Identification Of Nonlinear Wave Forces

Abstract

This paper is concerned with the time and frequency-domain analysis of nonlinear wave force data. In the first half of the paper, the time-domain NARMAX modelling technique is applied to a number of fluid loading data sets in order to establish if there exists a simple extension of Morison's equation which will predict forces with improved accuracy. Data from a variety of flow situations is considered, ranging from regular planar oscillatory flow in a U-tube, to unidirectional irregular waves in a wave flume, to directional seas in Christchurch Bay. The second half of the paper is concerned with the dual frequency-domain approach. A brief review is given of the basic theory of higher order Frequency Response Functions (FRFs) and the means of constructing them by the method of harmonic probing. Interpretation of the higher order FRFs discussed. A study is made of the higher order FRFs for the Morison equation and a proposed extension, the Morison-Duffing equation. Finally, the FRFs for the aforementioned NARMAX models are determined; it is shown that the fluid systems exhibit a consistent structure in their higher responses which is not predicted by Morison's equation.