URANS To Predict Ship Roll Damping Using Forced Roll And Free Decay Oscillations

An estimation of ship roll behavior and ship roll damping coefficients of S175container ship,by performing Unsteady RANS simulations on a general purpos e commercial CFD codeFLUENT, is carried out. The first part of the simulations consist of forced roll tests similar tothat performed in model tests by specifying sinusoidal roll amplitude. The second pa rt of thesimulations consists of ship roll free decay tests just like conducted in mode l tests by releasingthe mode l from an initial roll amplitude and then observing its roll behavior in six degree offreedom motion, which occurs at damped natural frequency of the ship.Multiphase technique VOF has been used to capture the free surface effect, Simple and PISOfor pressure-velocity coupling, dynamic mesh for roll motion, while SST k-ω model to capturethe turbulence.For forced roll simulated tests; bilge Keel effects are considered in the analysis, by performingsimulations with bare hull and then with the appended hull having bilge keel. Simulations arecompleted for a range of roll amplitudes from about6to31degrees, hence taking intoconsideration the large roll amplitude scenario. Frequency variation is also carried out bydoing forced roll tests on two different forcing frequencies.For ship roll free decay simulated tests; initial amplitude effect has been considered on the rollbehavior by releasing the model from two different roll amplitudes. Radius of gyration effecthas also taken into account by carrying o ut simulations on two different values. Finally centerof gravity effect has also be en take n in account by placing it on, abo ve and be low the freesurface and some interesting facts have come into knowledge.Numerical verification is do ne through Grid-indepe ndence and time-step-indepe nde ncestudies. Finally, the results validation is done by comparing numerically obtained rolldamping coefficients with the experimental ones. The results at lower frequency are moreaccurate than those at higher frequencies while amplitude variation has little effect on CFDresults accuracy in a relative sense. The free decay simulations have captured damping withgood accuracy while natural frequency of the ship roll has also been predicted satisfactorily.Vortex shedding has been captured through simulations for ship roll motion visualization.From this work, it is derived that Unsteady RANS simulations prove to be a useful tool forship roll damping estimation and can work in tandem with mode l tests.