Analysis of U-Type Anti-Roll Tanks using URANS. Sensitivity and Validation

The roll motion of ships operating in a seaway is often limiting operations. These limits could be due to, e.g. maximum acceleration, green water, capsize risk or just comfort. Therefore additional roll damping is desired to prevent uncontrolled roll motion. Different means are available to decrease the roll motion of a ship, amongst other these include bilge keels, active fin stabilizers (either for forward or zero speed) and U-shape or free surface anti-roll tanks (ART). The amplitude and phase of the roll opposing moment resulting from the water that moves inside the ART are a function of the geometry of the tank and especially its internal damping. Due to the complex and non-linear nature of this flow, the use of Computational Fluid Dynamics (CFD) was chosen to analyse the details of the flow inside the tank and its anti-roll performance. The present paper focuses on the sensitivity and validation of the anti-roll performances of passive U-type ART using CFD. For this, the incompressible Unsteady Reynolds Averaged Navier-Stokes (URANS) code ReFRESCO was used. The sensitivity on the results for the U-tank is analysed by varying the grid resolution and the numerical time step. The two-dimensional (2D) full-scale and Froude based model-scale ReFRESCO results are compared to 2D and 3D full-scale CFD results of Delaunay (2012) and Thanyamanta and Molyneux (2012) and validated with model-scale experimental results of Field and Martin (1975) and MARIN experimental results by Gunsing et al. (2014). This paper shows the influence of the convective scheme for capturing the free-surface interface and provides recommendations for a time step and grid resolution to effectively calculate the roll damping of an ART.