ABSTRACT To predict the kinematics of an irregular wave train accurately, it is essential to understand the effects of wave modulation on the elevation and kinematics. The understanding of the wave modulation leads to a more realistic and hence more rational wave model using hybrid wave-mode functions. In a hybrid model for an irregular wave train, long-wavelength waves are described by conventional wave-mode functions and shortwavelength waves by modulated wave-mode functions which describe the modulation of the short-wavelength waves directly and explicitly. A simple hybrid wave model is used to decompose a dual component wave and compute the kinematics under its crests and troughs. The satisfactory comparison between the numerical and experimental results indicates that a hybrid wave model can be extended to simulate an irregular wave train and lead to a more accurate prediction of the wave kinematics.
INTRODUCTION Impacts of storm waves on offshore structures can have major influences on design and safety, especially for those in the open sea where large waves with wavelengths in the hundreds of meters and waveheights in the tens of meters have been well documented Furthermore, the costs of deep-sea structures are very sensitive to their structural design, steel weight, and foundation requirements. To design offshore structures safely and to avoid structure over-design, precise knowledge of the flow kinematics induced by ocean waves is desirable and essential. Although rapid progress in nonlinear wave dynamics has been made in the last two decades, there is still no generally acceptable method of proven accuracy for the prediction of irregular wave kinematics. The linear wave theory based on Fast Fourier Transform (FFT) algorithm is found to overpredict the wave kinematics. Consequently, many modifications were made to the linear wave theory based on the FFT, such as various stretching techniques (for references see Rodenbusch and Forristall 1986).