System Identification Techniques for the Modelling of Irregular Wave Kinematics

This paper presents the development of a parametric model linking the free-surface elevations with the orbital velocities field under an irregular wave. The system identification procedures are applied to estimate the parameters of the model based on data recorded in the wave tank. The free-surface time series are taken as input data and the output data are components of the particle velocity vector. A linear time-invariant model with the static nonlinearities at the input side incorporated is assumed. In the study different system identification procedures are applied for two cases—firstly when orbital velocities are modelled at points that are always submerged in the water and, secondly, in the surface zone where measurement points are not continuously submerged. This paper demonstrates the results of modelling the horizontal component of the orbital velocity in comparison with wave kinematics data taken in a wave flume using Laser Doppler Velocimetry specifically designed to give accurate measurements also in the vicinity of the mean water level. Comparison between the modelled and observed velocity time series presents a sufficiently good agreement to prove the effectiveness of the applied approach. The SI techniques applied to determine from the free-surface records the orbital velocities as well as other hydrodynamic parameters such as particle acceleration and forces may be very important and useful in engineering and oceanography.