Numerical prediction of roll damping of ships with and without bilge keel

Estimation of roll damping of ships from free roll decay experiments or using appropriate semi-empirical formulae is a common practice. The total roll damping of a ship or a barge can be strongly nonlinear consisting of linear and nonlinear components; especially when bilge keel is fitted to the hull as a damping device and also when angles of roll are large. CFD simulations of nonlinear roll motion with experimental validation is gaining acceptance in the recent past. The present work reports three dimensional simulations of roll decay of a ship model, both with and without bilge keel, with experimental validation. A good comparisons of roll damping, natural period and added mass with experiments are obtained. The use of large eddy simulation on this class of problems is explored and commented upon. Ikeda's semi-empirical method over-predicts roll damping at larger roll angles. The pressure aft of bilge keel is found to be deviating whereas the pressure in front of bilge keel showed a close resemblance with semi-empirical calculations. Effect of bilge keel thickness, degrees of freedom and scaling on roll motion and damping is found to be insignificant for a barge-like ship model chosen in this study.