Grid resolution for the simulation of sloshing using CFD

Sloshing occurs when a tank is partially filled with a liq-uid and subjected to an external excitation force [1]. Ships with large ballast tanks and liquid bulk cargo carriers, such as very large crude carriers (VLCCs), are at risk of expo

A verification and validation study of the application of computational fluid dynamics to the modelling of lateral sloshing

An understanding of liquid sloshing is of primary concern to the design and operation of Liquefied Natural Gas (LNG) carriers. Safe operation of LNG carriers requires the knowledge of global and local pressures imposed by the sloshing liquid. The most general method available to quantify such sloshing loads is the solution of the Navier Stokes system of equations using Computational Fluid Dynamics (CFD). Given the wide variety of modelling options available, as yet there is no consensus on the best modelling practice for such sloshing flows.This report seeks to address this issue, examining various models and identifying the most suitable combination. The work uses the commercial CFD code ANSYS superscript TM CFX-10.0 superscript TM but most of the findings are also relevant for similar other commercial codes. The physics of the sloshing problem are considered in order to identify the key modelling aspects. The correct application of CFD and how it can be used to model sloshing is considered. A suitable experimental dataset is described for use as a validation test case. The sloshing problem simulated is in a 1.2 m long and 0.6 m high tank with a 60 % filling level; excited at 95% of the first natural frequency with a maximum displacement of 1.25 % of the tank length.A space and time discretisation independence study is carried out to ascertain the applicability of the results. Subsequently, the effect of including either a k ? ? or Reynolds stress turbulence model as opposed to forcing laminar flow is examined. The choice of fluid (water and air) compressibility is investigated to determine its effects on model accuracy as well as the associated computational cost. Results are compared to experimental data and a computational reference case.It is found that a grid of 6000-7000 elements with an initial node wall offset of 1 mm is sufficient to achieve effective grid independence for sloshing in. The necessary time discretisation scheme was determined to be second order with a dynamic timestep adaptation scheme controlled by a root mean square Courant Number of 0.2. The flow regime should be considered as turbulent and the standard k ? ? turbulence model is suitable. Finally it is observed that a compressible-incompressible model combination for air and water respectively gives a near identical result to a fully compressible model with a 20% reduction in computational time

A simplified pump tower approach for realistic CFD simulation of sloshing in LNG tanks

The complexity of the sloshing analysis for liquefied natural gas carriers can be reduced by neglecting the pump tower. The validity of this assumption is examined by studying the effect of the pump tower, located near the aft bulkhead of a typical LNG tank on the sloshing flow evolution. Results are compared for surge-induced sloshing in a rectangular tank with pump tower to that without such an obstruction. A commercial flow solver is used to solve the unsteady Reynolds Averaged Navier Stokes equations for an inhomogeneous multiphase flow. Initial validation of the sloshing flow uses the experimental data of Hinatsu. It was found that a simplified pump tower consisting of a single vertical tube was suitable to capture the effect of the pump tower without the necessity of discretising the fine geometric detail of the pump tower structure. A suitable size for the simplified tower diameter was found using the total fluid force on a real pump tower in a steady flow for a similar range of Reynolds Number. This reduces the required mesh size by an order of magnitude. Although it is found that the effect of the pump tower on the overall force levels is small reductions of local impact pressures of up to 50% are observed and the sloshing flow develops a phase lag compared to the unobstructed tank

The effect of an internal pump tower on fluid sloshing in a rectangular container

The complexity of the sloshing analysis for liquefied natural gas carriers is usually reduced by neglecting the pump tower. This study examines the effect of the pump tower, located near the aft bulkhead of an LNG tank, on the sloshing flow evolution by comparing the results of surge-induced sloshing in a rectangular tank with pump tower to a clean 3D and 2D tank using an inhomogeneous multiphase CFD model implemented in the commercial CFD code ANSYS CFX-11. The results are validated against experimental data. A simplified pump tower consisting of a single vertical tube is developed using the total fluid force on a real pump tower, which reduces the required mesh size by an order of magnitude. The simplified pump tower is included in the sloshing simulation. It is found that the effect of the pump tower on the sloshing flow is small. Reductions of impact pressures by up to 50% are observed and the sloshing flow with the simplified pump tower develops a lag compared to the clean tank. The inclusion of a simplified pump tower in the CFD simulation gives a force history similar to the Morison equation with the flow field from the clean tank