• 대한조선학회논문집
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• 제44권6호
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• pp.564-571
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• 2007

In the present study, stern form optimization has been carried out using computational fluid dynamics (CFD) techniques. The viscous pressure drag has been minimized to optimize stern shape. Parametric modification function has been used to modify the shape of the hull. By the use of the parametric modification function and algebraic scheme to grid manipulation, the initial ship geometry was easily deformed according to change of design parameters. For purpose of illustration, KRISO 319K VLCC (KVLCC) is chosen for example ship to demonstrate stern form optimization. The numerical results indicate that the optimized hull yields a reduction in viscous resistance.

• 설비공학논문집
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• 제13권10호
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• pp.1025-1033
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• 2001

A scaling analysis is provided which predicts the sliding velocity of a liquid drop down an inclined surface. The analysis is based on the balance of the gravitational work rate that drives the drop sliding and the resistances by capillary and viscous forces. The capillary resistance is accounted for via the contact angle hysteresis, which is quantified by measuring the critical inclination causing the drop to start sliding. The sliding of the drop is governed by the rate of the viscous dissipation of the Stokes flow. The analysis result in its limit form for small contact angles is consistent with previous results. In the experiments to verify the analysis results, the measured sliding velocity of various liquid drops are shown to obey the predictions made in this study.

• 대한조선학회지
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• 제25권3호
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• pp.13-18
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• 1988

Three-dimensional turbulent flows over ship sterns are measured by using the hot-wire anemometer and static holes in the wind tunnel. A conventional stern and a barge-type stern are adopted for the present study. Three components of mean velocities, pressures on the hull and six components of Reynolds stresses are measured. Longitudinal velocity contours are more parallel to the hull surface and weak bilge vortices appear on the barges type stern rather than the conventional stern. Those viscous flow patterns may have close relations to improvements of the resistance and propulsion performance, which have been verified in the towing tests. Measured data files are valuable for the ensuing numerical studies.

• International Journal of Naval Architecture and Ocean Engineering
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• 제12권1호
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• pp.11-19
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• 2020

In recent years, oil prices have continued to be low owing to the development of unconventional resources such as shale gas, coalbed methane gas, and tight gas. However, shipping companies are still experiencing difficulties because of recession in the shipping market. Hence, they devote considerable effort toward reducing operating costs. One of the important parameters for reducing operating costs is the frictional resistance of vessels. Generally, a vessel is covered with paint for smoothing its surface. However, frictional resistance increases with time owing to surface roughness, such as that caused by fouling. To prevent this, shipping companies periodically clean or repaint the surfaces of vessels using analyzed operating data. In addition, studies using various methods have been continuously carried out to identify this phenomenon such as fouling for managing ships more efficiently. In this study, numerical simulation was used to analyze the change in the resistance performance of a ship owing to an increase in surface roughness using commercial software, i.e., Star-CCM+, which solves the continuity and Navier eStokes equations for incompressible and viscous flow. The conditions for numerical simulation were verified through comparison with experiments, and these conditions were applied to three ships to evaluate resistance performance according to surface roughness.

• 한국항해항만학회지
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• 제37권1호
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• pp.1-7
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• 2013

본 연구에서는 개발된 선미부에 수직날개를 부착한 선박의 조파저항성능을 예측할 수 있는 수치해석기법의 검증에 관한 것이다. 수치해석기법은 비점성 유동장 해석기법인 랜킨소오스 패널법과 와류격자법을 사용하여 개발하였으며, 자유수면 경계조건의 비선형성은 반복해법을 사용하여 만족시켰고, 선박의 트림과 침하량을 구하는 알고리즘을 포함하고 있다. 수치해석을 위한 선체표면의 패널을 생성하기 위하여 패널절단법을 사용하였다. 4000TEU 컨테이너 운반선을 대상 선박으로 하여 선미부 6개소의 서로 다른 위치에 수직날개를 부착하여 수치해석을 수행하였으며, 수치해석기법의 타당성을 검증하기 위하여 상용 점성 유동장 해석 프로그램인 FLUENT를 사용하여 선체 주위의 점성 유동장을 계산하였고, 모형시험을 수행하여 얻은 실험 결과를 수치해석 결과와 서로 비교하였다.

• Journal of Hydrospace Technology
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• 제2권2호
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• pp.44-56
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• 1996

The effects of the air on the reductions in resistance when supplied under the bottom of a semi-planing ship with a step are investigated in the present study. A 1.275m long FRP model is constructed and the pressure and viscous tangential stresses over the planing surface of the hull with and without air supply are measured through measuring holes carefully selected at the towing tank of Seoul National University. Locations of holes most suitable for air injection are surveyed in front of the planing surface of the model with careful examinations of the limiting streamlines and pressure distributions measured without air supply. At those locations, found to be just front of the step, air has been supplied into a wake region to form an air filled cavity of fixed type. Flow rates and pressure of the supplied air as well as the local pressure and shear stress distributions on the hull surface are measured to understand the physics involved as well as to determine the conditions most effective in resistance reduction at the design speed. It has been found that total resistance of the stepped semi-planing hull can be considerably reduced if an air cavity generated by an adequate air injection at the bottom of the hull near the step. After the cavity optimized at the given speed, air bubbles also have been generated right behind the point where dividing streamlines re-attach to further reduce the frictional resistance but found to be not so effective as the air cavity in resistance reductions.

• Selected Papers of The Society of Naval Architects of Korea
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• 제1권1호
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• pp.65-75
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• 1993

Hull form development for an AFRAMAX tanker characterized by the form parameters of $C_B\simeq0.8$, $L/B\simeq5.5$, $B/T\simeq3.5$, han been carried out by the application of ‘Composite Stern Frameline Concept’. The viscous resistance of the new form was much smaller than that of the conventional form. Form factor of the new form was only 0.18 compared to 0.30 for the conventional hull form, Nevertheless the propulsive efficiency was slightly lower and thus the required propulsion power was smaller by 5~6% at both full load and ballast condition. In addition, it is confirmed that introduction of the form factor method such as ITTC’78 method is highly advisable because there is a great risk of the underperdicting full scale resistance of the hull form whose form the extrapolation of moel resistance to full scale is to be based on Froude method with the correlation allowances usually applied to conventional hull forms.

• 대한조선학회지
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• 제24권2호
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• pp.11-19
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• 1987

A Series 60, $C_b=0.60$ model was tested in the towing tank of Seoul National University. Total resistance, hull attitude, wake distributions and wave measured at FR condition(free trim and sinkage) and FX condition(fixed trim and sinkage). From the measured data, residual, viscous and wave pattern resistance components were evaluated and compared. It is found that the changes in wetted surface area should be considered in predictions of frictional resistances, and can be easily found from hydrostatic data and measured mean sinkages without additional tests. Applications of the concept to the geosim tests of Series 60, Wigley, Lucy Ashton models show that the conventional extrapolation method can be improved considerably.

• International Journal of Naval Architecture and Ocean Engineering
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• 제10권5호
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• pp.629-643
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• 2018

For a sailing ship, the frictional resistance exerted on the hull of ship is due to viscous effect of the fluid flow, which is proportional to the wetted area of the hull and moving speed of ship. This resistance can be reduced through air bubble lubrication to the hull. The traditional way of introducing air to the wetted hull consumes extra energy to retain stability of air layer or bubbles. It leads to lower reduction rate of the net frictional resistance. In the present paper, a novel air bubble lubrication technique proposed by Kumagai et al. (2014), the Winged Air Induction Pipe (WAIP) device with opening hole on the upper surface of the hydrofoil is numerically investigated. This device is able to naturally introduce air to be sandwiched between the wetted hull and water. Propulsion system efficiency can be therefore increased by employing the WAIP device to reduce frictional drag. In order to maximize the device performance and explore the underlying physics, parametric study is carried out numerically. Effects of submerged depth of the hydrofoil and properties of the opening holes on the upper surface of the hydrofoil are investigated. The results show that more holes are favourable to reduce frictional drag. 62.85% can be achieved by applying 4 number of holes.

• 한국해양공학회지
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• 제18권5호
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• pp.57-63
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• 2004

Various propulsion systems, applicable for a G/T 47,000 class mid-size cruise ship, are discussed and a comparative study on stern forms and hull resistance characteristics is carried out, in relation to these propulsion systems. Based on shipyard production logs on similar cruise ships, a reference hull form of a single shaft propulsion system with center-skeg, is generated. Then two new stern hull forms are derived by using a hull transform technique: consisting of one stern form using a twin-skeg system and the other using the Azipod system. Using a CFD-based commercial flaw analysis program, WAVIS (WAve and VIScous flaw analysis system for hull form development), various hydrodynamic characteristics, including wave profiles and ship hull resistance, are compared for three hull forms.