Advances on numerical and experimental investigation of ship roll damping

Abstract

This paper presents recent developments towards efficient and reliable methods for roll damping estimation based on numerical simulations as well as model tests using the harmonic excited roll motion (HERM) technique. A newly designed automatic roll damping estimation procedure shows the advantage of a just-in-time post processing of experimental measurement results. Real-time analysis of the measured roll damping values permits a considerable shortening of the test times. Thus, a large number of investigations can be carried out with relatively manageable effort in order to determine the roll damping behavior of different keel configurations or at operating conditions, e.g., different sized keels or Froude numbers. In addition, HERM measurement method is applied to investigate the memory effect. For this purpose, different excitation schemes are introduced and the results are analyzed. Moreover, a study of the scale effect on the roll damping properties is conducted, in which experimental and numerical investigations are performed for two scales of a ship model. Furthermore, a method is developed that significantly reduces the effort of Reynolds average Navier-Stokes (RANS)-based simulations of roll motion. The reduction of simulation time is achieved by introducing an artificial damping. The obtained results show that the developed method is very well applicable for numerical as well as in experimental investigations. During the model tests using HERM technique, the model is free and the rudder is used to keep the straight-ahead course. The analysis of the numerical and experimental results shows that the influence of the rudder induced force and moment during HERM tests is not negligible and the contribution of the rudder must be taken into account by estimating the roll damping. Finally, a new concept is developed to investigate the parametric roll behavior of ships, which allows neglecting the consideration of the complex modelling of free surface waves in the simulations. During the RANS computations, a potential-based method is applied to compute the variation of restoring terms due the roll motion.