CFD aided modelling of anti-rolling tanks towards more accurate ship dynamics

Passive anti-roll tanks (ART) have been studied extensively to optimize their various design parameters. However, not much attention has been given to the accurate estimation of their damping. Here, we have developed the motion equation of water inside ART and proposed a CFD-based methodology to find the ART damping coefficient. Since our aim is to improve the modelling of flow inside ART, our analysis focuses on ART (without ship) to remove ship related complexities. Key findings regarding the damping are: (a) It is quadratic in the velocity and the damping coefficient can be expressed using a non-dimensional number, (b) one main part of the dissipation is bend losses, computed here using steady-state turbulent CFD, and (c) a second part of the dissipation is due to turbulence near the free surface, estimated here using a simple formula. We have calculated damping coefficients for five different designs of ARTs, and validated our results through both transient CFD simulations and experiments. The effect of these designs on ship motion has been discussed. The proposed model will help in better prediction of ship dynamics as far as inaccuracies due to ART modelling are concerned.