Sloshing Tank: UDF

Sloshing Tank, ANSYS Fluent CFD Simulation Training

Description

Experimental and numerical studies consistently demonstrate the importance of fluid sloshing within a tank on the maneuverability of floating vessels such as ships and boats. Several key laboratory studies inform this area of research, including measurements of sloshing dynamics within a tank, experimental-statistical analysis of sloshing wave impact loads on shell tank models, verification of numerical sloshing results against experimental data from scaled reservoir sections at different filling heights, investigation of long-term sloshing pressures accounting for impact-induced vibration, and experimental studies of pressure distribution from liquid sloshing in rectangular tanks.

The computational domain represents a tank containing LNG fuel and air, incorporating several rows of internal joints and walls designed to restrict fluid movement. This reduces friction between fluid layers, since the inertia of fluid moving within the tank can otherwise affect the stability of the fuel carrier vehicle.

The 2D tank geometry was modeled in Design Modeler, measuring 1 m in length and 0.7 m in width. Six rows of internal dividers, each 0.35 m high and 0.04 m thick, separate the fluid layers, with the geometry segmented to support a structured mesh. Since ANSYS Fluent relies on the finite volume method, mesh quality is critical; a structured mesh was accordingly generated for the tank using ANSYS Meshing.

Methodology

The simulation was set up using a pressure-based solver with absolute velocity formulation, run as a transient case with a 0.005 s time step. Gravity was enabled at -9.81 m/s² in the Y-direction, while the energy equation was disabled.

Tank motion was defined through a UDF applying an oscillating zone velocity along the X-direction following a sinusoidal function, with the rotation axis fixed along Z. All walls were assigned a no-slip condition, and the reference pressure point was set at X = 0.00 m, Y = 0.25 m.

The VOF multiphase model was used, with air as the primary phase and water as the secondary phase, applying implicit formulation, sharp interface modeling, and implicit body force, alongside an enabled level-set method for improved interface tracking; open channel flow was disabled. A surface tension coefficient of 0.0725 N/m was defined for the air-water interaction.

Solution methods included SIMPLE for pressure-velocity coupling, PRESTO! for pressure interpolation, and QUICK schemes for both momentum and level-set discretization, with a compressive scheme applied to VOF implementation. The domain was initialized uniformly at zero, followed by a patch applied over the region X: -10 m to +10 m, Y: 0 to 0.25 m, setting static pressure as ρ_w·g·(1 − y/H_w) and a water volume fraction of 1.0. Static pressure was monitored throughout the simulation at a point located at Y = 0.0525 m.

Conclusion

The simulation captures the oscillatory sloshing behavior of fluid within the tank as it responds to the applied motion, tracking the resulting pressure fluctuations at the monitored point over time. These results characterize how internal baffles and fluid inertia influence sloshing loads — information directly relevant to assessing structural and stability impacts on the fuel carrier vessel under realistic operating motion.