Hydrodynamic Interactions Between Two Bodies

Hydrodynamic interactions appear while several underwater bodies moving near each other. In the process of moving near free surface, near seabed, both deploying and retrieving of Undersea Unmanned Vehicle near parent submerged vehicle, the interactions result in the change of mechanical environment near UUV, which should be considered for the requirement of accurate motion and position control. The unsteady hydrodynamic interactions between two bodies during six degree of freedom movement in complex environment need to be investigated from classical fluid mechanics viewpoint to understand the interaction mechanism, changing rule, influence factors and describing methods.Two series of near-wall interaction experiments are conducted in Harbin Engineering University Ship Towing Tank and HEU Horizontal Plane Circulation Water Channel, comprising basic-shape body experiments and body-tail combination experiments. Interacting forces are measured in different gap ratio (ratio of wall clearance to thickness), angle of attack, Reynolds number and surface shapes. Some special characteristics are observed in basic-shape body tests, e.g. suction-drop phenomena, lifting effect of blunt-end body, influence of Re to forces, and difference among 2D and 3D interactions, etc. The interaction rules of body-tail combination near plane wall and curve wall are obtained in body-tail combination tests.Based on potential flow theory and viscous CFD method, theoretical research is given for hydrodynamic interactions between underwater body and wall. Mathematical model and predicting methods are presented for near wall interaction. Special characteristics revealed by tests are investigated, and influence of different factors to the interaction is analyzed. While for hydrodynamic interaction related to multi-body, six degree of freedom and unsteady movements difficult to simulate in tests, unsteady interacting forces are obtained by numerical methods.In the first part of this paper, the non-viscous mathematical models of hydrodynamic interactions are presented, to study the suction phenomena in zero angle of attack, lifting effect of blunt-end body, and near wall interaction of complex-shape body.Source distribution is used to forecast the interacting forces near wall in zero angle of attack. For gap ratios larger than the critical value, numerical results agree with test data well. From 2D Rankine oval, 3D flat ovoid, 3D ovoid to body-tail combination, the critical gap ratios decrease in sequence. For flows over a 2-D Rankine oval near plane wall, the numerical precisions of two force formula, i.e. formula in Lagrangian form and Bernoulli form, are compared. A simplified algorithm was presented for lifting effect of 2D Rankine oval based on experiments, in which the viscous influence of fluid was simulated by the changing position of surface flow meeting-point on afterbody and appropriate Kutta condition. The changing rule of meeting-point versus gap ratio and angle of attack was concluded based on tests. This viewpoint is also supported by the field visualization in viscous CFD computation. The lifting effect of 3D blunt-end body was simulated by vortex distribution on mid plane and wakes, while the wake shedding assumption was proved by viscous wake display. Using source-vortex singularity model, the interacting characteristics of body-tail combination near plane and curve wall were compared.Secondly, the viscous effect in hydrodynamic interactions was investigated by viscous CFD method, where interacting mechanisms are revealed.For flows over a circular cylinder near plane wall, detailed results of interacting forces and pressure distribution etc. are giving for gap ratio ranging from large value to zero value. Emphasis is focused on the viscous blocking effect in extremely small gap ratio. The flow characteristics, special rules in hydrodynamic interactions are also analyzed. For zero gap ratio, numerical results gained by different theoretical methods are compared with test results, from which discuss the influence of viscous to the pressure distribution and streamlines near touching-point. Through the interacting computations of 2D Rankine oval, 3D flat ovoid and 3D ovoid near wall, some special characteristics observed in experiments are explained, e.g. the lifting effect of blunt-end body, suction-drop phenomena and variation of drag. The influences of Re to hydrodynamic interactions are also discussed. Some differences between three cases are compared, including the quantities of interacting forces, pressure distribution and status of circumfluence zones. Finally, unsteady hydrodynamic interaction problem is investigated by numerical computation, corresponding to the unsteady movement of UUV. Unsteady effect of hydrodynamic interactions is analyzed from numerical results.For periodical movement, i.e. heaving oscillations and pitch oscillations, in unbounded flow field and near plane wall, the nonlinear unsteady interacting forces related to the evolution and movement of wakes are calculated using unsteady theoretical method, and the typical characteristics in unsteady interactions are revealed. When body-tail combination moving near plane or curve wall in specified paths (straight or curved path), the whole changing processes of hydrodynamic interactions are showed through theoretical prediction. The influences of different factors to unsteady interacting forces and wake shapes are investigated, consisting of: different moving paths, different moving manner (translating or rotating), and different moving parameters (geometry elevation, angle of velocity direction and angle of attack). The above results can be referred for path choosing and moving control of UUV during the deploying and retrieving process.