads and the wetted area compared to simplified (analytical) approaches. The effects of the gravity, usually missing in the approaches available in the literature, are also investigated, showing they are rather important, especially, in the exit phase.