The growing transport of LNG in partially filled tanks raises the demand to have accurate methods to predict the fluid behaviour in these sloshing tanks and the effect of the sloshing fluid on the tanker motion. To examine the motion of the sloshing fluid, model experiments have been carried out on a scale of 1:10. Different tank filling ratios and types of motion have been tested to study the sloshing fluid behaviour for various sea states. The model experiments have been carried out to provide extensive validation material for the numerical method ComFLOW. The details of this improved Volume Of Fluid (iVOF) method are presented in the paper. The method resolves the governing equations in both fluids, one of them being compressible. The compressibility of the second phase is especially important for more violent flow conditions, when two-phase phenomena such as air entrapment and air entrainment occur frequently. Particular attention in the numerical method has been paid to the treatment of the flow variables around the interface, especially the density. The fluid is convected by means of a first-or second-order upwind scheme. The behaviour of the sloshing fluid strongly depends upon the regularity of the tank motion and the filling ratio of the tank. Video frames, wave probes and pressure transducers have been used to compare the fluid flow of simulation and experiment. Two-phase effects such as air entrapment are more common for increasing tank filling ratios and for more irregular tank motion. A realistic simulation of these effects is possible by modeling two-phase flow, especially when using a relatively fine grid and applying the less-dissipative second-order upwind scheme. Compared to the earlier paper on the numerical simulation of sloshing in LNG tanks [8], where the numerical method was validated for regular sway motion, more extensive attention is paid to the accuracy of the applied discretisation schemes in space and time. The results of different schemes are now evaluated for both regular and irregular sway and roll motion of LNG tanks.

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