Abstract

The present work is focused on the comparative study of two numerical methods, i.e. MPS method and VOF method, for the prediction of sloshing in LNG tank. Numerical simulations are carried out by an in house meshless solver MLParticle-SJTU based on improved Moving Particle Semi-Implicit (IMPS) method, and VOF based CFD solver naoe-FOAM-SJTU developed on the open source platform OpenFOAM. Several sloshing conditions with different kinds of tanks are applied to validate the present two numerical methods. The time histories of impact pressure and flow patterns are presented and compared to the experimental data. For the rectangular tank, sway motion with different excitation periods are taken into account to nvestigate the sloshing performance and validate the two simulation methods. For the membrane-type LNG tank, pitch motion with different excitation periods are simulated. According to the numerical results, the two methods can both predict the impact pressure compared with the experiment data. The VOF method and the MPS method show different flow patterns when encountering with breaking waves.

Introduction

Generally, sloshing is the motion of fluid with free surface in partially filled tanks and is of significant importance in the field of ship and ocean engineering. Sloshing flow is a highly nonlinear problem, which may involve complicated phenomena, such as breaking wave, highspeed impact on tank wall and overturning of free surface. Violent liquid sloshing in an oil or liquefied natural gas (LNG) ship can cause local breakage and global instability to the ship hull, and even lead to leakage of oil, and capsizing of ship (Shao et al., 2012). The influence of sloshing depends on the amplitude and excitation period of the tank motion, liquid-fill depth, liquid properties and tank geometry. When the excitation frequency is close to the highest natural frequency of the liquid, sloshing will be significantly violent, which is called resonance phenomenon. As mentioned above, it is essential to avoid the ship first natural frequency being too close to the dominant frequency of the environment condition to achieve a good motion performance.

Since sloshing can be a significant factor for the safety and stabilization of ship, many researchers have conducted a lot of corresponding research work. Early studies on sloshing are usually theoretical method based strong hypothesis (Faltinsen, 1978), where flow is irrational and geometry of tank is simple. Thus, analytical solution is invalid for sloshing in membrane-type LNG tank, especially when the tank is oscillated in resonance frequency. Traditionally, the experimental researches for sloshing problems are widely used (Akyildiz and Ünal, 2005; Bulian et al., 2014; Kim et al., 2015; Lugni et al., 2006) and experimental results can validate the numerical solutions.

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