Ship Hull form Modification using Shift Method based on Hydrodynamic Performances Simulated by WAVIS

Abstract

The field of optimization is attracting widespread interest due to the fact that an optimum design provides economical and performance benefits. Consequently, the demand for optimum ship design based on computational fluid dynamics has increased since the ships that undergo an optimization process are more energy saving and competitive. Designers are making enormous efforts to find optimum ship design based on various constraints with different solvers. Generally, finding the optimum ship design is a process consisting of three modules; Computer-Aided Design/ Manufacturing (CAD/CAM) and modification module, computational fluid dynamic (CFD) module and optimization module. CAD/CAM module generates the geometry definition of ship hull. The modification module adjusts the ship hull according to the necessary conditions of the optimization process. The CFD module analyzes the flow around the ship and hydrodynamic performances of a ship hull with different solvers. The optimization module finds the minimum or maximum design under various constraints which are dependent on the designer’s preferences. Optimization can be considered as an iterative process that continuously modifies variables until a ‘perfect’ optimum achieved in other words, the condition which meet all the constraints. To get the reasonable geometry of a ship during an optimization process, efficient and effective modification methods play a crucial role. Among all the available modification methods this paper will focus on use of the shift method. The shift method is a technique extended from Lackenby’s approach and deals with the sectional area curve, modifying the ship hull geometry by moving the sections in a longitudinal direction. In describing the ship hull form, the sectional area curve is one of the most important aspects that influences on the geometry of hull. The author of this study selected the shift method for modification of ship hull form because this method has demonstrated to be simple and effective. Nowadays, most of the ship designers use this method because of its practicality. As the use for ships optimization based on CFD has increased, various CFD solvers such as OpenFOAM, ANSYS and SHIPFLOW have been developed despite the fact that each solver has limitations when connecting to the CAD software. Among these choices for CFD solvers, it is necessary to select which one is suitable to solve specific problems and for the required modifications. To reach to a continuous process of optimization, it is necessary to link the previously mentioned modules to achieve a computational efficiency. Since each one of the CFD solvers have some limitation on linking, there is a possibility for difficulties to occur such as compatibility and functionality problems. For example, OpenFOAM has its particular format to link with certain CAD modules. Based on this, the range of software options are limited. The aim of this study is to develop a program to link the modification module and CFD module to reduce the limitations that can appear during the optimization process of hull form. For the modification module, a shift method will be used and the CFD calculation will be simulated by WAVIS (version 1.3) as a potential flow solver in this study. The reason of using a potential solver lies in the fact that computation times are lower than viscous solvers. As an application example, KCS ship hull form is used for validity tests and KCS hull form was modified by the integrated program developed by the author of this paper.

최적화 과정을 거친 선박이 더 많은 에너지를 절약하고 경쟁력을 갖기 때문에 전산 유체 역학(CFD)에 기반한 최적의 선박 설계에 대한 요구가 증가하고 있다. 이러한 최적화 과정에서는 선형을 몇 개의 파라미터로 표시할 수 있는 기법이 필요하다. 즉, 최적화 루틴에서 변화된 파라미터로 새로운 선형이 생성되어야 하고 이를 CFD로 계산하여 좀더 나은 목적함수를 찾아가는 과정을 반복해야 한다. 일반적으로 최적의 선박 설계를 찾는 것은 CAD / CAM 및 수정 모듈, CFD 모듈 및 최적화 모듈의 세 가지 모듈로 구성된 프로세스이다. CAD / CAM 모듈은 선체의 형상 정의를 생성한다. 수정 모듈은 최적화 과정의 필요한 조건에 따라 선박 선체를 수정한다. CFD 모듈은 흐름과 선체의 유체 역학 성능을 계산한다. 최적화 과정에서 합리적인 기하학 구조를 얻으려면 효율적이고 효과적인 수정 방법이 중요한 역할을한다. 본 논문은 선형을 수정 기법으로Shift Method를 이용하여 단면적 곡선,Cp 곡선을 변환 시키는 방법을 적용하고 있다. 선박 선체 형태를 표현할 때, Cp 곡선은 선체의 기하학에 영향을 미치는 가장 중요한 측면 중 하나이다. 본 연구의 저자는 이 방법이 간단하고 효과적이기 때문에 선박 선체 형태의 수정 방법으로 선택했다. 그리고 CFD 기법으로는 WAVIS(version 1.3)를 사용하고, 최적화 기법으로는 SQP(Sequential Quadratic Programming)법을 사용하고 있다. 연속적인 최적화 프로세스에 도달하려면 앞서 언급 한 모듈을 연결하여 계산 효율성을 달성해야한다. 본 논문의 의의로서는 이러한 선형 변환 분야(CAD), CFD, 최적화 기법인 SQP를 통합하는 프로그램을 C++ 언어로 작성하였다는 것이다. 따라서 세부분 각각을 다른 프로그램으로 교체할 수 있는 능력이 생기게 되고, 이를 이용하여 계속 발전된 결과를 얻을 수 있다.