ORIGINAL ARTICLE Numerical study of free surface flow in a 3-dimensional FLNG tank under coupled rotational–heave excitations Yan Yan 1,2 • John M. Pfotenhauer 2 • Franklin Miller 2 • Zhonghua Ni 1 Received: 18 May 2016 / Accepted: 27 June 2017 / Published online: 24 August 2017 Ó JASNAOE 2017 Abstract In this paper, free surface flow in a full-sized 3D FLNG tank is numerically studied under coupled rota- tional–heave excitations. The numerical model uses the standard k–e turbulence model and the volume of fluid method to describe fluid flow and track free surface. The emphasis of this study is making use of a full-sized tank with coupled excitations. A mesh independence study and comparison with other experiment and numerical simula- tion are implemented to verify the computational model. By parametrically investigating the influence of the initial phase difference, heave frequency and filling ratio, it is found that an initial phase difference of 0.5p and 1p can result in a higher local pressure near the tank corner and 0.5p will lead to a bigger amplitude of surface sloshing. A heave frequency of 2 times the natural frequency makes the surface sloshing flow most violent, but a heave frequency larger than that will turn the sloshing pattern into an up- and-down oscillation. A low filling ratio is more sensitive to both single rotational excitation and coupled excitations. However, a high filling ratio is relatively stable under rotation alone, but becomes much more violent from an induced heave excitation. Keywords Coupling excitations  Volume of fluid method  Free surface flow  FLNG tank 1 Introduction In the past couple of years, due to the rapid expansion of the liquefied natural gas (LNG) market, the LNG regasifi- cation capacity has more than doubled and the number of LNG importing countries has tripled since 2000 [1]. To reduce the investment and building process, floating liq- uefied natural gas carrier (FLNG) is introduced, replacing land liquefaction factories, to liquefy and store natural gas. As LNG will be stored in FLNG for couple of months before LNG carriers come to transfer it to distant receiving terminals, it is necessary to keep the liquid stable inside the cargo containment system (CCS). This stability is threat- ened by ocean current, wave, wind and many other factors such as tank geometry, filling ratio, excitation frequency, etc. Considering the large capacity of newly designed vessels (generally more than 100,000 m 3 ), large-amplitude liquid sloshing can result in violent oscillations or damage to the tank shell, especially when the external excitation is close to the natural frequency. Therefore, many studies have been performed to char- acterize the fluid behavior of liquid sloshing in offshore vessels. Experiments and numerical simulations are two tools in this research area, with different advantages and disadvantages. Experiments can provide reliable measure- ments of tank shell stress using testing sensors installed on the tank walls. Experiments can also record the free surface motion of the LNG; information that can be utilized as reference data to validate numerical simulation results. However, experiments are constrained to the scaled struc- ture, rather than full-sized geometry. In general, a gentle flow can be easily filmed with a high-speed camera, but if the flow becomes violent or if the geometry is full-scale, numerical simulations provide a better option for predict- ing the surface motion. These numerical methods can be & Zhonghua Ni nzh2003@seu.edu.cn 1 School of Mechanical and Engineering, Southeast University, Nanjing 21000, China 2 Department of Mechanical Engineering, University of Wisconsin Madison, Madison 53706, USA 123 J Mar Sci Technol (2018) 23:333–348 DOI 10.1007/s00773-017-0467-5