| Underwater towed vehicle is an effective and widely used mobile ocean observation platform.The hydrodynamic and attitude control of underwater towed system,which shows strong nonlinearity,coupling and randomness,has always been the focus and difficulty of underwater towed system research.In this thesis,a new underwater towing cable dynamic model,which takes into account the nonlinear and random transient flow effects,is proposed.Based on this cable model,a new numerical model of underwater towed system is constructed,in which the underwater towed vehicle,towing cable,towing ship,and position and attitude control mechanism of the underwater towed system are placed in a same theoretical framework for coupling solution and multi-dimensional motion simulation.A new numerical algorithm coupling the new numerical model of underwater towed system and control factors is proposed.On the basis of the research on some basic scientific problems,such as the dynamic characteristics of the underwater towed system under non-autonomous or autonomous position and attitude control operations,a new prototype of a multi-degree-of-freedom controllable underwater towed vehicle with novel configuration and excellent performance is developed.The relevant theories were verified by numerical simulation comparison,towing tests,and sea tests.Specifically,the main contents of this thesis are listed as follows:(ⅰ)The prototype design of an underwater towed vehicle is being studied.Based on the analysis of previous research results on the control mode,cavity linetype,structural form and main materials of underwater towed vehicles,five structural layout design schemes of controllable underwater towed vehicles with independent stability capability,depth determination,and depth control capability,which have own intellectual property rights and meet the requirements of project R&D technical indicators,are proposed.The reliability and depth control ability is evaluated quantitatively and comprehensively,and the optimal scheme of underwater towed vehicle prototype is obtained.(ⅱ)A new numerical method of underwater towing cable is proposed,a new numerical model of underwater towed system,and a numerical algorithm coupling the hydrodynamics of underwater towng system and the position and attitude control of underwater towed vehicle are constructed to analyze and evaluate the dynamic response of the optimal underwater towed system under different working conditions.The numerical model of underwater towed system combines the solution of underwater towing cable with Computational Fluid Dynamics(CFD)method.The flow field around the underwater towed system,the hydrodynamic characteristics of the towing ship and the underwater towed vehicle are solved by CFD method;the underwater towing cable is discretized into finite flexible microelements with uniform mass distribution and connected by rigid nodes;the hydrodynamic forces on the underwater towing cable are defined on the rigid nodes,which are equal to one-half of the sums of the hydrodynamic forces on the two microelements connected to the rigid nodes.The hydrodynamic forces on the micro element are determined by Morison formula,and the flow field velocities are determined by the local instantaneous flow velocity at the rigid nodes obtained by CFD method.In the numerical model of the underwater towed system,except for the influence of the underwater towing cable on the flow field,the direct or indirect interface between the other components of the system is considered.The correctness and validity of the numerical model of underwater towed system proposed in this thesis are verified by literature verification,experimental verification,and other methods.(ⅲ)Research on the hydrodynamic characteristics of the non-autonomous motion control mechanism of different control strategies of the underwater towed vehicle is conducted.Firstly,the basic motion mechanism and law of the underwater towed system under freely towing operations are discussed,and the mechanical law between the towing speed and the motion of the underwater towed system is preliminarily summarized.Then,the numerical algorithm proposed in this thesis is used to analyze the basic motion control mechanism of the underwater towed system under five different position and attitude control mechanisms,i.e.,the yawcontrol tail,the yaw-control ducted propeller,the shifting weight,the induced heave-control hydrofoil,and the underwater towing cable.The hydrodynamic characteristics of the overall system under the control operation of the above five control mechanisms are preliminarily summarized,and the preliminary rules between the control operation and the underwater towed vehicle movement are preliminarily verified.The feasibility of the optimal scheme of underwater towed vehicles is numerically verified.(ⅳ)The hydrodynamic characteristics of the underwater towed system in the flow field of simple flows and ship propeller wakes under different autonomous depth control operations employing three different depth control strategies are discussed.The two-dimensional motions of the underwater towed system in the vertical plane in the simple flow conditions and the threedimensional motions of the underwater towed system in the wake of the ship propeller are considered,respectively.On the basis of preliminary discussion on the basic mechanism of three different depth control strategies,namely,the center of gravity adjustment based depth control,induced heave-control-hydrofoil based depth control,and cable retraction and release based depth control,are discussed employing the proposed numerical algorithm.A new numerical model of underwater towed system and a numerical algorithm coupling the hydrodynamics of underwater towng system and the position and attitude control of underwater towed vehicle are proposed.The basic scientific problem of hydrodynamic characteristics of the underwater towed system under autonomous depth control operation is investigated,the feasibility of autonomous depth control employing the above three control strategies is discussed,and the position and attitude stability of the optimal scheme of the underwater towed vehicle and the robustness of the above three control strategies are demonstrated,the independent stability and handling performance of the optimal scheme of the underwater towed vehicle under different working conditions are numerically verified.(ⅴ)On the basis of the relevant numerical verification of the underwater towed vehicle of the optimal scheme,the autonomous depth control strategy of the underwater towed vehicle is proposed,research on the prototype of the underwater towed vehicle,equipment selection,towing tests and sea tests are conducted to verify the performance of the prototype of the underwater towed vehicle and the feasibility of the numerical algorithm proposed in this thesis.The parameters of the underwater towed vehicle obtained from the previous numerical simulations under different working conditions,including the maximum tension at both ends of the underwater towing cable,the maximum hydrodynamic torque of the induced heave-control hydrofoil,etc.,are made as the references for the principle prototype development,equipment selection,etc.After the manufacture of the prototype of the underwater towed finished,the towing test of the prototype was carried out in a towing tank to further verify the correctness and effectiveness of the numerical model of underwater towed system proposed in this thesis.The sea tests of depth control operations of the prototype of the underwater towed vehicle principle are conducted.The research results show that the prototype of the underwater towed vehicle has excellent performance in towing stability and depth control accuracy under real sea conditions,and the proposed numerical algorithm coupling of hydrodynamic forces and control factors of underwater towed system is effective.This thesis can provide a useful reference for the development and application of new-type underwater towed system,and the proposed numerical method can also provide a reliable research and analysis tool for the study of the hydrodynamic and control characteristics of underwater towed system. |
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