A Numerical Study On Nonlinear Hydrodynamic Characteristics Of Cable-towed Acoustic Array

The cable-towed acoustic arrays are widely adopted in science, industry and defense, and it’s of fundamental importance to investigate the hydrodynamic characteristics for the design and operation of the high-technology-intensive systems. Two kinds of cable-towed acoustic array(single-line array and multi-branches array) are studied numerically in this dissertation. The main work is listed as:(1)The cable-towed acoustic arrays are outlined firstly. The major achievements of marine cable dynamics are reviewed, and the existing popular modeling approaches are summarized and discussed.(2)The 3-D cable dynamics governing equations are derived in Lagrangian reference frame based on micro element method. The partial differential governing equations are formulated to(a) take the nonlinearities of geometries, materials, boundaries, and loadings into consideration,(b) model bending-stiffness and torsional stiffness. The frame transitions in derivation are achieved by Euler-angles and Euler parameters separately.(3)Matlab codes are written to solve the derived governing equations. A computation procedure of relaxation method is written after the treatment of the governing equations by finite difference box scheme.(4)The hydrodynamic response of a single-line towed array are analyzed with the written Matlab codes. Dynamic response of a single-line towed array during steady straight-tow and 360° loop maneuvers are computed. The effects of tow speed, current, and loop radius are intensively examined via numerical simulation. Two new concepts, close-loop effect and close-loop radius, are proposed to anlyze the transient behavior of towed array during loop maneuvers.(5)Dynamics of a multi-branches towed array is studied numerically. A model of symmetric multi-branches towed array is built by Orcaflex and the sensitivity analysis on tow speed, wave height during steady straight-tow maneuvers are performed.Several remarkable conclusions are summarized as:(1)In finite difference method of cable dynamics modeling, the introductions of the Euler parameter formulation and the nonlinear stress-stain relation enhance the generality of the cable dynamics model.(2)The multi-branches towed array has a similar dynamic performance compared to that of the single-line array during straight-tow maneuvers. As the wave height increases, the spacing between arrays will be decreased.(3)The depth-time relationship curve shows a figure-V variation when the tow speed decreases to a specific value. This figure-V variation trend will recede as the ship’s loop radius increases. The close-loop radius of the towed array is proportional to ship’s turning radius.(4)Oscillations will be generated for tension and depth in time history under current. The oscillation frequencies of depth and tension are both inversely proportional to the value of turning radius. As the turning radius decreases, the oscillation amplitude of tension is also decreased, while the oscillation amplitude of depth shows an increasement.(5)The close-loop radius of the towed array will decreased due to the introduction of ocean current. The extent of the deduction is dependent on the value of the current. The phases of tension and depth depend on the current direction which delays or advances the time when the close-loop effect is generated.