• Journal of the Society of Naval Architects of Korea
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• 제51권6호
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• pp.480-488
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• 2014

Vortex shedding which is the dominant feature of body wakes and of direct relevance to practical engineering problems, has been intensively studied for flows past a circular cylinder. In contrast, vortex shedding from a hydrofoil trailing edge has been studied to much less extent despite numerous practical applications. The physics of the problem is still poorly understood. The present study deals with $K{\acute{a}}rm{\acute{a}}n$ vortex shedding from a truncated trailing-edge hydrofoil in relatively high Reynolds number flows. The objectives of this paper are twofold. First, we aim to simulate unsteady turbulent flows past a two dimensional hydrofoil through a hybrid particle-mesh method and penalization method. The vortex-in-cell (VIC) method offers a highly efficient particle-mesh algorithm that combines Lagrangian and Eulerian schemes, and the penalization method enables to enforce body boundary conditions by adding a penalty term to the momentum equation. The second purpose is to investigate shedding frequencies of vortices behind a NACA 0009 hydrofoil operating at a zero angle of attack.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제24권2호
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• pp.177-187
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• 2000

Measurements of temperature fields and film-cooling effectiveness have been conducted for a shower-head film cooling on the leading edge of a blunt body, which simulates a first-stage turbine stator. In this study, three injection cases are employed for an average blowing ratio based on freestream velocity, M, of 0.5, 1.0 and 1.5. Two (Case 1), four (Case 2) and six (Case 3) rows of normal holes are symmetrically drilled on the three tested circular-cylinder leading edges. The measurements show that regardless of M, the film-cooling effectiveness increases as the injection row is situated at farther downstream location. In Case 1, the film-cooling effectiveness is highest for M = 0.5 and lowest for M = 1.5. On the contrary, in Case 3, the film-cooling effectiveness is highest for M = 1.0 and lowest for M = 0.5. When M = 0.5, the film coverage by the first row of the injection holes deteriorates as the number of the injection row increases. In particular, the film-cooling effectiveness due to the injection through the first row of the holes in Case 3, has a nearly zero value.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 춘계학술대회논문집
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• pp.245-251
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• 2009

Numerical algorithms for solving the incompressible Navier-Stokes equations using P2P1 finite element are compared regarding the eigenvalues of matrices. P2P1 element allocates pressure at vertex nodes and velocity at both vertex and mid nodes. Therefore, compared to the P1P1 element, the number of pressure variables in the P2P1 element decreases to 1/4 in the case of two-dimensional problems and to 1/8 in the three-dimensional problems. Fully-implicit-integrated, semi-implicit- integrated and semi-segregated finite element formulations using P2P1 element are compared in terms of elapsed time, accuracy and eigenvlue distribution (condition number). For the comparison,they have been applied to the well-known benchmark problems. That is, the two-dimensional unsteady flows around a fixed circular cylinder and decaying vortex flow are adopted to check spatial accuracy.

• Aerospace Engineering and Technology
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• 제2권2호
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• pp.124-132
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• 2003

This paper described the structural design and the fabrication procedure of KSR-III composite pressure tank. The type of the composite pressure tank was COPV(Composite Overwrapped Pressure Vessel). A non-load sharing liner was made of aluminum 6061-0 and the liner provided a helium gas seal. The composite pressure tank was winded using T700 carbon/epoxy on the liner. Because the aluminum liner was thin, multiple cure cycles were applied to the filament winding technique. The multiple cure cycles prevented the liner-cylinder from losing a circular shape. A fitting force at the metallic boss was spread to the carbon fiber by a boss ring. The boss ring also prevented a local deformation at the boss part.

• Journal of electromagnetic engineering and science
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• 제8권3호
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• pp.100-109
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• 2008

The finite volume time domain(FVTD) technique faces serious limitations in simulating electromagnetic scattering at high frequencies due to requirements related to discretization. A modified FVTD method is proposed for electrically large, perfectly conducting scatterers by partially incorporating a time-domain physical optics(PO) approximation for the surface current. Dominant specular returns in the modified FVTD method are modeled using a PO approximation of the surface current allowing for a much coarser discretization at high electrical sizes compared to the original FVTD scheme. This coarse discretization can be based on the minimum surface resolution required for a satisfactory numerical evaluation of the PO integral for the scattered far-field. Non-uniform discretization and spatial accuracy can also be used in the context of the modified FVTD method. The modified FVTD method is aimed at simulating electromagnetic scattering from geometries containing long smooth illuminated sections with respect to the incident wave. The computational efficiency of the modified FVTD method for higher electrical sizes are shown by solving two-dimensional test cases involving electromagnetic scattering from a circular cylinder and a symmetric airfoil.

• Journal of Advanced Marine Engineering and Technology
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• 제29권7호
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• pp.801-807
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• 2005

Any failure of intake and exhaust valves of marine diesel engine must be regarded as serious, and any steps which can be taken to prevent such failure are desirable. The purposes of this study is to investigate and to analyse the failure causes of intake and exhaust valves for marine diesel engine during sea trial after completion of overhauling. In this study, to analyse the failure causes, we have carried out on board inspection, fractography test and discussion based on the specimen and repairing report provided by the ship owner. From the results of above inspection, test and discussion, it has been considered reasonable to conclude that the causes of damaged valves of the ship are as follow ; 1) During operation, the stick or seizure of valve spindle occurred and hence the movement of exhaust valve spindle was to be resisted and subsequently the engine was to be operated under an unappropriated valve timing and the exhaust valve sustained the repeated loads exceeding the fatigue strength of valve material. 2) By the loads above described, the fatigue fracture was initiated at the structural noncontinuous part of exhaust valve spindle, and then the valve head was finally fractured and dropped in the cylinder. 3) The fractured exhaust valve head impacted the intake valve at various direction to be bent or damaged.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제26권6호
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• pp.779-787
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• 2002

The cooling heat transfer by impinging water spray jets on a rotating roll with a relatively large diameter has been investigated under various experimental conditions with 3 different sizes of flat type nozzle. The local heat transfer coefficients were calculated by finite difference method using measured surface temperatures of the circular cylinder as boundary conditions. Results show that a peak value of the heat transfer coefficient is located at the center of sprayed area and there may be a secondary peak at the downstream. The average heat transfer coefficients on the sprayed area were found to be 10 to 22 ㎾/$m^2$$^{\circ}C$, and were not related to spraying pressure, but approximately linearly to flow rate of sprayed water. Also it is found that increasing the distance from roll to nozzle could improve the cooling efficiency by increasing the sprayed area.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• 제30권3호
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• pp.87-95
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• 2002

원주의 항력 감소와 원주 주위 유동의 제어에 대하여 실험적으로 연구하였다. 원주 주위 유동을 제어하기 위하여 작은 제어봉을 원주 전방에 설치하였다. 실험시 원주 직경(D=30mm)에 기초한 레이놀즈수는 $Re=2 {/times} 10^4$이었다. 제어봉의 직경(d)은 4mm, 5mm, 6mm, 7mm로 변화시켰으며, 원주와 제어봉 사이의 거리(L)를 조절하면서 원주의 표면압력분포와 후류속도를 측정하였다. 제어봉으로부터의 와 유출이 사라지는 임계거리 (critical distance) $ L_C$는 $ L_C$ /D=1.5+0.83d이었는데, $ L_C$를 전후로 유동특성과 항력값이 크게 바뀌었다. 특히 원주와 제어봉 사이의 거리가 임계거리보다 짧은 경우에는 항력이 감소하다가 $ L_C$보다 크게 되면 증가하는 경향을 보였다. 제어봉의 직경이 d/D=0.233인 경우, L/D=2.013의 조건에서 원주는 약 29%의 항력 감소를 보였고, 전체 항력은 L/D=1.833조건에서 약 25% 항력이 감소하였다.

• Korea-Australia Rheology Journal
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• 제16권1호
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• pp.47-54
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• 2004

Low Reynolds number, dilute, and surfactant-free bubble suspensions are prepared by mechanical mixing after introducing carbon dioxide bubbles into a Newtonian liquid, polyol. The apparent shear viscosity is measured with a wide-gap parallel plate rheometer by imposing a simple shear flow of capillary numbers(Ca) of the order of $10^{-2}$ ~ $10^{-1}$ and for various gas volume fractions ($\phi$). Effects of capillary numbers and gas volume fractions on the viscosity of polyol foam are investigated. At high capillary number, viscosity of the suspension increases as the gas volume fraction increases, while at low capillary number, the viscosity decreases as the gas volume fraction increases. An empirical constitutive equation that is similar to the Frankel and Acrivos equation is proposed by fitting experimental data. A numerical simulation for deformation of a single bubble suspended in a Newtonian fluid is conducted by using a newly developed two-dimensional numerical code using a finite volume method (FVM). Although the bubble is treated by a circular cylinder in the two dimensional analysis, numerical results are in good agreement with experimental results.

• Transactions of the Korean Society of Mechanical Engineers A
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• 제32권3호
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• pp.267-273
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• 2008

The circular cylinder pipes are used in the many industrial areas. In this paper, the acoustic wave propagation in the pipe containing a gas is researched. First of all, the theory for the coupled acoustic wave propagation in a pipe is investigated. Acoustic wave propagation in pipe can not be occurred independently between the wave of the fluid and the shell. It requires complicated analysis. However, as a special case, the coupled wave in a high density pipe containing a light density medium is corresponded closely to the uncoupled in-vacuo shell waves and to the rigid-walled duct fluid waves. The coincidence frequencies of acoustic and shell modes contribute to the predominant energy transmission. The coincidence frequency means the frequency corresponding to the coincidence of the wavenumber in both acoustic and shell. In this paper, it is assumed that the internal medium is much lighter than the pipe shell. After the uncoupled acoustic wave in the internal medium and uncoupled shell wave are considered, the coincidence frequencies are found. The analysis is successfully confirmed by the verification of the experiment using the real long steel pipe. This work verifies that the coupled wave characteristic of the shell and the fluid is occurred as predominant energy transmission at the coincidence frequencies.