Kyung-Kyu Yang

Systematic Experimental and Numerical Analyses on Added Resistance in Waves

국제적으로 친환경에 대한 관심이 증가하고, 국제해사기구 (International Maritime Organization, IMO)가 선박의 온실가스 방출을 규제하기 위해서 2013년부터 선박연비제조지수 (Energy Efficiency Design Index, EEDI)를 도입하면서 조선 및 해운업계 에서는 선박의 운항 효율이 주요 관심사가 되었다. 선박의 운항 효율을 향상시키기 위한 많은 방법 중에 하나로 파랑 중 저항을 감소시키는 방법이 고려되고 있다. 부가저항이란 선박이 실제 해 상에서 운항하는 경우 바람이나 파랑에 의해 정수 중 저항보다 증가하는 저항을 말하며, 이러한 부가저항은 선박에 따라서 정수 중에서 운항하는 경우의 저항보다 15~30%까지 커지는 경우도 있다. 따라서 부가저항을 정확히 예측하고 이를 효과적으로 감소 시키고자 하는 연구들이 수행되고 있다. 본 논문에서는 최근 수행되었던 부가저항의 예측을 위한 수조 실험 및 수치계산 기법에 대한 연구들에 대하여 체계적으로 살펴 보고, 그 결과들을 비교하여 여러 기법들에 대한 검증과 장단점, 그리고 실선 적용에 대한 활용성 등을 다루고자 한다. 파랑 중 부가저항의 추정을 위해 실험 및 수치적 연구는 1970~1980년대에 많이 수행이 되었으나 한동안 주목을 받지 못 하다가 최근 그린쉽에 대한 관심이 고조되며 근래들어 여러 연구 들이 수행되고 있다. 부가저항의 초기 연구는 실험에 기반하여 진행되었다. 부가저항 실험은 1970년대부터 Series 60 선형 (Gerritsma & Beukelman, 1972; Storm-Tejsen, et al., 1973), S175 컨테이너선 모델 (Fujii & Takahashi, 1975; Nakamura & Naito, 1977), Wigley 모델 (Journee, 1992) 등에 대한 연구가 진행되었다. 최근 Kuroda, et al. (2011)은 컨테이너선 기본선형 을 기준으로 선수부의 형상을 다양하게 변화시켜가면서 이에 따 른 부가저항 변화를 살펴보았다. 또한, Lee, et al. (2013)도 최 pISSN:1225-1143, Vol. 51, No. 6, pp. 459-479, December 2014

Analysis of Large-Amplitude Ship Motions Using a Cartesian-Gridbased Computational Method

In this study, a Cartesian-grid method based on finite volume approach is applied to simulate the ship motions in large amplitude waves. Fractional step method is applied for pressure-velocity coupling and TVD limiter is used to interpolate the cell face value for the discretization of convective term. Water, air, and solid phases are identified by using the concept of volume-fraction function for each phase. In order to capture the interface between air and water, the tangent of hyperbola for interface capturing (THINC) scheme is used with weighed line interface calculation (WLIC) method which considers multidimensional information. The volume fraction of solid body embedded in the Cartesian grid system is calculated using a level-set based algorithm, and the body boundary condition is imposed by a volume weighted formula. Numerical simulations for the two-dimensional barge type model and Wigley hull in linear waves have been carried out to validate the newly developed code. To demonstrate the applicability for highly nonlinear wave-body interactions such as green water on the deck, numerical analysis on the large-amplitude motion of S175 containership is conducted and all computational results are compared with experimental data.

Analysis of Added Resistance using a Cartesian-Grid-based Computational Method

In this paper, an Euler equation solver based on a Cartesian-grid method and non-uniform staggered grid system is applied to predict the ship motion response and added resistance in waves. Water, air, and solid domains are identified by a volume-fraction function for each phase and in each cell. For capturing the interface between air and water, the tangent of hyperbola for interface capturing (THINC) scheme is used with a weighed line interface calculation (WLIC) method. The volume fraction of solid body embedded in a Cartesian-grid system is calculated by a level-set based algorithm, and the body boundary condition is imposed by volume weighted formula. Added resistance is calculated by direct pressure integration on the ship surface. Numerical simulations for a Wigley III hull and an S175 containership in regular waves have been carried out to validate the newly developed code, and the ship motion responses and added resistances are compared with experimental data. For S175 containership, grid convergence test has been conducted to investigate the sensitivity of grid spacing on the motion responses and added resistances.

Analysis of Added Resistance in Short Waves

In this study, the added resistance of ships in short waves is systematically studied by using two different numerical methods - Rankine panel method and Cartesian grid method - and existing asymptotic and empirical formulae. Analysis of added resistance in short waves has been preconceived as a shortcoming of numerical computation. This study aims to observe such preconception by comparing the computational results, particularly based on two representative three-dimensional methods, and with the existing formulae and experimental data. In the Rankine panel method, a near-field method based on direct pressure integration is adopted. In the Cartesian grid method, the wave-body interaction problem is considered as a multiphase problem, and volume fraction functions are defined in order to identify each phase in a Cartesian grid. The computational results of added resistance in short waves using the two methods ar e systematically compared with experimental data for several ship models, including S175 containership, KVLCC2 and Series 60 hulls (CB = 0.7, 0.8). The present study includes the comparison with the established asymptotic and empirical formulae in short waves.

Analysis of Added Resistance: Comparative Study on Different Methodologies

This paper considers the comparative study on added resistance for different methodologies. An accurate prediction of added resistance and resultant power increase becomes an important issue in greenship design. There are several methodologies for the prediction of added resistance, and most of them are based on frequency domain approaches such as slender-body theory or wave Green-function approach. As the time-domain approaches becomes an alternative method for seakeeping analysis, the time-domain approaches are also applicable for added resistance prediction.In this paper, a few approaches have been applied for the prediction of added resistance on different hull forms. The methods to be considered in this study are (i) slender-body method, (ii) Rankine panel methods, (iii) Cartesian-grid-based Euler solver, and (iv) short-wave approximations. Both the far- and near-field formations are considered in the slender-body and Rankine panel methods, while the direct pressure integration is applied for the CFD method. The computational results are validated by comparing them with experimental data on Wigley hull, Series 60 hull, and S175 containership, showing reasonable agreements for all models. The study is extended to the analysis of added resistance in short wavelengths.© 2013 ASME

Numerical Simulation of 3D Free-Surface Flows by Using CIP-based and FV-based Methods

In this paper, three-dimensional free-surface flows are simulated by using two different numerical methods, the constrained interpolation profile (CIP)-based and finite volume (FV)-based methods. In the CIP-based method, the governing equations are solved on stationary staggered Cartesian grids by a finite difference method, and an immersed boundary technique is applied to deal with wave-body interactions. In the FV-based method, the governing equations are solved by applying collocated finite volume discretization, and body-fitted meshes are used. A free-surface boundary is considered as the interface of the multi-phase flow with air and water, and a volumeof-fluid (VOF) approach is applied to trace the free surface. Among many variations of the VOF-type method, the tangent of hyperbola for interface capturing (THINC) and the compressive interface capturing scheme for arbitrary meshes (CICSAM) techniques are used in the CIP-based method and FV-based method, respectively. Numerical simulations have been carried out for dam-breaking and wave-body interaction problems. The computational results of the two methods are compared with experimental data and their differences are observed.

Comparative Study on Sloshing Impact Flows between PIV and CFD

In this study, experimental and numerical methods were applied to observe sloshing impact phenomena. A two-dimensional rectangular tank filled with water and air was considered with a specific excitation condition that induced a hydrodynamic impact without an air pocket at the top corner of the tank. High-speed cameras and a pressure measurement system were synchronized, and a particle image velocimetry (PIV) technique was applied to measure the velocity field and corresponding pressure. The experimental condition was implemented in a numerical computation to solve incompressible two-phase flows using a Cartesian-grid method. The discretized solution was obtained using the finite difference and constraint-interpolation-profile (CIP) methods, which adopt a fractional step scheme for coupling the pressure and velocity. The tangent of the hyperbola for interface capturing (THINC) scheme was used with the weighed line interface calculation (WLIC) method to capture the interface between the air and water. The calculated impact pressures and velocity fields were compared with experimental data, and the relationship between the local velocity and pressure was investigated based on the computational results.

Numerical Analysis of Violent Sloshing Problems by CCUP Method

In the present paper, a numerical method based on the constraint interpolation profile (CIP) method is applied for simulating two-dimensional violent sloshing problems. The free surface boundary value problem is considered as a multiphase problem which includes water and air. A stationary Cartesian grid system is adopted, and an interface capturing method is used to trace the shape of free surface profile. The CIP combined unified procedure (CCUP) scheme is applied for flow solver, and the tangent of hyperbola for interface capturing (THINC) scheme is used for interface capturing. Numerical simulations have been carried out for partially-filled 2D tanks under forced sway and roll motions at various filling depths and frequencies. The computational results are compared with experiments and/or the other numerical results to validate the present numerical method.

A Conceptual Framework for Assessing the Potential of Ocean Mining Sites

Ocean mining is considered to be the new offshore frontier and has been the center of research and commercial focus over the past few years. The further development of the ocean mining industry posts challenges in many fields including engineering, economics, environment, law, logistics and supply chain. This research aims to understand the challenges and link these fields by developing a framework for assessing the potential of ocean mining sites. Seabed resources and associated exploration and exploitation technologies are reviewed. Based on this review, it identifies the most promising ocean mining sites, the massive sulphide deposits in inactive hydrothermal vents, along the oceanic ridge in the Exclusive Economic Zone. An online survey is conducted to obtain a broader academic and industrial view on ocean mining. The world’s first commercial ocean mining project developed by Nautilus Minerals Inc. is also analyzed as a case study. Based on the seabed resources review, online survey and case study, the major challenges in ocean mining are presented, covering engineering systems, environmental risk mitigation, economic feasibility, law, logistics and supply chain.Copyright