Research On The Parametric Design Method For Complex Ship Hull Form

As optimization technique such as Simulation Based Design(SBD)and Multidisciplinary Design Optimization(MDO)become more and more widely used in the field of ship design,moreover,the parametric model of ship hull surface,which provides model foundation for the entire process of ship optimization design,plays an important role in the practical application of optimization technology.So how to establish an effective ship hull parametric model is the key issue needed to be resolved.The hull surface modeling can typically be divided into three different types: conventional CAD modeling,partial parametric modeling and full parametric modeling.Where conventional CAD modeling method is based on discrete offsets data of the hull,and then uses the so-called "Point-Curve-Surface" modeling method to model the hull surface.Partial parametric modeling method,which is based on the surface model provided by conventional CAD modeling method,applies parametric transformation to the surface model,which means the applicability of partial parametric modeling method basically rely on the base model.Full parametric modeling method,which is completely defined by the parameters of hull surface,is free of traditional offsets,i.e.the hull surface is uniquely defined by a set of form parameters.Since full parametric modeling method adopts the high level descriptor—form parameter,it is currently the most advanced modeling method of hull surface.Although SBD and MDO have gradually applied in ship design,but because there still exist great difficulties in parametric modeling of ship hull,especially for complex ship hull form with bow and stern feature that contain large curvature variation,which greatly limits the application and promotion of the ship optimization design.The thesis,consequently,make an intensive study of parametric design of complex ship hull surface including accurate reconstruction method of complex ship hull surface,full parametric modeling method of complex ship hull form,construction and analysis of fairness invariant and fairness functional.The main contents include the following aspects:1.Improvement of hull surface reconstruction methodCurrently most designers use traditional modeling methods to model the hull,which relies heavily on designer’s experience.The traditional method also has big shortage on the modeling of complex hull form,the reasons of which include curvature variation is large in bow and stern and offsets of hull are discrete data,so there is a big problem in the expression of surfaces of these areas.This paper,on the basis of traditional interpolation and approximation theory,obtain the profile curves of hull surface by constructing generalized station ordinates,then using an improved surface lofting method to get the accurate hull surface model.2.Full parametric modeling method of complex ship hull formThis part of content is based on the full parametric modeling method of simple ship hull form proposed previously by predecessors.The complete set of form parameters of all kinds of cross-sectional curves of complex ship hull form are firstly constructed,different section types corresponding to different types of form parameters.Since form parameters are not completely independent between each other,the incidence and coupling relation between those parameters are studied so that designers can rationally selected parameter values and their feasible domain in the process of modeling and the subsequent optimization.The parametric modeling method of hull curve,based on the NURBS expression method,is proposed by using the form parameter sets obtained in the last step.The mapping relationship between form parameters and hull curves are constructed and the form parameter extraction method based on the geometric characteristics of parent ship is presented.The applicability,flexibility and fairness of the parametric modeling method are demonstrated by giving design examples of various types of cross-sectional curve.According to the hull curve constructed by the parametric modeling method,the hull surface division criterion and cross-sectional curves position configuration are analyzed firstly;Then the hull surface model are obtained by utilizing improved surface lofting method;Finally the effectiveness of the hull surface parametric modeling method is verified by several examples.3.Construction of fairness invariant and fairness functional of curve and surfaceIn view of potential problems of approximate linearized and discretized fairness functional,the second order and third order behavior of curve and surface are analyzed,which takes into account normal curvature and normal curvature variation along all directions of the tangent plane at a given surface point.Two surface geometric invariants are proposed as fairness measures of ship hull,based on which the fairness functional is constructed for hull surface design.The designers are guided to select appropriate fairness functional according to different design requirements by means of analyzing the impact of different functional on surface shape.