Vibration Analysis of Underwater Towed System

Abstract

The characteristics of vibration of underwater towed system, consisting of a faired cable and a fish having round bell-like configuration and connected with the cable through gimbals, are discussed both by eigenvalue analysis based on the linearized equations of the coupled motion of cable and fish of six degrees of freedom and by frequency response analysis of experimentally measured data of sea trials using auto regressive model.The accuracy of the numerical analysis is checked by comparing with analysed results of natural periods for longitudinal and lateral motion of towed cable and neutrally stable fish in air.The damping ratio of experimentally measured pitch and roll motion of fish is estimated by curve-fitting the transfer function of a linear mechanical system of one degree of freedom with the transfer function of AR model.The computed significant damping ratios and those corresponding undamped natural frequencies of fish motion are shown to be in fairly good agreement with the experimental results. The computed results show that(1) the coupling effect between cable motion and fish motion in the longitudinal direction is dominant, while the fish motion in the lateral direction is almost subject to the characteristics of fish alone.(2) the increase of tow speed less than 6 knots decreases the damping ratio and it increases the undamped natural frequency.(3) the characteristics of fish vibration are hardly influenced by the cable length less than 150 m and(4) as the distance between the origin of fish and vertical location of the center of gravity increases, the natural frequency of the pitch motion increases.