Irregular Wave Force On Bodies With Impulse Function Method

A method of time domain analysis in linear system with linear wave theory was used to calculate the velocity, acceleration and wave forces caused by vogue waves. Through inverse Fourier Transform, an analytical expression of the impulse response function was presented. The velocity of vogue waves was calculated by the convolution of wave history and the impulse response function, for the case where only wave history of vogue waves was known. By a comparison with superposition method, the proposed approach has been valid.For a small-scale cylinder, the wave forces were calculated by Morison formula. In the condition of weakly nonlinearity, results of spectrum analysis show that the wave forces spectrums of the two methods are close to each other. For the velocity forces, with the increase of nonlinearity, energy from impulse response method experienced an increase in the low and high frequency region of spectrums and the peak values continue to increase. Meanwhile, the spectral width of the main energy region experienced a growth as well. For the inertial forces, the spectrums are identical with the two approaches.For an arbitrary 3-D body, based on potential flow theory, by introducing the Rankine source as the Green function, a boundary integral equation for the velocity potential of an arbitrary point in the wave field was obtained in the time-domain. With the use of perturbation expansion theory, expanding the velocity potential, wave elevation, displacement and pressure to first-order, a linear boundary value problem was set up. To ensure there were no reflective waves in the domain, a damping layer was arranged at the boundaries. A higher order boundary element method was utilized to reduce the computation load. Besides the velocity potential and the instantaneous wave elevation, motion equations for objects were updated with fourth-order Runge-Kutta method. Based on the theory above, hydrodynamic analysis of a truncated cylinder was calculated, including wave forces and displacements. The velocity potential, be it regular wave or vogue wave, was obtained through impulse response function method. Comparison between Isaacson & Cheung and present showed the method in this paper has been valid. At last, the hydrodynamic characteristics of a Spar action with vogue wave where only the wave elevation was known were investigated...