Numerical Analysis Of Planing-hull Surface Shape On Resistance And Sailing Attitude

As one of the earliest hydrodynamic support vessels, the planing boat, for its characteristics such as small size, high speed, and flexible maneuverability, has been widely used in both military and civil areas including patrol, detection, debarkation, transportation, and rescue work. Therefore, it has broad application prospect. The hydrodynamic performance of the planing boat is different from conventional displacement vessels, as most of its hull will be raised above the water when sailing at high speed. And its resistance, corresponding resistance laws and sailing attitude will vary from lower static state to high speed state. While the planing hull is at high speed, only a limited portion of its bottom will contact with the water surface. The dynamic equilibrium of the planing boat is kept mainly by the hydrodynamic lift and the moment of force as provided by the planing surface. To design planing hulls with excellent drag performance, it is necessary to know the rules of the influence of planing surface shape on its drag and sailing attitude. The performance and characteristics are thus totally different from those of the conventional displacement hulls, and making the theoretical simulation much more difficult than the latter. The common used techniques and corresponding treatments for conventional vessels are simply useless, that is why, at present, many works are limited on model test, or relied on some valuable but limited empirical formulae. But these methods are very hard to be used for finding rules or making detail analysis of the effects of planing surface on the performance as mentioned.The aim of this work is to exploit, adopting CFD technique, through validation, the suitable and reliable ways of numerical simulation, for the prediction with some satisfaction the resistance and sailing attitude of planing hulls, in order to provide reference for the hull form development and analysis.The author has thus conducted the following works.1. Establishing the structured grid discretization to suit the simulation of viscous flow resulting from planing hull under high speed planing states. According to the features of the structured grids, combining the requirements for the numerical simulation of hull bottom dynamic pressure distribution of the planing hull, and the requirement of high accuracy of surface elevation capturing, exploiting the three factors like the height of the first layer grids off the hull surface, gridding of the hull surface, and the distribution coefficient of the grid nodes in the flow, and their influence on drag simulation accuracy, rate of convergence and calculation stability.2. Determining through careful numerical analysis the turbulence model that suitable to planing hull sailing and analyzing the effects of time steps on simulation accuracy and capturing of flow field. Since different turbulence models have different assumptions on Reynolds stress, and they have distinct ways of treatment, they have their unique extents in application, it is required to analyze the effects of selection from turbulence models on the simulation of planing hull hydrodynamic performance, with regard to the calculation convergence, resistance simulation accuracy, and precise capturing of physical flow features. With detailed comparisons of the effects of time step on such unsteady problem like disturbed flow with free surface by planing hull, on simulation stability, convergence rate and accuracy. Considering the characters of the great changes of the attitude at different speeds, verified the alternatives of numerical calculation with quite some model test states, the robustness of the determined alternative is assured.3. Realizing the free motion simulation for the planing hull in dynamic equilibrium state, and the accurate simulation of corresponding resistance and attitude. To realize the accurate prediction of both drag and attitude of planing hulls, based upon the numerical scheme and alternatives as derived above, applying motion field method, conducting the free motion of the planing hull according to the conditions of dynamic equilibrium. Calculating respectively with captive model, free model, and Savisky method, to predict the drag, trim and heaving for a simple prismatic form planing hull, at different speeds, and comparing the results with that from model tests, showing the advantages and weakness of different methods in simulation accuracy and convergence rates. Comparing between the results by Free Motion and Savisky methods in dealing with drag and attitude at different speeds for planing hulls with complicated planing surface, validating the calculation with the test results, investigating the reliability of CFD methods in the calculation of planing-hull’s drag and sailing attitude, and their adaptability to different hull forms. Relevant measures being adopted in the research, resulting in the reduction of negative effects in simulation convergence and free surface elevation capturing when using the Motion Field Method.4. Exploiting the rule of the influences of some planing surface parameters on planing-hull drag and sailing attitude, such as width, dead rise, longitudinal and transverse curvatures. In the process of changing hull forms, in addition to keeping the displacement and the location of the longitudinal center of gravity, other constraints are imposed, to cope with some limitations of the empirical formula. With respect to the investigation of width effect, the consideration is mainly about the effect of stern-width to midship-width ratio on planing hull drag and sailing attitude, under the conditions that the hull length, length-to-width ratio and the average widths at midship and stern are fixed. Comparing the effect of different angles of uniform dead rises in after body, and the effect of changing the ratio between the dead rise at stern and midship, while keeping their average unchanged, and analyzing the reason of significant increase in case that the planing surface being seriously twisted in form. Comparing the effects of the changing of the longitudinal curvature while the dead rise unchanged, the coupling change of dead rise and longitudinal curvature, as well the distribution of bottom pressure of different hull forms. Dividing the transverse section into portions of keel, main planning and knuckle, and analyzing the effects of their change in form parameters.5. Summarizing the influence rules of bottom steps on planing-hull drag and attitude. Comparing effect of different heights of the steps, analyzing the reason of the different effects on drag reduction due to different height of the steps. Comparing the effects of bottom step parameters like that of oblige steps, arc-shaped steps. Investigating also the effects of the matching of the longitudinal locations of the steps and that of the center of gravity, comparing coupling changes of these longitudinal locations.