Experimental and numerical analysis of resonant sloshing flow in a membrane tank of different filling levels

Abstract

In an effort to better understand the sloshing behaviour in LNG tanks, experimental and numerical modelling of a prismatic tank have been carried out for different filling levels. An analytical approach is first applied to identify the eigen-frequencies at the intermediate filling levels from 25% to 70%, where resonant sloshing flows are likely to occur. Model tests are carried out where pure sway excitation is applied on a 1:70 scale prismatic tank, for the two filling levels at the respective eigen-frequencies for filling levels 35% and 70%. From the experiments, it is observed that swirling flows can be induced even when the tanks are initialised to a horizontal position and subjected to sway excitation where both the initial horizontal levelling of the tank, as well as the pure sway excitations are ensured. This phenomenon is also investigated numerically using a variable acceleration solver based on the fixed mesh in OpenFOAM for five filling levels from 25% to 70%. Good agreement is also observed between the experiments and the computed results. The swirling phenomena are captured in the OpenFOAM simulations. The ratio between Fx and Fy can be used as an indication of swirling flows, where we note that swirling flows are observed for the cases with Fx/Fy around 0.20, for the filling ratios above 25%. There is a clear influence of the excitation amplitudes and filling ratio on the occurrence of swirling flows. Additional load cases to investigate the sensitivity of the tank sloshing to different inclination angles are also carried out. From this study, we observe that off-axis forcing could be induced on LNGCs even when the excitations are purely translational.