Effect of Forward Speed on Roll Damping of a Container Ship Using URANS Simulations

Abstract

Ships are prone to large roll motions in beam and oblique seas at encounter frequencies near the design frequency of the vessel. The nonlinearity of roll damping and roll motion has been investigated in the past by various researchers. The prediction of roll damping and roll motion of ships becomes difficult by simplified approaches. Ship roll motion is highly influenced by viscous flow around the hull. Vortex formation and its shedding from the hull and appendages have larger contribution to roll damping. Therefore, the popular approach for prediction of roll damping and roll motion of ships is with the help of model experiments and more recently URANS (Unsteady Reynolds-averaged Navier–Stokes)-based simulations. Free roll decay experiments in calm water conditions give a good estimate of roll damping of ships at natural frequency. The flow characteristics around the hull may not be the same when the ship is moving at a forward speed. Hence, it is important to take into consideration the effect of forward speed on roll damping obtained from free roll decay of the ships. This paper addresses the effect of forward speed on roll damping of a 1:100 (Froude) scaled container ship model. The model is 2.88 m in length, 0.345 m in beam and was loaded to a draft of 0.12 m; free roll decay experiments at zero forward speed were carried out in a wave flume at Department of Ocean Engineering, IIT Madras. The wave flume is 4 m wide, 90 m long and has water depth of 2.5 m. The held-over free roll decay tests were carried out by subjecting the model to known initial heel and releasing it. The roll angle was measured using inclinometer via data acquisition CPU. The URANS-based simulations of the free roll decay experiments at zero forward speed were carried out in a commercial computational fluid dynamics (CFD) software and validated. The CFD model was then used to carry out the free roll decay simulations at two forward speeds of the ship model. The effect of forward speed on roll damping of the ship model was assessed from the results of CFD simulations.