Identification of Nonlinear Wave Forces. Part II. Frequency-Domain Analysis

In part 1 of this study, concerned with time-domain results, a number of NARMAX time-series models were obtained for a range of situations of wave loading on slender cylinders. The dual frequency-domain approach is pursued here. First, a review is given of the basic theory of the Volterra-expansion for nonlinear systems. The associated higher order Frequency Response Functions (FRF's)are presented. The means of constructing them by the method of harmonic probing is outlined and their interpretation is discussed. A study is made of the higher order FRF's for the Morison equation and a proposed extension, the Morison-Duffing equation.Finally, the FRF's for the aforementioned NARMAX models are determined; it is shown that the fluid systems exhibit a consistent structure in their higher order responses which is not predicted by Morison's equation.