Investigation on nonlinear rolling dynamics of amphibious vehicle under wind and wave load

Nonlinear amphibious vehicle rolling under regular waves and wind load is analyzed by a single degree of freedom system.Considering nonlinear damping and restoring moments,a nonlinear rolling dynamical equation of amphibious vehicle is established.The Hamiltonian function of the nonlinear rolling dynamical equation of amphibious vehicle indicate when subjected to joint action of periodic wave excitation and crosswind,the nonlinear rolling system degenerates into being asymmetric.The threshold value of excited moment of wave and wind is analyzed by the Melnikov method.Finally,the nonlinear rolling motion response and phase portrait were simulated by four order Runge-Kutta method at different excited moment parameters.

Estimation of Nonlinear Roll Damping by Analytical Approximation of Experimental Free-Decay Amplitudes

Damping is critical for the roll motion response of a ship in waves. A common method for the assessment of damping in a ship’s rolling motion is to perform a free-decay experiment in calm water. In this paper, we propose an approach for estimating nonlinear damping that involves a linear exponential analytical approximation of the experimental roll free-decay amplitudes, fol- lowed by parametric identification based on the asymptotic method. The restoring moment can be strongly nonlinear. To validate this method, we first analyzed numerically simulated roll free-decay data using rolling equations with two alternative parametric forms: linear-plus-quadratic and linear-plus-cubic damping. By doing so, we obtained accurate estimates of nonlinear damping coefficients, even for large initial roll amplitudes. Then, we applied the proposed method to real free-decay data obtained from a scale model of a bulk barrier, and found the simulated results to be in good agreement with the experimental data. Using only free-decay peak data, the proposed method can be used to estimate nonlinear roll-damping coefficients for conditions with a strongly nonlinear restoring moment and large initial roll amplitudes.

Nonlinear roll damping of a barge with and without liquid cargo in spherical tanks

Damping plays a significant role on the maximum amplitude of a vessel’s roll motion,in particular near the resonant frequency.It is a common practice to predict roll damping using a linear radiation-diffraction code and add that to a linearized viscous damping component,which can be obtained through empirical,semi-empirical equations or free decay tests in calm water.However,it is evident that the viscous roll damping is nonlinear with roll velocity and amplitude.Nonlinear liquid cargo motions inside cargo tanks also contribute to roll damping,which when ignored impedes the accurate prediction of maximum roll motions.In this study,a series of free decay model tests is conducted on a barge-like vessel with two spherical tanks,which allows a better understanding of the nonlinear roll damping components considering the effects of the liquid cargo motion.To examine the effects of the cargo motion on the damping levels,a nonlinear model is adopted to calculate the damping coefficients.The liquid cargo motion is observed to affect both the linear and the quadratic components of the roll damping.The flow memory effect on the roll damping is also studied.The nonlinear damping coefficients of the vessel with liquid cargo motions in spherical tanks are obtained,which are expected to contribute in configurations involving spherical tanks.